Hi Ville,
Sorry for the late reply.
(Cross-posting to the linux-fbdev and linux-media mailing lists, as the topics
I'm about to discuss are of interest to everybody)
On Wednesday 16 November 2011 19:42:23 ville.syrjala@linux.intel.com wrote:
I decided to go all out with the pixel format definitions. Added pretty
much all of the possible RGB/BGR variations. Just left out ones with
16bit components and floats. Also added a whole bunch of YUV formats,
and 8 bit pseudocolor for good measure.
I'm sure some of the fourccs now clash with the ones used by v4l2,
but that's life.
Disclaimer: I realize this is a somehow controversial topic, and I'm not
trying to start a flame war :-)
What about defining a common set of fourccs for both subsystem (and using them
in FBDEV, which currently uses the V4L2 fourccs) ?
There's more and more overlap between DRM, FBDEV and V4L2, which results in
confusion for the user and mess in the kernel. I believe cleaning this up will
become more and more important with time, and also probably more and more
difficult if we don't act now.
There's no way we will all quickly agree on a big picture unified architecture
(I don't even know what it should look like), so I'm thinking about starting
small and see where it will lead us. Sharing fourccs would be such a first
step, and would make it easier for drivers to implement several of the DRM,
FBDEV and V4L2 APIs.
Another step I'd like to take would be working on sharing the video mode
definitions between subsystems. DRM has struct drm_mode_modeinfo and FBDEV has
struct fb_videomode. The former can't be modified as it's used in userspace
APIs, but I believe we should create a common in-kernel structure to represent
modes. This would also make it easier to share the EDID parser between DRM and
FBDEV.
One issue here is names. I understand that using names from another subsystem
in a driver that has nothing to do with it (like using V4L2_PIX_FMT_* in DRM,
or drm_mode_modeinfo in FBDEV) can be frustrating for many developers, so I'd
like to propose an alternative. We have a media-related kernel API that is
cross-subsystem, that's the media controller. It uses the prefix media_. We
could use more specific versions of that preview (such as media_video_ and
media_display_) as a neutral ground.
Another issue is control. It's quite natural to be suspicious about subsystems
we're not familiar with, and giving up control on things we consider as highly
important to other subsystems feels dangerous and wrong. Once again, media_*
could be an answer to this problem. Instead of trying to push other subsystems
to use our APIs (which are, obviously, the best possible ever, as they're the
ones we work on :-)) with the promise that we will extend them to fullfill
their needs if necessary, we could design new shared structures and core
functions together with a media_ prefix, and make sure they fullfill all the
needs from the start. What I like about this approach is that each of the 3
video-related subsystems would then benefit from the experience of the other
2.
To make it perfectly clear, I want to emphasize that I'm not trying to replace
DRM, FBDEV and V4L2 with a new shared subsystem. What I would like to see in
the (near future) is collaboration and sharing of core features that make
sense to share. I believe we should address this from a "pain point" point of
view. The one that lead me to writing this e-mail is that developing a driver
that implements both the DRM and FBDEV APIs for video output currently
requires various similar structures, and code to translate between all of
them. That code can't be shared between multiple drivers, is error-prone, and
painful to maintin.
I can (and actually would like to) submit an RFC, but I would first like to
hear your opinion on the topic.
If anyone has problems with the way the formats are defined, please
speak up now! Since only Jesse has bothered to comment on my rantings
I can only assume people are happy with my approach to things.
These patches should apply on top of Jesse's v3+v5 set... I think.
I sort of lost track of things when the patches started having
different version numbers. At least we're not yet into two digits
numbers ;)
--
Regards,
Laurent Pinchart
On Tuesday 29 November 2011 13:10:35 Laurent Pinchart wrote:
Hi Ville,
Sorry for the late reply.
(Cross-posting to the linux-fbdev and linux-media mailing lists, as the
topics I'm about to discuss are of interest to everybody)
On Wednesday 16 November 2011 19:42:23 ville.syrjala@linux.intel.com wrote:
quoted
I decided to go all out with the pixel format definitions. Added pretty
much all of the possible RGB/BGR variations. Just left out ones with
16bit components and floats. Also added a whole bunch of YUV formats,
and 8 bit pseudocolor for good measure.
I'm sure some of the fourccs now clash with the ones used by v4l2,
but that's life.
Disclaimer: I realize this is a somehow controversial topic, and I'm not
trying to start a flame war :-)
What about defining a common set of fourccs for both subsystem (and using
them in FBDEV, which currently uses the V4L2 fourccs) ?
