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

["Goel, Akash" <akash.goel@intel.com>]

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) ??

Best regards
Akash

> -           obj->base.write_domain != I915_GEM_DOMAIN_CPU &&
> -           cpu_write_needs_clflush(obj)) {
> +           (obj->base.filp == NULL ||
> +            (obj->base.write_domain != I915_GEM_DOMAIN_CPU &&
> +             cpu_write_needs_clflush(obj)))) {
>                  ret = i915_gem_gtt_pwrite_fast(dev_priv, obj, args, file);
>                  /* Note that the gtt paths might fail with non-page-backed user
>                   * pointers (e.g. gtt mappings when moving data between
> @@ -1125,7 +1126,7 @@ i915_gem_pwrite_ioctl(struct drm_device *dev, void *data,
>          if (ret == -EFAULT || ret == -ENOSPC) {
>                  if (obj->phys_handle)
>                          ret = i915_gem_phys_pwrite(obj, args, file);
> -               else
> +               else if (obj->filp)
>                          ret = i915_gem_shmem_pwrite(dev, obj, args, file);
>          }
>
>
> to enable pwrite access to stolen.
> -Chris
>
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