DRM Format Modifiers in v4l2

[Hans Verkuil]

On 08/24/17 13:14, Brian Starkey wrote:
> Hi Hans,
> 
> On Mon, Aug 21, 2017 at 06:36:29PM +0200, Hans Verkuil wrote:
>> On 08/21/2017 06:01 PM, Daniel Vetter wrote:
>>> On Mon, Aug 21, 2017 at 5:52 PM, Brian Starkey <brian.starkey at arm.com> wrote:
>>>> Hi all,
>>>>
>>>> I couldn't find this topic talked about elsewhere, but apologies if
>>>> it's a duplicate - I'll be glad to be steered in the direction of a
>>>> thread.
>>>>
>>>> We'd like to support DRM format modifiers in v4l2 in order to share
>>>> the description of different (mostly proprietary) buffer formats
>>>> between e.g. a v4l2 device and a DRM device.
>>>>
>>>> DRM format modifiers are defined in include/uapi/drm/drm_fourcc.h and
>>>> are a vendor-namespaced 64-bit value used to describe various
>>>> vendor-specific buffer layouts. They are combined with a (DRM) FourCC
>>>> code to give a complete description of the data contained in a buffer.
>>>>
>>>> The same modifier definition is used in the Khronos EGL extension
>>>> EGL_EXT_image_dma_buf_import_modifiers, and is supported in the
>>>> Wayland linux-dmabuf protocol.
>>>>
>>>>
>>>> This buffer information could of course be described in the
>>>> vendor-specific part of V4L2_PIX_FMT_*, but this would duplicate the
>>>> information already defined in drm_fourcc.h. Additionally, there
>>>> would be quite a format explosion where a device supports a dozen or
>>>> more formats, all of which can use one or more different
>>>> layouts/compression schemes.
>>>>
>>>> So, I'm wondering if anyone has views on how/whether this could be
>>>> incorporated?
>>>>
>>>> I spoke briefly about this to Laurent at LPC last year, and he
>>>> suggested v4l2_control as one approach.
>>>>
>>>> I also wondered if could be added in v4l2_pix_format_mplane - looks
>>>> like there's 8 bytes left before it exceeds the 200 bytes, or could go
>>>> in the reserved portion of v4l2_plane_pix_format.
>>>>
>>>> Thanks for any thoughts,
>>>
>>> One problem is that the modifers sometimes reference the DRM fourcc
>>> codes. v4l has a different (and incompatible set) of fourcc codes,
>>> whereas all the protocols and specs (you can add DRI3.1 for Xorg to
>>> that list btw) use both drm fourcc and drm modifiers.
>>>
>>> This might or might not make this proposal unworkable, but it's
>>> something I'd at least review carefully.
>>>
>>> Otherwise I think it'd be great if we could have one namespace for all
>>> modifiers, that's pretty much why we have them. Please also note that
>>> for drm_fourcc.h we don't require an in-kernel user for a new modifier
>>> since a bunch of them might need to be allocated just for
>>> userspace-to-userspace buffer sharing (e.g. in EGL/vk). One example
>>> for this would be compressed surfaces with fast-clearing, which is
>>> planned for i915 (but current hw can't scan it out). And we really
>>> want to have one namespace for everything.
>>
>> Who sets these modifiers? Kernel or userspace? Or can it be set by both?
>> I assume any userspace code that sets/reads this is code specific for that
>> hardware?
> 
> I think normally the modifier would be set by userspace. However it
> might not necessarily be device-specific code. In DRM the intention is
> for userspace to query the set of modifiers which are supported, and
> then use them without necessarily knowing exactly what they mean
> (insofar as that is possible).
> 
> e.g. if I have two devices which support MODIFIER_FOO, I could attempt
> to share a buffer between them which uses MODIFIER_FOO without
> necessarily knowing exactly what it is/does.
> 
>>
>> I think Laurent's suggestion of using a 64 bit V4L2 control for this makes
>> the most sense.
>>
>> Especially if you can assume that whoever sets this knows the hardware.
>>
>> I think this only makes sense if you pass buffers from one HW device to another.
>>
>> Because you cannot expect generic video capture code to be able to interpret
>> all the zillion different combinations of modifiers.
> 
> I don't quite follow this last bit. The control could report the set
> of supported modifiers.

What I mean was: an application can use the modifier to give buffers from
one device to another without needing to understand it.

But a generic video capture application that processes the video itself
cannot be expected to know about the modifiers. It's a custom HW specific
format that you only use between two HW devices or with software written
for that hardware.

> 
> However, in DRM the API lets you get the supported formats for each
> modifier as-well-as the modifier list itself. I'm not sure how exactly
> to provide that in a control.

We have support for a 'menu' of 64 bit integers: V4L2_CTRL_TYPE_INTEGER_MENU.
You use VIDIOC_QUERYMENU to enumerate the available modifiers.

So enumerating these modifiers would work out-of-the-box.

Regards,

	Hans