There's more and more overlap between DRM, FBDEV and V4L2, which results in
confusion for the user and mess in the kernel. I believe cleaning this up
will become more and more important with time, and also probably more and
more difficult if we don't act now.
There's no way we will all quickly agree on a big picture unified
architecture (I don't even know what it should look like), so I'm thinking
about starting small and see where it will lead us. Sharing fourccs would
be such a first step, and would make it easier for drivers to implement
several of the DRM, FBDEV and V4L2 APIs.
Another step I'd like to take would be working on sharing the video mode
definitions between subsystems. DRM has struct drm_mode_modeinfo and FBDEV
has struct fb_videomode. The former can't be modified as it's used in
userspace APIs, but I believe we should create a common in-kernel
structure to represent modes. This would also make it easier to share the
EDID parser between DRM and FBDEV.
One issue here is names. I understand that using names from another
subsystem in a driver that has nothing to do with it (like using
V4L2_PIX_FMT_* in DRM, or drm_mode_modeinfo in FBDEV) can be frustrating
for many developers, so I'd like to propose an alternative. We have a
media-related kernel API that is cross-subsystem, that's the media
controller. It uses the prefix media_. We could use more specific versions
of that preview (such as media_video_ and media_display_) as a neutral
ground.
I agree.
Another issue is control. It's quite natural to be suspicious about
subsystems we're not familiar with, and giving up control on things we
consider as highly important to other subsystems feels dangerous and
wrong. Once again, media_* could be an answer to this problem. Instead of
trying to push other subsystems to use our APIs (which are, obviously, the
best possible ever, as they're the ones we work on :-)) with the promise
that we will extend them to fullfill their needs if necessary, we could
design new shared structures and core functions together with a media_
prefix, and make sure they fullfill all the needs from the start. What I
like about this approach is that each of the 3 video-related subsystems
would then benefit from the experience of the other 2.
To make it perfectly clear, I want to emphasize that I'm not trying to
replace DRM, FBDEV and V4L2 with a new shared subsystem. What I would like
to see in the (near future) is collaboration and sharing of core features
that make sense to share. I believe we should address this from a "pain
point" point of view. The one that lead me to writing this e-mail is that
developing a driver that implements both the DRM and FBDEV APIs for video
output currently requires various similar structures, and code to
translate between all of them. That code can't be shared between multiple
drivers, is error-prone, and painful to maintin.
I can (and actually would like to) submit an RFC, but I would first like to
hear your opinion on the topic.
I have added Jesse Barker to the CC list. I think that to achieve closer
cooperation between the various subsystems we need to organize a few days
of talks between the main developers to 1) understand each others subsystem,
2) to discover what functionality would be good to share, and 3) figure out
some sort of roadmap on how to do this.
Linaro might be a suitable organization that can assist with at least the
organizational part of setting up such a meeting. They did a good job in the
past (in Budapest and Cambourne) with the buffer sharing discussions.
One week of discussions/brainstorming between a small(ish) group of developers
can be very, very productive in my experience.
Regards,
Hans
quoted
If anyone has problems with the way the formats are defined, please
speak up now! Since only Jesse has bothered to comment on my rantings
I can only assume people are happy with my approach to things.
These patches should apply on top of Jesse's v3+v5 set... I think.
I sort of lost track of things when the patches started having
different version numbers. At least we're not yet into two digits
numbers ;)
On Tue, 2011-11-29 at 13:10 +0100, Laurent Pinchart wrote:
To make it perfectly clear, I want to emphasize that I'm not trying to replace
DRM, FBDEV and V4L2 with a new shared subsystem. What I would like to see in
the (near future) is collaboration and sharing of core features that make
sense to share. I believe we should address this from a "pain point" point of
view. The one that lead me to writing this e-mail is that developing a driver
that implements both the DRM and FBDEV APIs for video output currently
requires various similar structures, and code to translate between all of
them. That code can't be shared between multiple drivers, is error-prone, and
painful to maintin.
We've been in the same situation with OMAP display driver for years. The
same low level display driver is used by FB, V4L2 and now DRM drivers,
so I didn't have much choice but to implement omapdss versions for
things like video timings and color formats.
I've been planning to write code for this almost as long as omapdss has
had this problem, but there's always been
yet-another-important-thing-to-do. So good that somebody finally brought
this up =).
I'm happy to test any code related to this with omapdss.
Tomi