Re: [PATCH] drm: Explicitly compute the last cacheline for clflush on range

[Imre Deak <imre.deak@intel.com>]

On Fri, 2015-10-16 at 20:55 +0100, Chris Wilson wrote:
> Fixes regression from
> 
> 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()
> 
> I'm stumped. Looking at the loop we should be iterating over every cache
> line until we reach the start of the cacheline after the end of the
> virtual range. Evidence says otherwise.
> 
> More bizarely, I stored the last address to be clflushed and found it to
> be equal to the start of the cacheline containing the last byte. Doubly
> purplexed.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92501
> Testcase: gem_tiled_partial_pwrite_pread/reads
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Imre Deak <imre.deak@intel.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> ---
>  drivers/gpu/drm/drm_cache.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_cache.c b/drivers/gpu/drm/drm_cache.c
> index 6743ff7dccfa..7c909bc8b68a 100644
> --- a/drivers/gpu/drm/drm_cache.c
> +++ b/drivers/gpu/drm/drm_cache.c
> @@ -131,10 +131,13 @@ drm_clflush_virt_range(void *addr, unsigned long length)
>  #if defined(CONFIG_X86)
>  	if (cpu_has_clflush) {
>  		const int size = boot_cpu_data.x86_clflush_size;
> -		void *end = addr + length;
> -		addr = (void *)(((unsigned long)addr) & -size);
> +		void *end;
> +
> +		end = (void *)(((unsigned long)addr + length - 1) & -size);
> +		addr = (void *)((unsigned long)addr & -size);
> +
>  		mb();
> -		for (; addr < end; addr += size)
> +		for (; addr <= end; addr += size)

Hm, I can't see how could this make any difference. The old way still
looks ok to me and the new version would flush the exact same cache
lines as the old one using the same addresses (beginning of each cache
line).

>  			clflushopt(addr);
>  		mb();
>  		return;


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