Re: [PATCH] drm/i915/gtt: Trust the uncached store to flush wcb

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Chris Wilson <chris@chris-wilson.co.uk> writes:

> Quoting Mika Kuoppala (2018-05-08 13:03:13)
>> Not all architectures guarantee that uncached read will
>> flush the write combining buffer. So marking it explicitly
>> is recommended [1].
>> 
>> However we know the architecture we are operating on
>> and can avoid wmb as the UC store will flush the wcb [2].
>> 
>> Omit the wmb() before invalidate as redudant.
>> 
>> v2: squash combining and removal (Chris)
>> 
>> References: http://yarchive.net/comp/linux/write_combining.html [1]
>> References: http://download.intel.com/design/PentiumII/applnots/24442201.pdf [2]
>> Cc: Chris Wilson <chris@chris-wilson.co.uk>
>> Cc: Matthew Auld <matthew.auld@intel.com>
>> Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
>> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
>> ---
>>  drivers/gpu/drm/i915/i915_gem_gtt.c | 6 ++----
>>  1 file changed, 2 insertions(+), 4 deletions(-)
>> 
>> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
>> index c879bfd9294f..2126358761a5 100644
>> --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
>> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
>> @@ -110,7 +110,8 @@ i915_get_ggtt_vma_pages(struct i915_vma *vma);
>>  
>>  static void gen6_ggtt_invalidate(struct drm_i915_private *dev_priv)
>>  {
>> -       /* Note that as an uncached mmio write, this should flush the
>> +       /*
>> +        * Note that as an uncached mmio write, this will flush the
>>          * WCB of the writes into the GGTT before it triggers the invalidate.
>>          */
>>         I915_WRITE(GFX_FLSH_CNTL_GEN6, GFX_FLSH_CNTL_EN);
>> @@ -2418,8 +2419,6 @@ static void gen8_ggtt_insert_entries(struct i915_address_space *vm,
>>         for_each_sgt_dma(addr, sgt_iter, vma->pages)
>>                 gen8_set_pte(gtt_entries++, pte_encode | addr);
>>  
>> -       wmb();
>> -
>>         /* This next bit makes the above posting read even more important. We
>
> Yeah, we should rewrite this comment as well; just skip the first
> obsolete sentence.

v3 pushed. Thanks for review.

-Mika
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