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 _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
