DRM Format Modifiers in v4l2

Thu Aug 31 14:36:43 UTC 2017

Hi Brian,

On Tuesday, 29 August 2017 12:19:43 EEST Brian Starkey wrote:
> On Fri, Aug 25, 2017 at 10:14:03AM +0200, Hans Verkuil wrote:
> >On 24/08/17 14:26, Brian Starkey wrote:
> >> On Thu, Aug 24, 2017 at 01:37:35PM +0200, Hans Verkuil wrote:
> >>> On 08/24/17 13:14, Brian Starkey wrote:
> >>>> 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 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.
> >> 
> >> Yes, makes sense.
> >> 
> >>>> 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.
> >> 
> >> Right. So I guess the supported set of formats could be somehow
> >> enumerated in the menu item string. In DRM the pairs are (modifier +
> >> bitmask) where bits represent formats in the supported formats list
> >> (commit db1689aa61bd in drm-next). Printing a hex representation of
> >> the bitmask would be functional but I concede not very pretty.
> >
> > So this patch limits the number of formats to 64 (being the size of
> > the bit mask).
> 
> It's not limited to 64 formats. Right now no DRM drivers support more
> than 64 formats, but when they do, the "offset" field in struct
> drm_format_modifier can be set to 64 and then the bit mask represents
> formats 65 through 128 (see the comment on that struct):
> 
>   * If the number formats grew to 128, and formats 98-102 are
>   * supported with the modifier:
>   *
>   * 0x0000003c00000000 0000000000000000
>   *                  ^
>   *                  |__offset = 64, formats = 0x3c00000000
> 
> > I was hoping these modifiers applied to all formats,
> > but unfortunately that isn't the case apparently.
> 
> Yeah, if only it were so simple :-)
> 
> > How it would work with my proposal is that the integer menu control
> > would reflect the list of supported modifiers for the currently selected
> > format. If you change format, then the available modifier list changes
> > as well.
> 
> Ah yes, I need to get used to thinking about stateful APIs - that
> works.

No, you don't, we need to make enumeration stateless :-)

> > The advantage is that there is no '64 formats' limitation,
> > something that I feel very uncomfortable about since some devices support
> > a *lot* of formats. The disadvantage is that it is harder to get a quick
> > overview of all combinations for formats and modifiers.
> >
> > This has more to do with limitations in the V4L2 API than with supporting
> > modifiers in general. We need something better to give userspace a quick
> > overview of all combinations of pixelformats, framesizes, frameintervals
> > and now modifiers. However, that's our problem :-)

-- 
Regards,

Laurent Pinchart