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Date:   Tue, 23 Apr 2019 18:45:27 +0300
From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Maxime Ripard <maxime.ripard@bootlin.com>,
        Daniel Vetter <daniel.vetter@intel.com>,
        David Airlie <airlied@linux.ie>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Sean Paul <seanpaul@chromium.org>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Sakari Ailus <sakari.ailus@linux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Paul Kocialkowski <paul.kocialkowski@bootlin.com>,
        Hans Verkuil <hans.verkuil@cisco.com>,
        Thomas Petazzoni <thomas.petazzoni@bootlin.com>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@vger.kernel.org>
Subject: Re: [PATCH 00/20] drm: Split out the formats API and move it to a
 common place
Message-ID: <20190423154527.GH16111@pendragon.ideasonboard.com>
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Hi Daniel,

On Tue, Apr 23, 2019 at 09:25:54AM +0200, Daniel Vetter wrote:
> On Sun, Apr 21, 2019 at 01:59:04AM +0300, Laurent Pinchart wrote:
> > On Thu, Apr 18, 2019 at 12:07:44PM +0200, Daniel Vetter wrote:
> >> On Thu, Apr 18, 2019 at 11:02 AM Maxime Ripard wrote:
> >>> On Thu, Apr 18, 2019 at 09:52:10AM +0200, Daniel Vetter wrote:
> >>>> On Thu, Apr 18, 2019 at 8:22 AM Maxime Ripard wrote:
> >>>>> On Wed, Apr 17, 2019 at 05:41:21PM +0200, Daniel Vetter wrote:
> >>>>>> On Wed, Apr 17, 2019 at 09:54:26AM +0200, Maxime Ripard wrote:
> >>>>>>> Hi,
> >>>>>>>
> >>>>>>> DRM comes with an extensive format support to retrieve the various
> >>>>>>> parameters associated with a given format (such as the subsampling, or the
> >>>>>>> bits per pixel), as well as some helpers and utilities to ease the driver
> >>>>>>> development.
> >>>>>>>
> >>>>>>> v4l2, on the other side, doesn't provide such facilities, leaving each
> >>>>>>> driver reimplement a subset of the formats parameters for the one supported
> >>>>>>> by that particular driver. This leads to a lot of duplication and
> >>>>>>> boilerplate code in the v4l2 drivers.
> >>>>>>>
> >>>>>>> This series tries to address this by moving the DRM format API into lib and
> >>>>>>> turning it into a more generic API. In order to do this, we've needed to do
> >>>>>>> some preliminary changes on the DRM drivers, then moved the API and finally
> >>>>>>> converted a v4l2 driver to give an example of how such library could be
> >>>>>>> used.
> >>>>>>>
> >>>>>>> Let me know what you think,
> >>>>>>> Maxime
> >>>>>>>
> >>>>>>> Changes from RFC:
> >>>>>>>   - Rebased on next
> >>>>>>>   - Fixed the various formats mapping
> >>>>>>>   - Added tags
> >>>>>>>   - Did most of the formats functions as inline functions
> >>>>>>>   - Used a consistent prefix for all the utilities functions
> >>>>>>>   - Fixed the compilation breakages, and did a run of allmodconfig for arm,
> >>>>>>>     arm64 and x86_64
> >>>>>>>   - Fixed out of array bounds warnings in the image_format_info_block_*
> >>>>>>>     functions
> >>>>>>>   - Added License and copyright headers on missing files
> >>>>>>>
> >>>>>>> Maxime Ripard (20):
> >>>>>>>   drm: Remove users of drm_format_num_planes
> >>>>>>>   drm: Remove users of drm_format_(horz|vert)_chroma_subsampling
> >>>>>>>   drm/fourcc: Pass the format_info pointer to drm_format_plane_cpp
> >>>>>>>   drm/fourcc: Pass the format_info pointer to drm_format_plane_width/height
> >>>>>>>   drm: Replace instances of drm_format_info by drm_get_format_info
> >>>>>>>   lib: Add video format information library
> >>>>>>>   drm/fb: Move from drm_format_info to image_format_info
> >>>>>>>   drm/malidp: Convert to generic image format library
> >>>>>>>   drm/client: Convert to generic image format library
> >>>>>>>   drm/exynos: Convert to generic image format library
> >>>>>>>   drm/i915: Convert to generic image format library
> >>>>>>>   drm/ipuv3: Convert to generic image format library
> >>>>>>>   drm/msm: Convert to generic image format library
> >>>>>>>   drm/omap: Convert to generic image format library
> >>>>>>>   drm/rockchip: Convert to generic image format library
> >>>>>>>   drm/tegra: Convert to generic image format library
> >>>>>>>   drm/fourcc: Remove old DRM format API
> >>>>>>>   lib: image-formats: Add v4l2 formats support
> >>>>>>>   lib: image-formats: Add more functions
> >>>>>>>   media: sun6i: Convert to the image format API
> >>>>>>
> >>>>>> In the interest of making myself unpopular: Why move this out of drm?
> >>>>>>
> >>>>>> We do have a bunch of other such shared helpers already (mostly in
