Re: [PATCH 4/4] drm/i915: Support for pread/pwrite from/to non shmem backed objects

[Ankitprasad Sharma <ankitprasad.r.sharma@intel.com>]

On Fri, 2015-07-31 at 18:46 +0530, Goel, Akash wrote:
> 
> On 7/22/2015 8:09 PM, Chris Wilson wrote:
> > On Wed, Jul 22, 2015 at 07:21:49PM +0530, ankitprasad.r.sharma@intel.com wrote:
> >>   static int
> >>   i915_gem_shmem_pread(struct drm_device *dev,
> >>   		     struct drm_i915_gem_object *obj,
> >> @@ -754,17 +850,20 @@ i915_gem_pread_ioctl(struct drm_device *dev, void *data,
> >>   		goto out;
> >>   	}
> >>
> >> -	/* prime objects have no backing filp to GEM pread/pwrite
> >> -	 * pages from.
> >> -	 */
> >> -	if (!obj->base.filp) {
> >> -		ret = -EINVAL;
> >> -		goto out;
> >> -	}
> >> -
> >>   	trace_i915_gem_object_pread(obj, args->offset, args->size);
> >>
> >> -	ret = i915_gem_shmem_pread(dev, obj, args, file);
> >> +	/* pread for non shmem backed objects */
> >> +	if (!obj->base.filp) {
> >> +		if (obj->tiling_mode == I915_TILING_NONE)
> >
> > pread/pwrite is defined as a cpu linear, the restriction upon tiling is
> > a simplification of handling swizzling.
> >
> >> +			ret = i915_gem_gtt_pread_pwrite(dev, obj, args->size,
> >> +							args->offset,
> >> +							args->data_ptr,
> >> +							false);
> >> +		else
> >> +			ret = -EINVAL;
> >> +	} else {
> >> +		ret = i915_gem_shmem_pread(dev, obj, args, file);
> >> +	}
> >>
> >>   out:
> >>   	drm_gem_object_unreference(&obj->base);
> >> @@ -1105,17 +1204,22 @@ i915_gem_pwrite_ioctl(struct drm_device *dev, void *data,
> >>   		goto out;
> >>   	}
> >>
> >> -	/* prime objects have no backing filp to GEM pread/pwrite
> >> -	 * pages from.
> >> -	 */
> >> -	if (!obj->base.filp) {
> >> -		ret = -EINVAL;
> >> -		goto out;
> >> -	}
> >> -
> >>   	trace_i915_gem_object_pwrite(obj, args->offset, args->size);
> >>
> >>   	ret = -EFAULT;
> >> +
> >> +	/* pwrite for non shmem backed objects */
> >> +	if (!obj->base.filp) {
> >> +		if (obj->tiling_mode == I915_TILING_NONE)
> >> +			ret = i915_gem_gtt_pread_pwrite(dev, obj, args->size,
> >> +							args->offset,
> >> +							args->data_ptr,
> >> +							true);
> >> +		else
> >> +			ret = -EINVAL;
> >> +
> >> +		goto out;
> >
> > The fast pwrite code always works for obj->base.filp == NULL. To easily
> > make it generic and handle the cases where we cannot fallback to shem,
> > undo the PIN_NONBLOCK.
> >
> > Then you just want something like
> > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > index d3016f37cd4d..f2284a27dd6d 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -1114,8 +1114,9 @@ i915_gem_pwrite_ioctl(struct drm_device *dev, void *data,
> >           * perspective, requiring manual detiling by the client.
> >           */
> >          if (obj->tiling_mode == I915_TILING_NONE &&
> 
> Since the suggestion is to reuse the gtt_pwrite_fast function only for 
> non-shmem backed objects, can we also relax the Tiling constraint here, 
> apart from removing the PIN_NONBLOCK flag. As anyways there is a 
> put_fence already being done in gtt_pwrite_fast function.
> 
> Also currently the gtt_pwrite_fast function is non-tolerant to faults, 
> incurred while accessing the User space buffer, so can that also be 
> relaxed (at least for the non-shmem backed objects) ??

Even if we reuse the gtt_pwrite_fast we will have to handle page-faults
for non-shmem backed objects, that will contradict the purpose of
gtt_pwrite_fast. The gtt_pread_pwrite will still be around for pread
purpose.

Also, I think it should be ok to relax the tiling constraint as well, as
we put_fence everytime we try to pread/pwrite from/to a non-shmem-backed
object here.

Thanks,
Ankit

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