[PATCH/RFC 0/5] HDMI driver for Samsung S5PV310 platform

[Alex Deucher]

On Tue, Feb 8, 2011 at 5:47 PM, Andy Walls <awalls at md.metrocast.net> wrote:
> On Tue, 2011-02-08 at 10:28 -0500, Alex Deucher wrote:
>> On Tue, Feb 8, 2011 at 4:47 AM, Hans Verkuil <hansverk at cisco.com> wrote:
>> > Just two quick notes. I'll try to do a full review this weekend.
>> >
>> > On Tuesday, February 08, 2011 10:30:22 Tomasz Stanislawski wrote:
>> >> ==============
>> >>  Introduction
>> >> ==============
>> >>
>> >> The purpose of this RFC is to discuss the driver for a TV output interface
>> >> available in upcoming Samsung SoC. The HW is able to generate digital and
>> >> analog signals. Current version of the driver supports only digital output.
>> >>
>> >> Internally the driver uses videobuf2 framework, and CMA memory allocator.
>> > Not
>> >> all of them are merged by now, but I decided to post the sources to start
>> >> discussion driver's design.
>
>> >
>> > Cisco (i.e. a few colleagues and myself) are working on this. We hope to post
>> > an RFC by the end of this month. We also have a proposal for CEC support in
>> > the pipeline.
>>
>> Any reason to not use the drm kms APIs for modesetting, display
>> configuration, and hotplug support?  We already have the
>> infrastructure in place for complex display configurations and
>> generating events for hotplug interrupts.  It would seem to make more
>> sense to me to fix any deficiencies in the KMS APIs than to spin a new
>> API.  Things like CEC would be a natural fit since a lot of desktop
>> GPUs support hdmi audio/3d/etc. and are already using kms.
>>
>> Alex
>
> I'll toss one out: lack of API documentation for driver or application
> developers to use.
>
>
> When I last looked at converting ivtvfb to use DRM, KMS, TTM, etc. (to
> possibly get rid of reliance on the ivtv X video driver
> http://dl.ivtvdriver.org/xf86-video-ivtv/ ), I found the documentation
> was really sparse.
>
> DRM had the most documentation under Documentation/DocBook/drm.tmpl, but
> the userland API wasn't fleshed out.  GEM was talked about a bit in
> there as well, IIRC.
>
> TTM documentation was essentially non-existant.
>
> I can't find any KMS documentation either.
>
> I recall having to read much of the drm code, and having to look at the
> radeon driver, just to tease out what the DRM ioctls needed to do.
>
> Am I missing a Documentation source for the APIs?
>

Documentation is somewhat sparse compared to some other APIs.  Mostly
inline kerneldoc comments in the core functions.  It would be nice to
improve things.   The modesetting API is very similar to the xrandr
API in the xserver.

At the moment a device specific surface manager (Xorg ddx, or some
other userspace lib) is required to use kms due to device specific
requirements with respect to memory management and alignment for
acceleration.  The kms modesetting ioctls are common across all kms
drm drivers, but the memory management ioctls are device specific.
GEM itself is an Intel-specific memory manager, although radeon uses
similar ioctls.  TTM is used internally by radeon, nouveau, and svga
for managing memory gpu accessible memory pools.  Drivers are free to
use whatever memory manager they want; an existing one shared with a
v4l or platform driver, TTM, or something new.  There is no generic
userspace kms driver/lib although Dave and others have done some work
to support that, but it's really hard to make a generic interface
flexible enough to handle all the strange acceleration requirements of
GPUs.  kms does however provide a legacy kernel fb interface.

While the documentation is not great, the modesetting API is solid and
it would be nice to get more people involved and working on it (or at
least looking at it) rather than starting something equivalent from
scratch or implementing a device specific modesetting API.  If you
have any questions about it, please ask on dri-devel (CCed).

Alex

>
>
> For V4L2 and DVB on ther other hand, one can point to pretty verbose
> documentation that application developers can use:
>
>        http://linuxtv.org/downloads/v4l-dvb-apis/
>
>
>
> Regards,
> Andy
>
>
>