Re: [PATCH] drm/i915: Always mark the object as dirty when used by the GPU

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Aug 31, 2015 at 03:10:39PM +0100, Chris Wilson wrote:
> There have been many hard to track down bugs whereby userspace forgot to
> flag a write buffer and then cause graphics corruption or a hung GPU
> when that buffer was later purged under memory pressure (as the buffer
> appeared clean, its pages would have been evicted rather than preserved
> and any changes more recent than in the backing storage would be lost).
> In retrospect this is a rare optimisation against memory pressure,
> already the slow path. If we always mark the buffer as dirty when
> accessed by the GPU, anything not used can still be evicted cheaply
> (ideal behaviour for mark-and-sweep eviction) but we do not run the risk
> of corruption. For correct read serialisation, userspace still has to
> notify when the GPU writes to an object. However, there are certain
> situations under which userspace may wish to tell white lies to the
> kernel...
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: Kristian Høgsberg <krh@bitplanet.net>
> Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
> Cc: "Goel, Akash" <akash.goel@intel.co>
> Cc: Michał Winiarski <michal.winiarski@intel.com>
> Cc: stable@vger.kernel.org

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

> ---
>  drivers/gpu/drm/i915/i915_gem_execbuffer.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> index 923a3c4bf0b7..a953d4975b8c 100644
> --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> @@ -1032,6 +1032,7 @@ i915_gem_execbuffer_move_to_active(struct list_head *vmas,
>  		u32 old_read = obj->base.read_domains;
>  		u32 old_write = obj->base.write_domain;
>  
> +		obj->dirty = 1; /* be paranoid  */
>  		obj->base.write_domain = obj->base.pending_write_domain;
>  		if (obj->base.write_domain == 0)
>  			obj->base.pending_read_domains |= obj->base.read_domains;
> @@ -1039,7 +1040,6 @@ i915_gem_execbuffer_move_to_active(struct list_head *vmas,
>  
>  		i915_vma_move_to_active(vma, req);
>  		if (obj->base.write_domain) {
> -			obj->dirty = 1;
>  			i915_gem_request_assign(&obj->last_write_req, req);
>  
>  			intel_fb_obj_invalidate(obj, ORIGIN_CS);
> -- 
> 2.5.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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