Re: [PATCH 00/20] drm: Split out the formats API and move it to a common place

[Paul Kocialkowski]

Hi,

On Tue, 2019-04-23 at 09:30 +0200, Daniel Vetter wrote:
> On Sun, Apr 21, 2019 at 01:40:45AM +0300, Laurent Pinchart wrote:
> > Hi Paul,
> > 
> > On Thu, Apr 18, 2019 at 01:49:54PM +0200, Paul Kocialkowski wrote:
> > > On Thu, 2019-04-18 at 11:02 +0200, Maxime Ripard wrote:
> > > > On Thu, Apr 18, 2019 at 09:52:10AM +0200, Daniel Vetter wrote:
> > > > > And a lot of people pushed for the "fourcc is a standard", when
> > > > > really it's totally not.
> > > > 
> > > > Even if it's not a standard, having consistency would be a good thing.
> > > > 
> > > > And you said yourself that DRM fourcc is now pretty much an authority
> > > > for the fourcc, so it definitely looks like a standard to me.
> > > 
> > > I think trying to make the V4L2 and DRM fourccs converge is a lost
> > > cause, as it has already significantly diverged. Even if we coordinate
> > > an effort to introduce new formats with the same fourcc on both sides,
> > > I don't see what good that would be since the formats we have now are
> > > still plagued by the inconsistency.
> > > 
> > > I think we always need an explicit translation step from either v4l2 or
> > > drm to the internal representation and back, without ever assuming that
> > > formats might be compatible because they share the same fourcc.
> > 
> > I don't agree. APIs evolve, and while we can't switch from one set of
> > 4CCs to another in existing APIs, we could in new APIs. Boris is working
> > on new ioctls to handle formats in V4L2, and while 4CC unification could
> > be impopular from a userspace developers point of view there, I don't
> > think we have ruled it out completely. The move to the request API is
> > also an area where a common set of 4CCs could be used, as it will depart
> > from the existing V4L2 ioctls. To summarize my opinion, we're not there
> > yet, but I wouldn't rule it out completely for the future.
> > 
> > > It looks like so far, V4L2 pixel formats describe a DRM pixel format +
> > > modifier.
> > 
> > DRM modifiers are mostly about tiling and compression, and we hardly
> > support these in V4L2. What are the modifiers you think are hardcoded in
> > 4CCs in V4L2 ?
> 
> Hm maybe it was a drm one that didn't come from v4l or anywhere else
> really, but the NV12MT one is nv12 + some tiling. I think we managed to
> uapi-bend that one into shape in at least drm.

The one I had in mind is V4L2_PIX_FMT_SUNXI_TILED_NV12 which translates
to DRM_FORMAT_NV12 + DRM_FORMAT_MOD_ALLWINNER_TILED. Seems to be a
pretty similar case to the Mediatek one indeed.

In our cause, that's because the video decoding engine produces its
destination buffers in a specific tiled format, that the display engine
can take in directly.

