Commit 77f2ea2f authored by Bjorn Helgaas's avatar Bjorn Helgaas

DMA-API: Clarify physical/bus address distinction

The DMA-API documentation sometimes refers to "physical addresses" when it
really means "bus addresses."  Sometimes these are identical, but they may
be different if the bridge leading to the bus performs address translation.
Update the documentation to use "bus address" when appropriate.

Also, consistently capitalize "DMA", use parens with function names, use
dev_printk() in examples, and reword a few sections for clarity.

No functional change; documentation changes only.
Signed-off-by: default avatarBjorn Helgaas <bhelgaas@google.com>
Acked-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: default avatarArnd Bergmann <arnd@arndb.de>
Acked-by: default avatarJames Bottomley <jbottomley@Parallels.com>
Acked-by: default avatarRandy Dunlap <rdunlap@infradead.org>
parent c9eaa447
......@@ -9,16 +9,76 @@ This is a guide to device driver writers on how to use the DMA API
with example pseudo-code. For a concise description of the API, see
DMA-API.txt.
Most of the 64bit platforms have special hardware that translates bus
addresses (DMA addresses) into physical addresses. This is similar to
how page tables and/or a TLB translates virtual addresses to physical
addresses on a CPU. This is needed so that e.g. PCI devices can
access with a Single Address Cycle (32bit DMA address) any page in the
64bit physical address space. Previously in Linux those 64bit
platforms had to set artificial limits on the maximum RAM size in the
system, so that the virt_to_bus() static scheme works (the DMA address
translation tables were simply filled on bootup to map each bus
address to the physical page __pa(bus_to_virt())).
CPU and DMA addresses
There are several kinds of addresses involved in the DMA API, and it's
important to understand the differences.
The kernel normally uses virtual addresses. Any address returned by
kmalloc(), vmalloc(), and similar interfaces is a virtual address and can
be stored in a "void *".
The virtual memory system (TLB, page tables, etc.) translates virtual
addresses to CPU physical addresses, which are stored as "phys_addr_t" or
"resource_size_t". The kernel manages device resources like registers as
physical addresses. These are the addresses in /proc/iomem. The physical
address is not directly useful to a driver; it must use ioremap() to map
the space and produce a virtual address.
I/O devices use a third kind of address: a "bus address" or "DMA address".
If a device has registers at an MMIO address, or if it performs DMA to read
or write system memory, the addresses used by the device are bus addresses.
In some systems, bus addresses are identical to CPU physical addresses, but
in general they are not. IOMMUs and host bridges can produce arbitrary
mappings between physical and bus addresses.
Here's a picture and some examples:
CPU CPU Bus
Virtual Physical Address
Address Address Space
Space Space
+-------+ +------+ +------+
| | |MMIO | Offset | |
| | Virtual |Space | applied | |
C +-------+ --------> B +------+ ----------> +------+ A
| | mapping | | by host | |
+-----+ | | | | bridge | | +--------+
| | | | +------+ | | | |
| CPU | | | | RAM | | | | Device |
| | | | | | | | | |
+-----+ +-------+ +------+ +------+ +--------+
| | Virtual |Buffer| Mapping | |
X +-------+ --------> Y +------+ <---------- +------+ Z
| | mapping | RAM | by IOMMU
| | | |
| | | |
+-------+ +------+
During the enumeration process, the kernel learns about I/O devices and
their MMIO space and the host bridges that connect them to the system. For
example, if a PCI device has a BAR, the kernel reads the bus address (A)
from the BAR and converts it to a CPU physical address (B). The address B
is stored in a struct resource and usually exposed via /proc/iomem. When a
driver claims a device, it typically uses ioremap() to map physical address
B at a virtual address (C). It can then use, e.g., ioread32(C), to access
the device registers at bus address A.
If the device supports DMA, the driver sets up a buffer using kmalloc() or
a similar interface, which returns a virtual address (X). The virtual
memory system maps X to a physical address (Y) in system RAM. The driver
can use virtual address X to access the buffer, but the device itself
cannot because DMA doesn't go through the CPU virtual memory system.
In some simple systems, the device can do DMA directly to physical address
Y. But in many others, there is IOMMU hardware that translates bus
addresses to physical addresses, e.g., it translates Z to Y. This is part
of the reason for the DMA API: the driver can give a virtual address X to
an interface like dma_map_single(), which sets up any required IOMMU
mapping and returns the bus address Z. The driver then tells the device to
do DMA to Z, and the IOMMU maps it to the buffer at address Y in system
RAM.
