Re: [Intel-gfx] [PATCH] drm/i915: return DRIVER_NAME for the fence driver name

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Jun 16, 2021 at 01:28:33PM +0100, Matthew Auld wrote:
> The first tracepoint for a request is trace_dma_fence_init which is
> called in the ctor before we have properly setup the request->engine. So
> if it's a non-recycled request the rq->engine might be NULL, or some
> garbage value, which leads to a crash.

I'd hammer it more in here that we cannot move the dma_fence_init out of
the slab constructor (aka ctor), because that breaks the magic request
recycling through SLAB_TYPESAFE_BY_RCU. You do explain it all, but in a
rather dense fashion. More verbosity here would be good.

> Since we are not permitted to use kmem_cache_zalloc() here with

This part I'm not really clear on ... what would zalloc break? Ideally
references to the commits/bugs/functions in the code that would blow up
would be good here.

> SLAB_TYPESAFE_BY_RCU, one approach is simply to return DRIVER_NAME. We
> can then revisit this later if we decide to get rid of
> SLAB_TYPESAFE_BY_RCU.
> 
> Fixes: 855e39e65cfc ("drm/i915: Initialise basic fence before acquiring seqno")
> Signed-off-by: Matthew Auld <matthew.auld@intel.com>
> Cc: Michael Mason <michael.w.mason@intel.com>
> Cc: Daniel Vetter <daniel@ffwll.ch>

As a stop-gab the code change looks reasonable, but I think the commit
message needs work and much more of the history/background we're hitting
here.

Thanks, Daniel

> ---
>  drivers/gpu/drm/i915/i915_request.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> index 1014c71cf7f5..55fa94bde22e 100644
> --- a/drivers/gpu/drm/i915/i915_request.c
> +++ b/drivers/gpu/drm/i915/i915_request.c
> @@ -61,7 +61,7 @@ static struct i915_global_request {
>  
>  static const char *i915_fence_get_driver_name(struct dma_fence *fence)
>  {
> -	return dev_name(to_request(fence)->engine->i915->drm.dev);
> +	return DRIVER_NAME;
>  }
>  
>  static const char *i915_fence_get_timeline_name(struct dma_fence *fence)
> -- 
> 2.26.3
> 

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