Re: [PATCH] drm: Avoid the double clflush on the last cache line in drm_clflush_virt_range()

[Dave Gordon <david.s.gordon@intel.com>]

On 10/06/15 15:58, Chris Wilson wrote:
> As the clflush operates on cache lines, and we can flush any byte
> address, in order to flush all bytes given in the range we issue an
> extra clflush on the last byte to ensure the last cacheline is flushed.
> We can can the iteration to be over the actual cache lines to avoid this
> double clflush on the last byte.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Imre Deak <imre.deak@intel.com>
> ---
>  drivers/gpu/drm/drm_cache.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_cache.c b/drivers/gpu/drm/drm_cache.c
> index 9a62d7a53553..6743ff7dccfa 100644
> --- a/drivers/gpu/drm/drm_cache.c
> +++ b/drivers/gpu/drm/drm_cache.c
> @@ -130,11 +130,12 @@ 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);

Should this cast be to uintptr_t? Or intptr_t, as size has somewhat
strangely been defined as signed? To complete the mix, x86_clflush_size
is 'u16'! So maybe we should write

+		const size_t size = boot_cpu_data.x86_clflush_size;
+		const size_t mask = ~(size - 1);
 		void *end = addr + length;
+		addr = (void *)(((uintptr_t)addr) & mask);

>  		mb();
> -		for (; addr < end; addr += boot_cpu_data.x86_clflush_size)
> +		for (; addr < end; addr += size)
>  			clflushopt(addr);
> -		clflushopt(end - 1);
>  		mb();
>  		return;
>  	}
> 

.Dave.
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