So that Linux can use the dynamic DMA mapping, it needs some help from the
drivers, namely it has to take into account that DMA addresses should be
......@@ -29,17 +89,17 @@ The following API will work of course even on platforms where no such
hardware exists.
Note that the DMA API works with any bus independent of the underlying
microprocessor architecture. You should use the DMA API rather than
the bus specific DMA API (e.g. pci_dma_*).
microprocessor architecture. You should use the DMA API rather than the
bus-specific DMA API, i.e., use the dma_map_*() interfaces rather than the
pci_map_*() interfaces.
First of all, you should make sure
#include <linux/dma-mapping.h>
is in your driver. This file will obtain for you the definition of the
dma_addr_t (which can hold any valid DMA address for the platform)
type which should be used everywhere you hold a DMA (bus) address
returned from the DMA mapping functions.
is in your driver, which provides the definition of dma_addr_t. This type
can hold any valid DMA or bus address for the platform and should be used
everywhere you hold a DMA address returned from the DMA mapping functions.
What memory is DMA'able?
......@@ -123,9 +183,9 @@ Here, dev is a pointer to the device struct of your device, and mask
is a bit mask describing which bits of an address your device
supports. It returns zero if your card can perform DMA properly on
the machine given the address mask you provided. In general, the
device struct of your device is embedded in the bus specific device
struct of your device. For example, a pointer to the device struct of
your PCI device is pdev->dev (pdev is a pointer to the PCI device
device struct of your device is embedded in the bus-specific device
struct of your device. For example, &pdev->dev is a pointer to the
device struct of a PCI device (pdev is a pointer to the PCI device
struct of your device).
If it returns non-zero, your device cannot perform DMA properly on
......@@ -147,8 +207,7 @@ exactly why.
The standard 32-bit addressing device would do something like this:
if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32))) {
printk(KERN_WARNING
"mydev: No suitable DMA available.\n");
dev_warn(dev, "mydev: No suitable DMA available\n");
goto ignore_this_device;
}
......@@ -170,8 +229,7 @@ all 64-bits when accessing streaming DMA:
} else if (!dma_set_mask(dev, DMA_BIT_MASK(32))) {
using_dac = 0;
} else {
printk(KERN_WARNING
"mydev: No suitable DMA available.\n");
dev_warn(dev, "mydev: No suitable DMA available\n");
goto ignore_this_device;
}
......@@ -187,8 +245,7 @@ the case would look like this:
using_dac = 0;
consistent_using_dac = 0;
} else {
printk(KERN_WARNING
"mydev: No suitable DMA available.\n");
dev_warn(dev, "mydev: No suitable DMA available\n");
goto ignore_this_device;
}
......@@ -201,8 +258,7 @@ Finally, if your device can only drive the low 24-bits of
address you might do something like:
if (dma_set_mask(dev, DMA_BIT_MASK(24))) {
printk(KERN_WARNING
"mydev: 24-bit DMA addressing not available.\n");
dev_warn(dev, "mydev: 24-bit DMA addressing not available\n");
goto ignore_this_device;
}
......@@ -232,14 +288,14 @@ Here is pseudo-code showing how this might be done:
card->playback_enabled = 1;
} else {
card->playback_enabled = 0;
printk(KERN_WARNING "%s: Playback disabled due to DMA limitations.\n",
dev_warn(dev, "%s: Playback disabled due to DMA limitations\n",
card->name);
}
if (!dma_set_mask(dev, RECORD_ADDRESS_BITS)) {
card->record_enabled = 1;
} else {
card->record_enabled = 0;
printk(KERN_WARNING "%s: Record disabled due to DMA limitations.\n",
dev_warn(dev, "%s: Record disabled due to DMA limitations\n",
card->name);
}
......@@ -331,7 +387,7 @@ context with the GFP_ATOMIC flag.
Size is the length of the region you want to allocate, in bytes.
This routine will allocate RAM for that region, so it acts similarly to
__get_free_pages (but takes size instead of a page order). If your
__get_free_pages() (but takes size instead of a page order). If your
driver needs regions sized smaller than a page, you may prefer using
the dma_pool interface, described below.
