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

[Nicolas Dufresne <nicolas@ndufresne.ca>]

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Le mardi 23 avril 2019 à 17:09 +0200, Daniel Vetter a écrit :
> On Tue, Apr 23, 2019 at 4:28 PM Nicolas Dufresne <nicolas@ndufresne.ca> wrote:
> > Le mardi 23 avril 2019 à 14:33 +0200, Paul Kocialkowski a écrit :
> > > 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.
> > 
> > We also have the Samsung tiling (Z pattern) as mentioned here, but also
> > linear 16x16 tile placement (also from Samsung ?) and I believe Amlogic
> > CODEC patches is bringing another tiling (unavoidable on older Meson8,
> > with 64bytes swaps). All these should be expressed as NV12 + mod in DRM
> > space.
> > 
> > What is very often not enabled, but affect the performance on mainline
> > media drivers is the ARM frame buffer compression. I know that RK chips
> > have support for this, and that you can't achieve the maximum
> > throughput without that. This one is not documented anywhere, but I
> > understand that there is multiple variants that HW vendor licence.
> > Though, in general, each SoC are likely running a single variant, so a
> > single mod would make sense.
> 
> We have AFBC modifiers now in drm_fourcc.h, jointly developed by
> display engineers from ARM and mali gpu reverse engineer people doing
> the panfrost driver. So should be covered.
> 
> > So all this to say that V4L2 equally needs supports for these. What I
> > understood through DRM API is that a buffer allocated for let's say
> > NV12 + mod, is compatible with linear NV12. That could be used to
> > simplify some code, but at the same time, a common API that deals with
> > the padding and alignment of each format + mod independently would do
> > that same as long as this is not variable depending on which target HW
> > uses that same format.
> 
> Not sure why you mean with NV12 + mod is compatible with linear NV12.
> In general fourcc + modifier != fourcc = linear modifier, size, number
> of planes, alignment constraints and everything else can be changed by
> a modifier (and there's examples for all of these, maybe not yet in
> all cases for NV12, but I think NV12 + AFBC modifiers gives some
> pretty interesting results). In generally you need to think of the
> (drm fourcc, modifier) as the pair identified the pixel format, each
> part individually is fairly meanigless. We have lots of modifiers
> where the exact tiling mode/layout changes depending upon the fourcc
> code.

I only meant that the NV12 + mod have the same number of planes, and
should be large enough to store a linear NV12 equivalent. Not that it
would render correctly (even though I found it useful to be able to
render them when I needed to reverse it).

> 
> The only exception is legacy interfaces, i.e. when you create a
> framebuffer with only the drm fourcc and not a modifier. In that case
> you get driver specific behaviour, but modifier aware drivers tend to
> change that into a specific (fourcc, modifier) pair again (at least
> i915.ko, and it's what I recommend).
> 
> Oh and we have some legacy modifiers that depend upon the target hw,
> but it's very much not recommended (we did it in i915 to make things
> easier on really old platforms, on some of them we don't even know the
> exact tiling mode since it's not documented nor correctly
> reverse-engineered).
> 
> Another fun case are some of the recent non-byte-aligned formats, for
> which you need to have a modifier to be able to use them with anything
> - there's not really a real linear layout for them, they just serve as
> an index/parameter into the modifier space.
> 
> > I think DRM + mod reduce the amount of dedicated formats that needs to
> > be added, and simplify the documentation of each formats. I was looking
> > at the Amlogic Axi configurations on Amlogic S905x recently, and for
> > each well known format, there is a bitmask that let you do arbitrary
> > swapping of bits, so effectively if we start exposing all these with
> > V4L2 style, the list would become very long and hard to maintained.
> 
> See above, modifiers aren't really simple ...
> -Daniel
> 
> > > 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
> 
> 

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