Re: [Intel-gfx] [PATCH] drm/i915/gem: Drop relocation support on all new hardware (v4)

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

On 11/03/2021 18:17, Jason Ekstrand wrote:
> The Vulkan driver in Mesa for Intel hardware never uses relocations if
> it's running on a version of i915 that supports at least softpin which
> all versions of i915 supporting Gen12 do.  On the OpenGL side, Gen12+ is
> only supported by iris which never uses relocations.  The older i965
> driver in Mesa does use relocations but it only supports Intel hardware
> through Gen11 and has been deprecated for all hardware Gen9+.  The
> compute driver also never uses relocations.  This only leaves the media
> driver which is supposed to be switching to softpin going forward.
> Making softpin a requirement for all future hardware seems reasonable.
> 
> There is one piece of hardware enabled by default in i915: RKL which was
> enabled by e22fa6f0a976 which has not yet landed in drm-next so this
> almost but not really a userspace API change for RKL.  If it becomes a
> problem, we can always add !IS_ROCKETLAKE(eb->i915) to the condition.
> 
> Rejecting relocations starting with newer Gen12 platforms has the
> benefit that we don't have to bother supporting it on platforms with
> local memory.  Given how much CPU touching of memory is required for
> relocations, not having to do so on platforms where not all memory is
> directly CPU-accessible carries significant advantages.
> 
> v2 (Jason Ekstrand):
>   - Allow TGL-LP platforms as they've already shipped
> 
> v3 (Jason Ekstrand):
>   - WARN_ON platforms with LMEM support in case the check is wrong
> 
> v4 (Jason Ekstrand):
>   - Call out Rocket Lake in the commit message
> 
> Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
> Acked-by: Keith Packard <keithp@keithp.com>
> Cc: Dave Airlie <airlied@redhat.com>
> Cc: Daniel Vetter <daniel.vetter@intel.com>
> ---
>   drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c | 15 ++++++++++++---
>   1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> index 99772f37bff60..b02dbd16bfa03 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> @@ -1764,7 +1764,8 @@ eb_relocate_vma_slow(struct i915_execbuffer *eb, struct eb_vma *ev)
>   	return err;
>   }
>   
> -static int check_relocations(const struct drm_i915_gem_exec_object2 *entry)
> +static int check_relocations(const struct i915_execbuffer *eb,
> +			     const struct drm_i915_gem_exec_object2 *entry)
>   {
>   	const char __user *addr, *end;
>   	unsigned long size;
> @@ -1774,6 +1775,14 @@ static int check_relocations(const struct drm_i915_gem_exec_object2 *entry)
>   	if (size == 0)
>   		return 0;
>   
> +	/* Relocations are disallowed for all platforms after TGL-LP */
> +	if (INTEL_GEN(eb->i915) >= 12 && !IS_TIGERLAKE(eb->i915))
> +		return -EINVAL;

I still recommend ENODEV as more inline with our established error 
codes. (Platform does not support vs dear userspace you messed up your 
flags, modes, whatever.)

> +
> +	/* All discrete memory platforms are Gen12 or above */
> +	if (WARN_ON(HAS_LMEM(eb->i915)))
> +		return -EINVAL;

What was the conclusion on value of supporting fake lmem?

Regards,

Tvrtko

> +
>   	if (size > N_RELOC(ULONG_MAX))
>   		return -EINVAL;
>   
> @@ -1807,7 +1816,7 @@ static int eb_copy_relocations(const struct i915_execbuffer *eb)
>   		if (nreloc == 0)
>   			continue;
>   
> -		err = check_relocations(&eb->exec[i]);
> +		err = check_relocations(eb, &eb->exec[i]);
>   		if (err)
>   			goto err;
>   
> @@ -1880,7 +1889,7 @@ static int eb_prefault_relocations(const struct i915_execbuffer *eb)
>   	for (i = 0; i < count; i++) {
>   		int err;
>   
> -		err = check_relocations(&eb->exec[i]);
> +		err = check_relocations(eb, &eb->exec[i]);
>   		if (err)
>   			return err;
>   	}
> 
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