......@@ -343,11 +399,11 @@ the consistent DMA mask has been explicitly changed via
dma_set_coherent_mask(). This is true of the dma_pool interface as
well.
dma_alloc_coherent returns two values: the virtual address which you
dma_alloc_coherent() returns two values: the virtual address which you
can use to access it from the CPU and dma_handle which you pass to the
card.
The cpu return address and the DMA bus master address are both
The CPU virtual address and the DMA bus address are both
guaranteed to be aligned to the smallest PAGE_SIZE order which
is greater than or equal to the requested size. This invariant
exists (for example) to guarantee that if you allocate a chunk
......@@ -359,13 +415,13 @@ To unmap and free such a DMA region, you call:
dma_free_coherent(dev, size, cpu_addr, dma_handle);
where dev, size are the same as in the above call and cpu_addr and
dma_handle are the values dma_alloc_coherent returned to you.
dma_handle are the values dma_alloc_coherent() returned to you.
This function may not be called in interrupt context.
If your driver needs lots of smaller memory regions, you can write
custom code to subdivide pages returned by dma_alloc_coherent,
custom code to subdivide pages returned by dma_alloc_coherent(),
or you can use the dma_pool API to do that. A dma_pool is like
a kmem_cache, but it uses dma_alloc_coherent not __get_free_pages.
a kmem_cache, but it uses dma_alloc_coherent(), not __get_free_pages().
Also, it understands common hardware constraints for alignment,
like queue heads needing to be aligned on N byte boundaries.
......@@ -381,29 +437,29 @@ type of data is "align" (which is expressed in bytes, and must be a
power of two). If your device has no boundary crossing restrictions,
pass 0 for alloc; passing 4096 says memory allocated from this pool
must not cross 4KByte boundaries (but at that time it may be better to
go for dma_alloc_coherent directly instead).
use dma_alloc_coherent() directly instead).
Allocate memory from a dma pool like this:
Allocate memory from a DMA pool like this:
cpu_addr = dma_pool_alloc(pool, flags, &dma_handle);
flags are SLAB_KERNEL if blocking is permitted (not in_interrupt nor
holding SMP locks), SLAB_ATOMIC otherwise. Like dma_alloc_coherent,
holding SMP locks), SLAB_ATOMIC otherwise. Like dma_alloc_coherent(),
this returns two values, cpu_addr and dma_handle.
Free memory that was allocated from a dma_pool like this:
dma_pool_free(pool, cpu_addr, dma_handle);
where pool is what you passed to dma_pool_alloc, and cpu_addr and
dma_handle are the values dma_pool_alloc returned. This function
where pool is what you passed to dma_pool_alloc(), and cpu_addr and
dma_handle are the values dma_pool_alloc() returned. This function
may be called in interrupt context.
Destroy a dma_pool by calling:
dma_pool_destroy(pool);
Make sure you've called dma_pool_free for all memory allocated
Make sure you've called dma_pool_free() for all memory allocated
from a pool before you destroy the pool. This function may not
be called in interrupt context.
......@@ -418,7 +474,7 @@ one of the following values:
DMA_FROM_DEVICE
DMA_NONE
One should provide the exact DMA direction if you know it.
You should provide the exact DMA direction if you know it.
DMA_TO_DEVICE means "from main memory to the device"
DMA_FROM_DEVICE means "from the device to main memory"
......@@ -489,14 +545,14 @@ and to unmap it:
dma_unmap_single(dev, dma_handle, size, direction);
You should call dma_mapping_error() as dma_map_single() could fail and return
error. Not all dma implementations support dma_mapping_error() interface.
error. Not all DMA implementations support the dma_mapping_error() interface.
However, it is a good practice to call dma_mapping_error() interface, which
will invoke the generic mapping error check interface. Doing so will ensure
that the mapping code will work correctly on all dma implementations without
that the mapping code will work correctly on all DMA implementations without
any dependency on the specifics of the underlying implementation. Using the
returned address without checking for errors could result in failures ranging
from panics to silent data corruption. A couple of examples of incorrect ways
to check for errors that make assumptions about the underlying dma
to check for errors that make assumptions about the underlying DMA
implementation are as follows and these are applicable to dma_map_page() as
well.
......@@ -516,12 +572,12 @@ Incorrect example 2:
goto map_error;
}
You should call dma_unmap_single when the DMA activity is finished, e.g.
You should call dma_unmap_single() when the DMA activity is finished, e.g.,
from the interrupt which told you that the DMA transfer is done.
