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

[Christian König]

Am 04.09.2018 um 12:15 schrieb 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.

The problem here is that at least for some parameters we actually don't 
know which formats are actually used.

The following are not real world examples, but just to explain the 
general problem.

The memory configuration for example can be not ASIC specific, but 
rather determined by whoever took the ASIC and glued it together with 
VRAM on a board. It is not likely that somebody puts all the VRAM chips 
on one channel, but it is still perfectly possible.

Same is true for things like harvesting, e.g. of 16 channels halve of 
them could be bad and we need to know which to actually use.

Regards,
Christian.

>
> 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
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