Re: [Intel-gfx] [PATCH 4/4] drm/i915/uapi: reject set_domain for discrete

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 15/07/2021 11:15, Matthew Auld wrote:
> The CPU domain should be static for discrete, and on DG1 we don't need
> any flushing since everything is already coherent, so really all this
> does is an object wait, for which we have an ioctl. Longer term the
> desired caching should be an immutable creation time property for the
> BO, which can be set with something like gem_create_ext.
> 
> One other user is iris + userptr, which uses the set_domain to probe all
> the pages to check if the GUP succeeds, however we now have a PROBE
> flag for this purpose.
> 
> v2: add some more kernel doc, also add the implicit rules with caching
> 
> Suggested-by: Daniel Vetter <daniel@ffwll.ch>
> Signed-off-by: Matthew Auld <matthew.auld@intel.com>
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Cc: Kenneth Graunke <kenneth@whitecape.org>
> Cc: Jason Ekstrand <jason@jlekstrand.net>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: Ramalingam C <ramalingam.c@intel.com>
> Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
> ---
>   drivers/gpu/drm/i915/gem/i915_gem_domain.c |  3 +++
>   include/uapi/drm/i915_drm.h                | 19 +++++++++++++++++++
>   2 files changed, 22 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_domain.c b/drivers/gpu/drm/i915/gem/i915_gem_domain.c
> index 43004bef55cb..b684a62bf3b0 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_domain.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_domain.c
> @@ -490,6 +490,9 @@ i915_gem_set_domain_ioctl(struct drm_device *dev, void *data,
>   	u32 write_domain = args->write_domain;
>   	int err;
>   
> +	if (IS_DGFX(to_i915(dev)))
> +		return -ENODEV;
> +
>   	/* Only handle setting domains to types used by the CPU. */
>   	if ((write_domain | read_domains) & I915_GEM_GPU_DOMAINS)
>   		return -EINVAL;
> diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
> index 2e4112bf4d38..04ce310e7ee6 100644
> --- a/include/uapi/drm/i915_drm.h
> +++ b/include/uapi/drm/i915_drm.h
> @@ -901,6 +901,25 @@ struct drm_i915_gem_mmap_offset {
>    *	- I915_GEM_DOMAIN_GTT: Mappable aperture domain
>    *
>    * All other domains are rejected.
> + *
> + * Note that for discrete, starting from DG1, this is no longer supported, and
> + * is instead rejected. On such platforms the CPU domain is effectively static,
> + * where we also only support a single &drm_i915_gem_mmap_offset cache mode,
> + * which can't be set explicitly and instead depends on the object placements,
> + * as per the below.
> + *
> + * Implicit caching rules, starting from DG1:
> + *
> + *	- If any of the object placements (see &drm_i915_gem_create_ext_memory_regions)
> + *	  contain I915_MEMORY_CLASS_DEVICE then the object will be allocated and
> + *	  mapped as write-combined only.

A note about write-combine buffer? I guess saying it is userspace 
responsibility to do it and how.

> + *
> + *	- Everything else is always allocated and mapped as write-back, with the
> + *	  guarantee that everything is also coherent with the GPU.

Haven't been following this so just a question on this one - it is not 
considered interesting to offer non-coherent modes, or even write 
combine, with system memory buffers, for a specific reason?

Regards,

Tvrtko

> + *
> + * Note that this is likely to change in the future again, where we might need
> + * more flexibility on future devices, so making this all explicit as part of a
> + * new &drm_i915_gem_create_ext extension is probable.
>    */
>   struct drm_i915_gem_set_domain {
>   	/** @handle: Handle for the object. */
> 
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