Re: [PATCH 0/1] [RFC] drm/fourcc: Add new unsigned R16_UINT/RG1616_UINT formats

[Dennis Tsiang <dennis.tsiang@arm.com>]

On 30/06/2022 08:47, Pekka Paalanen wrote:
> On Wed, 29 Jun 2022 14:53:49 +0000
> Simon Ser <contact@emersion.fr> wrote:
>
>> On Wednesday, June 29th, 2022 at 16:46, Dennis Tsiang <dennis.tsiang@arm.com> wrote:
>>
>>> Thanks for your comments. This is not intended to be used for KMS, where
>>> indeed there would be no difference. This proposal is for other Graphics
>>> APIs such as Vulkan, which requires the application to be explicit
>>> upfront about how they will interpret the data, whether that be UNORM,
>>> UINT .etc. We want to be able to import dma_bufs which create a VkImage
>>> with a "_UINT" VkFormat. However there is currently no explicit mapping
>>> between the DRM fourccs + modifiers combos to "_UINT" VkFormats. One
>>> solution is to encode that into the fourccs, which is what this RFC is
>>> proposing.
>>
>> As a general comment, I don't think it's reasonable to encode all of the
>> VkFormat information inside DRM FourCC. For instance, VkFormat has SRGB/UNORM
>> variants which describe whether pixel values are electrical or optical
>> (IOW, EOTF-encoded or not). Moreover, other APIs may encode different
>> information in their format enums.
>
> Yeah, do not add any of that information to the DRM pixel format codes.
>
> There is *so much* other stuff you also need to define than what's
> already mentioned, and which bits you need for the API at hand depends
> totally on the API at hand. After the API has defined some parts of the
> metadata, the API user has to take care of the remaining parts of the
> metadata in other ways, like dynamic range or color space.
>
> Besides, when you deal with dmabuf, you already need to pass a lot of
> metadata explicitly, like the pixel format, width, height, stride,
> modifier, etc. so it's better to add more of those (like we will be
> doing in Wayland, and not specific to dmabuf even) than to try make
> pixel formats a huge mess through combinatorial explosion and sometimes
> partial and sometimes conflicting image metadata.
>
> You might be able to get a glimpse of what all metadata there could be
> by reading
> https://gitlab.freedesktop.org/pq/color-and-hdr/-/blob/main/doc/pixels_color.md
> .
>
> Compare Vulkan formats to e.g.
> https://docs.microsoft.com/en-us/windows/win32/api/dxgicommon/ne-dxgicommon-dxgi_color_space_type
> and you'll see that while DXGI color space enumeration is mostly about
> other stuff, it also has overlap with Vulkan formats I think, at least
> the SRGB vs. not part.
>
> Btw. practically all buffers you see used, especially if they are 8
> bpc, they are almost guaranteed to be "SRGB" non-linearly encoded, but
> do you ever see that fact being explicitly communicated?
>
> Then there is the question that if you have an SRGB-encoded buffer, do
> you want to read out SRGB-encoded or linear values? That depends on
> what you are doing with the buffer, so if you always mapped dmabuf to
> Vulkan SRGB formats (or always to non-SRGB formats), then you need some
> other way in Vulkan for the app to say whether to sample encoded or
> linear (electrical or optical) values. And whether texture filtering is
> done in encoded or linear space, because that makes a difference too.
>
> IOW, there are cases where the format mapping depends on the user of the
> buffer and not only on the contents of the buffer.
>
> Therefore you simply cannot create a static mapping table between two
> format definition systems when the two systems are fundamentally
> different, like Vulkan and DRM fourcc.
>
>
> Thanks,
> pq

Thanks all for your comments. We'll look into an alternative approach to
achieve what we need.
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