Using cpu pointers like this for single mappings has a disadvantage,
Using cpu pointers like this for single mappings has a disadvantage:
you cannot reference HIGHMEM memory in this way. Thus, there is a
map/unmap interface pair akin to dma_{map,unmap}_single. These
map/unmap interface pair akin to dma_{map,unmap}_single(). These
interfaces deal with page/offset pairs instead of cpu pointers.
Specifically:
......@@ -550,7 +606,7 @@ Here, "offset" means byte offset within the given page.
You should call dma_mapping_error() as dma_map_page() could fail and return
error as outlined under the dma_map_single() discussion.
You should call dma_unmap_page when the DMA activity is finished, e.g.
You should call dma_unmap_page() when the DMA activity is finished, e.g.,
from the interrupt which told you that the DMA transfer is done.
With scatterlists, you map a region gathered from several regions by:
......@@ -588,18 +644,16 @@ PLEASE NOTE: The 'nents' argument to the dma_unmap_sg call must be
it should _NOT_ be the 'count' value _returned_ from the
dma_map_sg call.
Every dma_map_{single,sg} call should have its dma_unmap_{single,sg}
counterpart, because the bus address space is a shared resource (although
in some ports the mapping is per each BUS so less devices contend for the
same bus address space) and you could render the machine unusable by eating
all bus addresses.
Every dma_map_{single,sg}() call should have its dma_unmap_{single,sg}()
counterpart, because the bus address space is a shared resource and
you could render the machine unusable by consuming all bus addresses.
If you need to use the same streaming DMA region multiple times and touch
the data in between the DMA transfers, the buffer needs to be synced
properly in order for the cpu and device to see the most uptodate and
properly in order for the cpu and device to see the most up-to-date and
correct copy of the DMA buffer.
So, firstly, just map it with dma_map_{single,sg}, and after each DMA
So, firstly, just map it with dma_map_{single,sg}(), and after each DMA
transfer call either:
dma_sync_single_for_cpu(dev, dma_handle, size, direction);
......@@ -623,9 +677,9 @@ or:
as appropriate.
After the last DMA transfer call one of the DMA unmap routines
dma_unmap_{single,sg}. If you don't touch the data from the first dma_map_*
call till dma_unmap_*, then you don't have to call the dma_sync_*
routines at all.
dma_unmap_{single,sg}(). If you don't touch the data from the first
dma_map_*() call till dma_unmap_*(), then you don't have to call the
dma_sync_*() routines at all.
Here is pseudo code which shows a situation in which you would need
to use the dma_sync_*() interfaces.
......@@ -690,12 +744,12 @@ to use the dma_sync_*() interfaces.
}
}
Drivers converted fully to this interface should not use virt_to_bus any
longer, nor should they use bus_to_virt. Some drivers have to be changed a
little bit, because there is no longer an equivalent to bus_to_virt in the
Drivers converted fully to this interface should not use virt_to_bus() any
longer, nor should they use bus_to_virt(). Some drivers have to be changed a
little bit, because there is no longer an equivalent to bus_to_virt() in the
dynamic DMA mapping scheme - you have to always store the DMA addresses
returned by the dma_alloc_coherent, dma_pool_alloc, and dma_map_single
calls (dma_map_sg stores them in the scatterlist itself if the platform
returned by the dma_alloc_coherent(), dma_pool_alloc(), and dma_map_single()
calls (dma_map_sg() stores them in the scatterlist itself if the platform
supports dynamic DMA mapping in hardware) in your driver structures and/or
in the card registers.
......@@ -709,9 +763,9 @@ as it is impossible to correctly support them.
DMA address space is limited on some architectures and an allocation
failure can be determined by:
- checking if dma_alloc_coherent returns NULL or dma_map_sg returns 0
- checking if dma_alloc_coherent() returns NULL or dma_map_sg returns 0
- checking the returned dma_addr_t of dma_map_single and dma_map_page
- checking the dma_addr_t returned from dma_map_single() and dma_map_page()
by using dma_mapping_error():
dma_addr_t dma_handle;
......@@ -794,7 +848,7 @@ Example 2: (if buffers are allocated in a loop, unmap all mapped buffers when
dma_unmap_single(array[i].dma_addr);
}
Networking drivers must call dev_kfree_skb to free the socket buffer
Networking drivers must call dev_kfree_skb() to free the socket buffer
and return NETDEV_TX_OK if the DMA mapping fails on the transmit hook
(ndo_start_xmit). This means that the socket buffer is just dropped in
the failure case.
