• Luis R. Rodriguez's avatar
    x86/mm: Add ioremap_uc() helper to map memory uncacheable (not UC-) · e4b6be33
    Luis R. Rodriguez authored
    ioremap_nocache() currently uses UC- by default. Our goal is to
    eventually make UC the default. Linux maps UC- to PCD=1, PWT=0
    page attributes on non-PAT systems. Linux maps UC to PCD=1,
    PWT=1 page attributes on non-PAT systems. On non-PAT and PAT
    systems a WC MTRR has different effects on pages with either of
    these attributes. In order to help with a smooth transition its
    best to enable use of UC (PCD,1, PWT=1) on a region as that
    ensures a WC MTRR will have no effect on a region, this however
    requires us to have an way to declare a region as UC and we
    currently do not have a way to do this.
      WC MTRR on non-PAT system with PCD=1, PWT=0 (UC-) yields WC.
      WC MTRR on non-PAT system with PCD=1, PWT=1 (UC)  yields UC.
      WC MTRR on PAT system with PCD=1, PWT=0 (UC-) yields WC.
      WC MTRR on PAT system with PCD=1, PWT=1 (UC)  yields UC.
    A flip of the default ioremap_nocache() behaviour from UC- to UC
    can therefore regress a memory region from effective memory type
    WC to UC if MTRRs are used. Use of MTRRs should be phased out
    and in the best case only arch_phys_wc_add() use will remain,
    even if this happens arch_phys_wc_add() will have an effect on
    non-PAT systems and changes to default ioremap_nocache()
    behaviour could regress drivers.
    Now, ideally we'd use ioremap_nocache() on the regions in which
    we'd need uncachable memory types and avoid any MTRRs on those
    regions. There are however some restrictions on MTRRs use, such
    as the requirement of having the base and size of variable sized
    MTRRs to be powers of two, which could mean having to use a WC
    MTRR over a large area which includes a region in which
    write-combining effects are undesirable.
    Add ioremap_uc() to help with the both phasing out of MTRR use
    and also provide a way to blacklist small WC undesirable regions
    in devices with mixed regions which are size-implicated to use
    large WC MTRRs. Use of ioremap_uc() helps phase out MTRR use by
    avoiding regressions with an eventual flip of default behaviour
    or ioremap_nocache() from UC- to UC.
    Drivers working with WC MTRRs can use the below table to review
    and consider the use of ioremap*() and similar helpers to ensure
    appropriate behaviour long term even if default
    ioremap_nocache() behaviour changes from UC- to UC.
    Although ioremap_uc() is being added we leave set_memory_uc() to
    use UC- as only initial memory type setup is required to be able
    to accommodate existing device drivers and phase out MTRR use.
    It should also be clarified that set_memory_uc() cannot be used
    with IO memory, even though its use will not return any errors,
    it really has no effect.
      MTRR Non-PAT   PAT    Linux ioremap value        Effective memory type
                                                        Non-PAT |  PAT
      WC   000      WB      _PAGE_CACHE_MODE_WB            WC   |   WC
      WC   001      WC      _PAGE_CACHE_MODE_WC            WC*  |   WC
      WC   010      UC-     _PAGE_CACHE_MODE_UC_MINUS      WC*  |   WC
      WC   011      UC      _PAGE_CACHE_MODE_UC            UC   |   UC
    Signed-off-by: default avatarLuis R. Rodriguez <mcgrof@suse.com>
    Signed-off-by: default avatarBorislav Petkov <bp@suse.de>
    Acked-by: default avatarH. Peter Anvin <hpa@zytor.com>
    Cc: Andy Lutomirski <luto@amacapital.net>
    Cc: Antonino Daplas <adaplas@gmail.com>
    Cc: Bjorn Helgaas <bhelgaas@google.com>
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: Brian Gerst <brgerst@gmail.com>
    Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
    Cc: Dave Airlie <airlied@redhat.com>
    Cc: Davidlohr Bueso <dbueso@suse.de>
    Cc: Denys Vlasenko <dvlasenk@redhat.com>
    Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
    Cc: Juergen Gross <jgross@suse.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Mel Gorman <mgorman@suse.de>
    Cc: Mike Travis <travis@sgi.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Suresh Siddha <sbsiddha@gmail.com>
    Cc: Thierry Reding <treding@nvidia.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
    Cc: Toshi Kani <toshi.kani@hp.com>
    Cc: Ville Syrjälä <syrjala@sci.fi>
    Cc: Vlastimil Babka <vbabka@suse.cz>
    Cc: Will Deacon <will.deacon@arm.com>
    Cc: linux-fbdev@vger.kernel.org
    Link: http://lkml.kernel.org/r/1430343851-967-2-git-send-email-mcgrof@do-not-panic.com
    Link: http://lkml.kernel.org/r/1431332153-18566-9-git-send-email-bp@alien8.deSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
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