Cheers,

Paul

> -Daniel
> 
> > > I think Boris (CCed) is working to change that by allowing to
> > > pass a DRM modifier through V4L2. With that, we'd be in a situation
> > > where some formats are described by the v4l2 pixfmt alone and some
> > > formats are also described a modifier (but I looked at it from a
> > > distance so might have misunderstod). That feels better since it avoids
> > > the combinatory explosion from describing each format + modifier
> > > individually.
> > > 
> > > What do you think?
> > > 
> > > > > v4l tends to conflate pixel format with stuff that we tend to encode
> > > > > in modifiers a lot more.
> > > > 
> > > > Boris is working on adding the modifiers concept to v4l2, so we're
> > > > converging here, and we can totally have a layer in v4l2 to convert
> > > > between old v4l2 "format+modifiers" formats, and DRM style formats.
> > > > 
> > > > > There's a bunch of reasons we can't just use v4l, and they're as
> > > > > valid as ever:
> > > > > 
> > > > > - We overlap badly in some areas, so even if fourcc codes match, we
> > > > >   can't use them and need a duplicated DRM_FOURCC code.
> > > > 
> > > > Do yo have an example of one of those areas?
> > > > 
> > > > > - v4l encodes some metadata into the fourcc that we encode elsewhere,
> > > > >   e.g. offset for planar yuv formats, or tiling mode
> > > > 
> > > > As I was saying, this changes on the v4l2 side, and converging to
> > > > what DRM is doing.
> > > > 
> > > > > - drm fourcc code doesn't actually define the drm_format_info
> > > > >   uniquely, drivers can override that (that's an explicit design
> > > > >   intent of modifiers, to allow drivers to add another plane for
> > > > >   e.g. compression information). You'd need to pull that driver
> > > > >   knowledge into your format library.
> > > > 
> > > > I'm not sure how my patches are changing anything here. This is
> > > > litterally the same code, with the functions renamed.
> > > > 
> > > > If drivers want to override that, then yeah, fine, we can let them do
> > > > that. Just like any helper this just provides a default that covers
> > > > most of the cases.
> > > > 
> > > > > Iow there's no way we can easily adopt v4l fourcc, except if we do
> > > > > something like a new addfb flag.
> > > > 
> > > > For the formats that would be described as a modifier, sure. For all
> > > > the others (that are not yet supported by DRM), then I don't really
> > > > see why not.
> > > > 
> > > > > > And given how the current state is a mess in this regard, I'm not too
> > > > > > optimistic about keeping the formats in their relevant frameworks.
> > > > > > 
> > > > > > Having a shared library, governed by both, will make this far easier,
> > > > > > since it will be easy to discover the formats that are already
> > > > > > supported by the other subsystem.
> > > > > 
> > > > > I think a compat library that (tries to, best effort) convert between
> > > > > v4l and drm fourcc would make sense. Somewhere in drivers/video, next
> > > > > to the conversion functions for videomode <-> drm_display_mode
> > > > > perhaps. That should be useful for drivers.
> > > > 
> > > > That's not really what this series is about though. That series is
> > > > about sharing the (image|pixels) formats database across everyone so
> > > > that everyone can benefit from it.
> > > > 
> > > > > Unifying the formats themselves, and all the associated metadata is
> > > > > imo a no-go, and was a pretty conscious decision when we implemented
> > > > > drm_fourcc a few years back.
> > > > > 
> > > > > > If we want to keep the current library in DRM, we have two options
> > > > > > then:
> > > > > > 
> > > > > >   - Support all the v4l2 formats in the DRM library, which is
> > > > > >     essentially what I'm doing in the last patches. However, that
> > > > > >     would require to have the v4l2 developpers also reviewing stuff
> > > > > >     there. And given how busy they are, I cannot really see how that
> > > > > >     would work.
> > > > > 
> > > > > Well, if we end up with a common library then yes we need cross
> > > > > review. There's no way around that. Doesn't matter where exactly that
> > > > > library is in the filesystem tree, and adding a special MAINTAINERS
> > > > > entry for anything related to fourcc (both drm and v4l) to make sure
> > > > > they get cross-posted is easy. No file renaming needed.
> > > > 
> > > > That would create some governing issues as well. For example, if you
> > > > ever have a patch from one fourcc addition (that might or might not be
> > > > covered by v4l2), will you wait for any v4l2 developper to review it?
> > > > 
> > > > If it's shared code, then it should be shared, and every client
> > > > framework put on an equal footing.
> > > > 
> > > > > >   - Develop the same library from within v4l2. That is really a poor
> > > > > >     solution, since the information would be greatly duplicated
> > > > > >     between the two, and in terms of maintainance, code, and binary
> > > > > >     size that would be duplicated too.
> > > > > 
> > > > > It's essentially what we decided to do for drm years back.
> > > > 
> > > > And it was probably the right solution back then, but I'm really not
> > > > convinced it's still the right thing to do today.
> > > > 
> > > > > > Having it shared allows to easily share, and discover formats from the
> > > > > > other subsystem, and to have a single, unique place where this is
> > > > > > centralized.
> > > > > 
> > > > > What I think could work as middle ground:
> > > > > - Put drm_format stuff into a separate .ko
> > > > > - Add a MAINTAINERS entry to make sure all things fourcc are cross
> > > > > posted to both drm and v4l lists. Easy on the drm side, since it's all
> > > > > separate files. Not sure it's so convenient for v4l uapi.
> > > > > - Add a conversion library that tries to best-effort map between drm
> > > > > and v4l formats. On the drm side that most likely means you need
> > > > > offsets for planes, and modifiers too (since those are implied in some
> > > > > v4l fourcc). Emphasis on "best effort" i.e. only support as much as
> > > > > the drivers that use this library need.
> > > > > - Add drm_fourcc as-needed by these drivers that want to use a unified
> > > > > format space.
> > > > > 
> > > > > Forcing this unification on everyone otoh is imo way too much.
> > > > 
> > > > v4l2 is starting to converge with DRM, and we're using the DRM API
> > > > pretty much untouched for that library, so I'm not really sure how
> > > > anyone is hurt by that unification.
> > 
> > -- 
> > Regards,
> > 
> > Laurent Pinchart
-- 
Paul Kocialkowski, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com