......@@ -831,7 +885,7 @@ transform some example code.
DEFINE_DMA_UNMAP_LEN(len);
};
2) Use dma_unmap_{addr,len}_set to set these values.
2) Use dma_unmap_{addr,len}_set() to set these values.
Example, before:
ringp->mapping = FOO;
......@@ -842,7 +896,7 @@ transform some example code.
dma_unmap_addr_set(ringp, mapping, FOO);
dma_unmap_len_set(ringp, len, BAR);
3) Use dma_unmap_{addr,len} to access these values.
3) Use dma_unmap_{addr,len}() to access these values.
Example, before:
dma_unmap_single(dev, ringp->mapping, ringp->len,
......
......@@ -4,22 +4,26 @@
James E.J. Bottomley <James.Bottomley@HansenPartnership.com>
This document describes the DMA API. For a more gentle introduction
of the API (and actual examples) see
Documentation/DMA-API-HOWTO.txt.
of the API (and actual examples), see Documentation/DMA-API-HOWTO.txt.
This API is split into two pieces. Part I describes the API. Part II
describes the extensions to the API for supporting non-consistent
memory machines. Unless you know that your driver absolutely has to
support non-consistent platforms (this is usually only legacy
platforms) you should only use the API described in part I.
This API is split into two pieces. Part I describes the basic API.
Part II describes extensions for supporting non-consistent memory
machines. Unless you know that your driver absolutely has to support
non-consistent platforms (this is usually only legacy platforms) you
should only use the API described in part I.
Part I - dma_ API
-------------------------------------
To get the dma_ API, you must #include <linux/dma-mapping.h>
To get the dma_ API, you must #include <linux/dma-mapping.h>. This
provides dma_addr_t and the interfaces described below.
A dma_addr_t can hold any valid DMA or bus address for the platform. It
can be given to a device to use as a DMA source or target. A cpu cannot
reference a dma_addr_t directly because there may be translation between
its physical address space and the bus address space.
Part Ia - Using large dma-coherent buffers
Part Ia - Using large DMA-coherent buffers
------------------------------------------
void *
......@@ -33,20 +37,21 @@ to make sure to flush the processor's write buffers before telling
devices to read that memory.)
This routine allocates a region of <size> bytes of consistent memory.
It also returns a <dma_handle> which may be cast to an unsigned
integer the same width as the bus and used as the physical address
base of the region.
Returns: a pointer to the allocated region (in the processor's virtual
It returns a pointer to the allocated region (in the processor's virtual
address space) or NULL if the allocation failed.
It also returns a <dma_handle> which may be cast to an unsigned integer the
same width as the bus and given to the device as the bus address base of
the region.
Note: consistent memory can be expensive on some platforms, and the
minimum allocation length may be as big as a page, so you should
consolidate your requests for consistent memory as much as possible.
The simplest way to do that is to use the dma_pool calls (see below).
The flag parameter (dma_alloc_coherent only) allows the caller to
specify the GFP_ flags (see kmalloc) for the allocation (the
The flag parameter (dma_alloc_coherent() only) allows the caller to
specify the GFP_ flags (see kmalloc()) for the allocation (the
implementation may choose to ignore flags that affect the location of
the returned memory, like GFP_DMA).
......@@ -61,24 +66,24 @@ void
dma_free_coherent(struct device *dev, size_t size, void *cpu_addr,
dma_addr_t dma_handle)
Free the region of consistent memory you previously allocated. dev,
size and dma_handle must all be the same as those passed into the
consistent allocate. cpu_addr must be the virtual address returned by
the consistent allocate.
Free a region of consistent memory you previously allocated. dev,
size and dma_handle must all be the same as those passed into
dma_alloc_coherent(). cpu_addr must be the virtual address returned by
the dma_alloc_coherent().
Note that unlike their sibling allocation calls, these routines
may only be called with IRQs enabled.
Part Ib - Using small dma-coherent buffers
Part Ib - Using small DMA-coherent buffers
------------------------------------------
To get this part of the dma_ API, you must #include <linux/dmapool.h>
Many drivers need lots of small dma-coherent memory regions for DMA
Many drivers need lots of small DMA-coherent memory regions for DMA
descriptors or I/O buffers. Rather than allocating in units of a page
or more using dma_alloc_coherent(), you can use DMA pools. These work
much like a struct kmem_cache, except that they use the dma-coherent allocator,
much like a struct kmem_cache, except that they use the DMA-coherent allocator,
not __get_free_pages(). Also, they understand common hardware constraints
for alignment, like queue heads needing to be aligned on N-byte boundaries.