> >>>>>> drivers/video) for dt videomode and hdmi infoframes, and I'm not super
> >>>>>> sure that's going better than keeping it maintained in drm.
> > 
> > That's a bit of a different situation as both DRM and FBDEV address the
> > same features (display output), and FBDEV is deprecared and replaced by
> > DRM.
> > 
> > I'm not against maintaining a 4CC library in DRM (adding a third-party
> > maintainer would likely create more problems than it would solve), but
> > that doesn't mean the library has to live in drivers/gpu/, nor that it
> > needs to have the drm_ prefix. I would actually advocate to make it live
> > in a neutral directory, with a neutral prefix (kudos to anyone who can
> > propose a nice naming convention that would establish a new shared
> > ground for image/video-related Linux APIs), and maintained through the
> > DRM tree (possibly with extra entries in MAINTAINERS to ensure it
> > reaches all the related folks).
> > 
> >>>>>> Plus the uapi is already that you include drm_fourcc.h to get at these,
> >>>>>> dropping the drm prefix isn't going to help I think.
> >>>>>>
> >>>>>> Of course we'd need to make it a separate drm_formats.ko (so that v4l can
> >>>>>> use it without dragging in all of drm), and we need to add some fields for
> >>>>>> converting to v4l fourcc codes (which are different). But that should be
> >>>>>> all possible. And I don't think the drm_ prefix in v4l code is a problem,
> >>>>>> it's actually a feature: It makes it really clear that these are the drm
> >>>>>> fourcc codes, as allocated in drm_fourcc.h, plus their modifiers, and all
> >>>>>> that. That allocation authority is also already baked into various khr/ext
> >>>>>> standards, too.
> > 
> > There's one thing that V4L2 has and DRM hasn't for 4CCs: good
> > documentation. Look at
> > https://linuxtv.org/downloads/v4l-dvb-apis-new/uapi/v4l/pixfmt-packed-rgb.html
> > or
> > https://linuxtv.org/downloads/v4l-dvb-apis-new/uapi/v4l/yuv-formats.html
> > for instance. It's painful to write, painful to read, but defines the
> > 4CCs very clearly without ambiguity. I wouldn't be surprised if
> > different drivers used the same DRM 4CC for different formats due to the
> > lack of detailed documentation. Moving to a shared library for 4CCs
> > should also address the documentation side, and any new format added to
> > the kernel, whether from the V4L2 side or the DRM side, would be
> > required to provide detailed documentation. That would be a great
> > improvement for DRM 4CC handling.
> > 
> >>>>> The way I see it, there's a fundamental difference between the UAPI
> >>>>> and the kernel. I don't suggest we change anything about the UAPI: the
> >>>>> drm formats should keep their prefix, drm_fourcc.h can remain that
> >>>>> authority, it's all fine.
> >>>>>
> >>>>> Internally however, the long term goal is to share the fourcc's
> >>>>> between DRM and V4L2 for the same formats. It basically means that of
> >>>>> course v4l2 should be using the DRM fourcc when a format exists in DRM
> >>>>> and not v4l2, but also that DRM should use v4l2 fourcc when the format
> >>>>> exists in v4l2 but not DRM, and that is far more likely, given the
> >>>>> already extensive v4l2 formats support.
> >>>>
> >>>> Uh no. We did look at v4l fourcc extensively when deciding upon a drm
> >>>> format identifier space.
> >>>
> >>> No to what exactly?
> >>>
> >>>> 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.
> >> 
> >> drm fourcc is the authority for drm fourcc codes. Not for any of the
> >> others (and there's lots of them). Now it's used in a bunch of other
> >> places (khr standards, dri protocols in wayland/X11), but they're
> >> still only drm fourcc. There is no overall fourcc standard.
> >> 
> >>>> 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?
> >> 
> >> I think the rgb stuff was one of the big reasons to not reuse any
> >> existing fourcc standard (whether v4l, or another one from e.g. video
> >> container formats). We had initial patches to reuse the fourcc that
> >> existed, but the end result was really inconsistent, so we ditch that
> >> and rolled out a new set of entirely drm specific fourcc codes for
> >> rgba.
> > 
> > Could you give one or a couple of examples of V4L2 4CCs that are not
> > OCD-compatible ? :-)
> > 
> >>>> - 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.
> > 
> > That's a mistake in my opinion. We tried that in V4L2 to store metadata
> > in a separate plane, and had to go another route eventually as it
> > created a very bad mess.
> 
> Just quick clarification in the middle here: This is how the hw works.

