[PATCH v2] uapi/drm/drm_fourcc.h: Note on platform specificity for format modifiers

[Daniel Vetter]

On Wed, May 06, 2020 at 03:08:27PM +0300, Mika Kahola wrote:
> Make an additional note on DRM format modifiers for x and y tiling. These
> format modifiers are defined for BDW+ platforms and therefore definition
> is not valid for older gens. This is due to address swizzling for tiled
> surfaces is no longer used. For newer platforms main memory controller has
> a more effective address swizzling algorithm.
> 
> v2: Rephrase comment (Daniel)
> 
> Signed-off-by: Mika Kahola <mika.kahola at intel.com>

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

> ---
>  include/uapi/drm/drm_fourcc.h | 18 ++++++++++++------
>  1 file changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
> index 8bc0b31597d8..9e488d10f8b4 100644
> --- a/include/uapi/drm/drm_fourcc.h
> +++ b/include/uapi/drm/drm_fourcc.h
> @@ -354,9 +354,12 @@ extern "C" {
>   * a platform-dependent stride. On top of that the memory can apply
>   * platform-depending swizzling of some higher address bits into bit6.
>   *
> - * This format is highly platforms specific and not useful for cross-driver
> - * sharing. It exists since on a given platform it does uniquely identify the
> - * layout in a simple way for i915-specific userspace.
> + * Note that this layout is only accurate on intel gen 8+ or valleyview chipsets.
> + * On earlier platforms the is highly platforms specific and not useful for
> + * cross-driver sharing. It exists since on a given platform it does uniquely
> + * identify the layout in a simple way for i915-specific userspace, which
> + * facilitated conversion of userspace to modifiers. Additionally the exact
> + * format on some really old platforms is not known.
>   */
>  #define I915_FORMAT_MOD_X_TILED	fourcc_mod_code(INTEL, 1)
>  
> @@ -369,9 +372,12 @@ extern "C" {
>   * memory can apply platform-depending swizzling of some higher address bits
>   * into bit6.
>   *
> - * This format is highly platforms specific and not useful for cross-driver
> - * sharing. It exists since on a given platform it does uniquely identify the
> - * layout in a simple way for i915-specific userspace.
> + * Note that this layout is only accurate on intel gen 8+ or valleyview chipsets.
> + * On earlier platforms the is highly platforms specific and not useful for
> + * cross-driver sharing. It exists since on a given platform it does uniquely
> + * identify the layout in a simple way for i915-specific userspace, which
> + * facilitated conversion of userspace to modifiers. Additionally the exact
> + * format on some really old platforms is not known.
>   */
>  #define I915_FORMAT_MOD_Y_TILED	fourcc_mod_code(INTEL, 2)
>  
> -- 
> 2.20.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch