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    Subject[PATCH v4 2/6] x86: document WC MTRR effects on PAT / non-PAT pages
    Date
    From: "Luis R. Rodriguez" <mcgrof@suse.com>

    As part of the effort to phase out MTRR use document
    write-combining MTRR effects on pages with different
    non-PAT page attributes flags and different PAT entry
    values. Extend arch_phys_wc_add() documentation to
    clarify power of two sizes / boundary requirements as
    we phase out mtrr_add() use.

    Lastly hint towards ioremap_uc() for corner cases on
    device drivers working with devices with mixed regions
    where MTRR size requirements would otherwise not
    enable write-combining effective memory types.

    Cc: Toshi Kani <toshi.kani@hp.com>
    Cc: Jonathan Corbet <corbet@lwn.net>
    Cc: Dave Hansen <dave.hansen@linux.intel.com>
    Cc: Andy Lutomirski <luto@amacapital.net>
    Cc: Suresh Siddha <sbsiddha@gmail.com>
    Cc: Ingo Molnar <mingo@elte.hu>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Juergen Gross <jgross@suse.com>
    Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
    Cc: Dave Airlie <airlied@redhat.com>
    Cc: Antonino Daplas <adaplas@gmail.com>
    Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
    Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
    Cc: Ville Syrjälä <syrjala@sci.fi>
    Cc: Mel Gorman <mgorman@suse.de>
    Cc: Vlastimil Babka <vbabka@suse.cz>
    Cc: Borislav Petkov <bp@suse.de>
    Cc: Davidlohr Bueso <dbueso@suse.de>
    Cc: linux-fbdev@vger.kernel.org
    Cc: linux-kernel@vger.kernel.org
    Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
    ---
    Documentation/x86/mtrr.txt | 18 +++++++++++++++---
    Documentation/x86/pat.txt | 40 +++++++++++++++++++++++++++++++++++++++-
    arch/x86/kernel/cpu/mtrr/main.c | 3 +++
    3 files changed, 57 insertions(+), 4 deletions(-)

    diff --git a/Documentation/x86/mtrr.txt b/Documentation/x86/mtrr.txt
    index cc071dc..a111a6c 100644
    --- a/Documentation/x86/mtrr.txt
    +++ b/Documentation/x86/mtrr.txt
    @@ -1,7 +1,19 @@
    MTRR (Memory Type Range Register) control
    -3 Jun 1999
    -Richard Gooch
    -<rgooch@atnf.csiro.au>
    +
    +Richard Gooch <rgooch@atnf.csiro.au> - 3 Jun 1999
    +Luis R. Rodriguez <mcgrof@do-not-panic.com> - April 9, 2015
    +
    +===============================================================================
    +Phasing MTRR use
    +
    +MTRR use is replaced on modern x86 hardware with PAT. Over time the only type
    +of effective MTRR that is expected to be supported will be for write-combining.
    +As MTRR use is phased out device drivers should use arch_phys_wc_add() to make
    +MTRR effective on non-PAT systems while a no-op on PAT enabled systems.
    +
    +For details refer to Documentation/x86/pat.txt.
    +
    +===============================================================================

    On Intel P6 family processors (Pentium Pro, Pentium II and later)
    the Memory Type Range Registers (MTRRs) may be used to control
    diff --git a/Documentation/x86/pat.txt b/Documentation/x86/pat.txt
    index cf08c9f..7e183e3 100644
    --- a/Documentation/x86/pat.txt
    +++ b/Documentation/x86/pat.txt
    @@ -34,6 +34,8 @@ ioremap | -- | UC- | UC- |
    | | | |
    ioremap_cache | -- | WB | WB |
    | | | |
    +ioremap_uc | -- | UC | UC |
    + | | | |
    ioremap_nocache | -- | UC- | UC- |
    | | | |
    ioremap_wc | -- | -- | WC |
    @@ -102,7 +104,43 @@ wants to export a RAM region, it has to do set_memory_uc() or set_memory_wc()
    as step 0 above and also track the usage of those pages and use set_memory_wb()
    before the page is freed to free pool.

    -
    +MTRR effects on PAT / non-PAT systems
    +-------------------------------------
    +
    +The following table provides the effects of using write-combining MTRRs when
    +using ioremap*() calls on x86 for both non-PAT and PAT systems. Ideally
    +mtrr_add() usage will be phased in favor of arch_phys_wc_add() which will
    +be a no-op on PAT enabled systems. The region over which a arch_phys_wc_add()
    +is made should already have be ioremap'd with write-combining page attributes
    +or PAT entries, this can be done by using ioremap_wc() / or respective helpers.
    +Devices which combine areas of IO memory desired to remain uncachable with
    +areas where write-combining is desirable and are restricted by the size
    +requirements of MTRRs should consider splitting up their IO memory space
    +cleanly with ioremap_uc() and ioremap_wc() followed by an arch_phys_wc_add()
    +encompassing both regions. Such use is nevertheless heavily discouraged as
    +the effective memory type is considered implementation defined. This strategy
    +should only be used as last resort on devices with size-contrained regions
    +where otherwise MTRR write-combining would not be effective.
    +
    +Note that you cannot use set_memory_wc() to override / whitelist IO remapped
    +memory space mapped with ioremap*() calls, set_memory_wc() can only be used
    +on RAM.
    +
    +----------------------------------------------------------------------
    +MTRR Non-PAT PAT Linux ioremap value Effective memory type
    +----------------------------------------------------------------------
    + Non-PAT | PAT
    + PAT
    + |PCD
    + ||PWT
    + |||
    +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
    +----------------------------------------------------------------------
    +
    +(*) denotes implementation defined and is discouraged

    Notes:

    diff --git a/arch/x86/kernel/cpu/mtrr/main.c b/arch/x86/kernel/cpu/mtrr/main.c
    index ea5f363..12abdbe 100644
    --- a/arch/x86/kernel/cpu/mtrr/main.c
    +++ b/arch/x86/kernel/cpu/mtrr/main.c
    @@ -538,6 +538,9 @@ EXPORT_SYMBOL(mtrr_del);
    * attempts to add a WC MTRR covering size bytes starting at base and
    * logs an error if this fails.
    *
    + * The caller should expect to need to provide a power of two size on an
    + * equivalent power of two boundary.
    + *
    * Drivers must store the return value to pass to mtrr_del_wc_if_needed,
    * but drivers should not try to interpret that return value.
    */
    --
    2.3.2.209.gd67f9d5.dirty


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