Re: [PATCH] i915: Drop relocation support on all new hardware (v3)

[Jason Ekstrand <jason@jlekstrand.net>]

+Zbigniew for review

On Wed, Mar 10, 2021 at 3:50 PM Jason Ekstrand <jason@jlekstrand.net> 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.
>
> Rejecting relocations starting with Gen12 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
>
> Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
> 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;
> +
> +       /* All discrete memory platforms are Gen12 or above */
> +       if (WARN_ON(HAS_LMEM(eb->i915)))
> +               return -EINVAL;
> +
>         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;
>         }
> --
> 2.29.2
>
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