......@@ -87,7 +92,7 @@ for alignment, like queue heads needing to be aligned on N-byte boundaries.
dma_pool_create(const char *name, struct device *dev,
size_t size, size_t align, size_t alloc);
The pool create() routines initialize a pool of dma-coherent buffers
dma_pool_create() initializes a pool of DMA-coherent buffers
for use with a given device. It must be called in a context which
can sleep.
......@@ -102,25 +107,26 @@ from this pool must not cross 4KByte boundaries.
void *dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags,
dma_addr_t *dma_handle);
This allocates memory from the pool; the returned memory will meet the size
and alignment requirements specified at creation time. Pass GFP_ATOMIC to
prevent blocking, or if it's permitted (not in_interrupt, not holding SMP locks),
pass GFP_KERNEL to allow blocking. Like dma_alloc_coherent(), this returns
two values: an address usable by the cpu, and the dma address usable by the
pool's device.
This allocates memory from the pool; the returned memory will meet the
size and alignment requirements specified at creation time. Pass
GFP_ATOMIC to prevent blocking, or if it's permitted (not
in_interrupt, not holding SMP locks), pass GFP_KERNEL to allow
blocking. Like dma_alloc_coherent(), this returns two values: an
address usable by the cpu, and the DMA address usable by the pool's
device.
void dma_pool_free(struct dma_pool *pool, void *vaddr,
dma_addr_t addr);
This puts memory back into the pool. The pool is what was passed to
the pool allocation routine; the cpu (vaddr) and dma addresses are what
dma_pool_alloc(); the cpu (vaddr) and DMA addresses are what
were returned when that routine allocated the memory being freed.
void dma_pool_destroy(struct dma_pool *pool);
The pool destroy() routines free the resources of the pool. They must be
dma_pool_destroy() frees the resources of the pool. It must be
called in a context which can sleep. Make sure you've freed all allocated
memory back to the pool before you destroy it.
......@@ -187,9 +193,9 @@ dma_map_single(struct device *dev, void *cpu_addr, size_t size,
enum dma_data_direction direction)
Maps a piece of processor virtual memory so it can be accessed by the
device and returns the physical handle of the memory.
device and returns the bus address of the memory.
The direction for both api's may be converted freely by casting.
The direction for both APIs may be converted freely by casting.
However the dma_ API uses a strongly typed enumerator for its
direction:
......@@ -198,31 +204,30 @@ DMA_TO_DEVICE data is going from the memory to the device
DMA_FROM_DEVICE data is coming from the device to the memory
DMA_BIDIRECTIONAL direction isn't known
Notes: Not all memory regions in a machine can be mapped by this
API. Further, regions that appear to be physically contiguous in
kernel virtual space may not be contiguous as physical memory. Since
this API does not provide any scatter/gather capability, it will fail
if the user tries to map a non-physically contiguous piece of memory.
For this reason, it is recommended that memory mapped by this API be
obtained only from sources which guarantee it to be physically contiguous
(like kmalloc).
Further, the physical address of the memory must be within the
dma_mask of the device (the dma_mask represents a bit mask of the
addressable region for the device. I.e., if the physical address of
the memory anded with the dma_mask is still equal to the physical
address, then the device can perform DMA to the memory). In order to
Notes: Not all memory regions in a machine can be mapped by this API.
Further, contiguous kernel virtual space may not be contiguous as
physical memory. Since this API does not provide any scatter/gather
capability, it will fail if the user tries to map a non-physically
contiguous piece of memory. For this reason, memory to be mapped by
this API should be obtained from sources which guarantee it to be
physically contiguous (like kmalloc).
Further, the bus address of the memory must be within the
dma_mask of the device (the dma_mask is a bit mask of the
addressable region for the device, i.e., if the bus address of
the memory ANDed with the dma_mask is still equal to the bus
address, then the device can perform DMA to the memory). To
ensure that the memory allocated by kmalloc is within the dma_mask,
the driver may specify various platform-dependent flags to restrict
the physical memory range of the allocation (e.g. on x86, GFP_DMA
guarantees to be within the first 16Mb of available physical memory,
the bus address range of the allocation (e.g., on x86, GFP_DMA
guarantees to be within the first 16MB of available bus addresses,
as required by ISA devices).
Note also that the above constraints on physical contiguity and
dma_mask may not apply if the platform has an IOMMU (a device which
supplies a physical to virtual mapping between the I/O memory bus and
the device). However, to be portable, device driver writers may *not*
assume that such an IOMMU exists.
maps an I/O bus address to a physical memory address). However, to be
portable, device driver writers may *not* assume that such an IOMMU
exists.
Warnings: Memory coherency operates at a granularity called the cache
line width. In order for memory mapped by this API to operate
......@@ -281,9 +286,9 @@ cache width is.
int
dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
In some circumstances dma_map_single and dma_map_page will fail to create
In some circumstances dma_map_single() and dma_map_page() will fail to create
a mapping. A driver can check for these errors by testing the returned
dma address with dma_mapping_error(). A non-zero return value means the mapping
DMA address with dma_mapping_error(). A non-zero return value means the mapping
could not be created and the driver should take appropriate action (e.g.
reduce current DMA mapping usage or delay and try again later).
......@@ -291,7 +296,7 @@ reduce current DMA mapping usage or delay and try again later).
dma_map_sg(struct device *dev, struct scatterlist *sg,
int nents, enum dma_data_direction direction)
Returns: the number of physical segments mapped (this may be shorter
Returns: the number of bus address segments mapped (this may be shorter
than <nents> passed in if some elements of the scatter/gather list are
physically or virtually adjacent and an IOMMU maps them with a single
entry).
......@@ -299,7 +304,7 @@ entry).
Please note that the sg cannot be mapped again if it has been mapped once.
The mapping process is allowed to destroy information in the sg.
As with the other mapping interfaces, dma_map_sg can fail. When it
As with the other mapping interfaces, dma_map_sg() can fail. When it
does, 0 is returned and a driver must take appropriate action. It is
critical that the driver do something, in the case of a block driver
aborting the request or even oopsing is better than doing nothing and
......@@ -335,7 +340,7 @@ must be the same as those and passed in to the scatter/gather mapping
API.
Note: <nents> must be the number you passed in, *not* the number of
physical entries returned.
bus address entries returned.
void
dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle, size_t size,
......@@ -391,10 +396,10 @@ The four functions above are just like the counterpart functions
without the _attrs suffixes, except that they pass an optional
struct dma_attrs*.
struct dma_attrs encapsulates a set of "dma attributes". For the
struct dma_attrs encapsulates a set of "DMA attributes". For the
definition of struct dma_attrs see linux/dma-attrs.h.
The interpretation of dma attributes is architecture-specific, and
The interpretation of DMA attributes is architecture-specific, and
each attribute should be documented in Documentation/DMA-attributes.txt.
If struct dma_attrs* is NULL, the semantics of each of these
......@@ -458,7 +463,7 @@ Note: where the platform can return consistent memory, it will
guarantee that the sync points become nops.
Warning: Handling non-consistent memory is a real pain. You should
only ever use this API if you positively know your driver will be
only use this API if you positively know your driver will be
required to work on one of the rare (usually non-PCI) architectures
that simply cannot make consistent memory.
......@@ -496,26 +501,26 @@ dma_declare_coherent_memory(struct device *dev, dma_addr_t bus_addr,
dma_addr_t device_addr, size_t size, int
flags)
Declare region of memory to be handed out by dma_alloc_coherent when
Declare region of memory to be handed out by dma_alloc_coherent() when
it's asked for coherent memory for this device.
bus_addr is the physical address to which the memory is currently
assigned in the bus responding region (this will be used by the
platform to perform the mapping).
device_addr is the physical address the device needs to be programmed
device_addr is the bus address the device needs to be programmed
with actually to address this memory (this will be handed out as the
dma_addr_t in dma_alloc_coherent()).
size is the size of the area (must be multiples of PAGE_SIZE).
flags can be or'd together and are:
flags can be ORed together and are:
DMA_MEMORY_MAP - request that the memory returned from
dma_alloc_coherent() be directly writable.
DMA_MEMORY_IO - request that the memory returned from
dma_alloc_coherent() be addressable using read/write/memcpy_toio etc.
dma_alloc_coherent() be addressable using read()/write()/memcpy_toio() etc.
One or both of these flags must be present.
......@@ -572,7 +577,7 @@ region is occupied.
Part III - Debug drivers use of the DMA-API
-------------------------------------------