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. 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. 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. > > 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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