[RFC] drm/amdgpu: Add macros and documentation for format modifiers.

[Daniel Stone]

Hi,

On Tue, 4 Sep 2018 at 11:05, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> On Tue, Sep 4, 2018 at 3:00 AM, Bas Nieuwenhuizen <bas at basnieuwenhuizen.nl> wrote:
> > +/* The chip this is compatible with.
> > + *
> > + * If compression is disabled, use
> > + *   - AMDGPU_CHIP_TAHITI for GFX6-GFX8
> > + *   - AMDGPU_CHIP_VEGA10 for GFX9+
> > + *
> > + * With compression enabled please use the exact chip.
> > + *
> > + * TODO: Do some generations share DCC format?
> > + */
> > +#define AMDGPU_MODIFIER_CHIP_GEN_SHIFT                 40
> > +#define AMDGPU_MODIFIER_CHIP_GEN_MASK                  0xff
>
> Do you really need all the combinations here of DCC + gpu gen + tiling
> details? When we had the entire discussion with nvidia folks they
> eventually agreed that they don't need the massive pile with every
> possible combination. Do you really plan to share all these different
> things?
>
> Note that e.g. on i915 we spec some of the tiling depending upon
> buffer size and buffer format (because that's how the hw works), not
> using explicit modifier flags for everything.

Right. The conclusion, after people went through and started sorting
out the kinds of formats for which they would _actually_ export real
colour buffers for, that most vendors definitely have fewer than
115,792,089,237,316,195,423,570,985,008,687,907,853,269,984,665,640,564,039,457,584,007,913,129,639,936
possible formats to represent, very likely fewer than
340,282,366,920,938,463,463,374,607,431,768,211,456 formats, probably
fewer than 72,057,594,037,927,936 formats, and even still generally
fewer than 281,474,976,710,656 if you want to be generous and leave 8
bits of the 56 available.

If you do use 256 bits in order to represent
115,792,089,237,316,195,423,570,985,008,687,907,853,269,984,665,640,564,039,457,584,007,913,129,639,936
modifiers per format, userspace would start hitting OOM pretty quickly
as it attempted to enumerate and negotiate acceptable modifiers.
Either that or we need to replace the fixed 64-bit modifier tokens
with some kind of eBPF script.

Cheers,
Daniel