The hardware takes parameters from a buffer, but it doesn't mandate how
that buffer is exposed to userspace :-) Using an extra plane is one
option, but other APIs are possible.

> It's not metadata that sw ever touches (in general, testcases to make sure
> we display these correctly excepted).
> 
> There has been some talking to add maybe a bit more mixed metadata, for
> fast-clear colors (which isn't used by any display engine afaik yet). That

What are fast-clear colors ?

> would generally be written by the cpu (in the gl stack), but again read by
> the hw (loaded as indirect state packet most likely, or something like
> that). So again hw specific layout, because the hw needs to read it.
> 
> Pure metadata only of interest for the cpu/sw stack has been shot down
> completely on the drm side too.
> 
> > There's a tendency in both subsystems to look at the other as a bit of a
> > retarded relative, and that's a shame, we have lots to learn from each
> > other's mistakes. That wouldn't be too difficult if people started
> > talking to each other.
> > 
> > A semi-related comment: DRM also carries other mistakes of its own, I'm
> > thinking about DRM_FORMAT_BIG_ENDIAN in particular
> 
> Yeah that one is hilarios, but in practice big endian is dead, except for
> a very few server chips, and there I think Gerd's work mostly fixed up
> that mess.
> 
> >>> 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.
> >> 
> >> See above, we tried that initially, didn't pass the ocd tests when
> >> reviewing. Maybe the situation is better outside of rbgx/a formats,
> >> and I think we do at least try to use the same fourcc codes there when
> >> there already is one. But then the details occasionally look
> >> different.
> >> 
> >>>>> 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.
> >> 
> >> Yeah I know. I still think leaving the drm fourcc with the drm prefix
> >> would be good, since there's really no standard here.
> >> 
> >>>> 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?
> >> 
> >> None of this is fixed by code renaming or locations. Either way we
> >> need to figure that out.
> >> 
> >> And yes part of the reasons for not moving this out of drm is that I'm
> >> not a fan of boutique trees for small stuff. If sharing means we need
> >> to split the drm_fourcc code and library out of drm trees, I'm not
> >> sure that's a good idea. We're already super liberal with merging
> >> anything through driver trees with acks, and integrating them quickly
> >> into drm-next. This would still be workable if v4l sends regular pull
> >> requests to drm-next (every 1-2 weeks, like the other big gpu trees
> >> do). If we can only sync up once per merge window with a shared
> >> boutique tree for formats only, life is going to be painful.
> > 
> > That should be solved by the proposal above, maintaining the shared
> > library in the DRM tree. We would need to be careful there, and ideally
> > maintain that in a separate branch that could be merged in both DRM and
> > V4L2 without having to merge most of the other subsystem's pending
> > changes at the same time, but I think it's doable without any big issue.
> > 
> >>> 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.
> >> 
> >> It might make sense to regularly pull v4l updates into drm-next then
> >> anyway. That would also remove the need to have the format library
> >> somewhere else.
> > 
> > Are you saying it should the live in V4L2 ? ;-)
> 
> Maybe a few clarifications on how the drm shared core thing usually works,
> and why I'm a sticker here. Bottom reply since I'm not sure where to put
> it:
> 
> - Refactorings usually go in through drm-misc (at least since a few
>   years).
> 
> - Small patches go in through the relevant driver tree (which is often
>   drm-misc, but not always), with an Ack from drm maintainers.
> 
> - No topic branches, everyone just pushes patches where it's most
>   convenient.
> 
> We get away with this mess because everyone sends regular pull requests to
> drm, where the entire history is baked in and others can backmerge/fast
> forward/rebase. Worst case you wait one month (around the merge window,
> when drm-next is closed for features), but usually it's just 1-2 weeks.
> Plus we tend to have fairly big trees, with good chances that the next
> patch series lands in the same tree again and no work at all is needed.
> 
> If we start regularly sharing lots of code with v4l (which seems to be the
> long term goal here), then I think we need something equally convenient
> for all that.
> 
> We're not going to be able to teach some complicated topic branch scheme
> to the 50+ submaintainers/committers we have in drm - a lot much more
> basic stuff already takes lots of work to get it to stick. If the proposal
> is "to be careful" and "maintain it in a separate branch", I'm not in
> favour because I think that just wouldn't work.

Why not ? It can be a fast-moving branch that gets merged in drm-misc
as often as you want (even at every commit if that's desired). We're
dealing with a limited amount of code here, and there's no more reason
that V4L2 should pull in drm-misc to get 4CC updates than DRM should
pull V4L2 for the same. I have no objection against a 4CC branch being
officially maintained under the DRM umbrella, but I think the code
should live elsewhere than drivers/gpu/drm/, have a neutral prefix, and
not require pulling an entire subsystem in.


-- 
Regards,

Laurent Pinchart