Re: [PATCH 06/64] drm: Restore double clflush on the last partial cacheline

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

Daniel Vetter <daniel@ffwll.ch> writes:

> On Thu, Jul 07, 2016 at 09:41:12AM +0100, Chris Wilson wrote:
>> This effectively reverts
>> 
>> commit afcd950cafea6e27b739fe7772cbbeed37d05b8b
>> Author: Chris Wilson <chris@chris-wilson.co.uk>
>> Date:   Wed Jun 10 15:58:01 2015 +0100
>> 
>>     drm: Avoid the double clflush on the last cache line in drm_clflush_virt_range()
>> 
>> as we have observed issues with serialisation of the clflush operations
>> on Baytrail+ Atoms with partial updates. Applying the double flush on the
>> last cacheline forces that clflush to be ordered with respect to the
>> previous clflush, and the mfence then protects against prefetches crossing
>> the clflush boundary.
>> 
>> The same issue can be demonstrated in userspace with igt/gem_exec_flush.
>> 
>> Fixes: afcd950cafea6 (drm: Avoid the double clflush on the last cache...)
>> Testcase: igt/gem_concurrent_blit
>> Testcase: igt/gem_partial_pread_pwrite
>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92845
>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>> Cc: dri-devel@lists.freedesktop.org
>> Cc: Akash Goel <akash.goel@intel.com>
>> Cc: Imre Deak <imre.deak@intel.com>
>> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
>> Cc: Jason Ekstrand <jason.ekstrand@intel.com>
>> Cc: stable@vger.kernel.org
>> Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
>
> It's duct-tape, but oh well. Applied to drm-misc.

We were confused how many layers we need.

-Mika

> -Daniel
>
>> ---
>>  drivers/gpu/drm/drm_cache.c | 1 +
>>  1 file changed, 1 insertion(+)
>> 
>> diff --git a/drivers/gpu/drm/drm_cache.c b/drivers/gpu/drm/drm_cache.c
>> index 059f7c39c582..a7916e5f8864 100644
>> --- a/drivers/gpu/drm/drm_cache.c
>> +++ b/drivers/gpu/drm/drm_cache.c
>> @@ -136,6 +136,7 @@ drm_clflush_virt_range(void *addr, unsigned long length)
>>  		mb();
>>  		for (; addr < end; addr += size)
>>  			clflushopt(addr);
>> +		clflushopt(end - 1); /* force serialisation */
>>  		mb();
>>  		return;
>>  	}
>> -- 
>> 2.8.1
>> 
>
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
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