Date: Fri, 26 Oct 2001 19:52:59 +0200
From: Gerd Knorr <kraxel@bytesex.org>
Message-Id: <200110261752.f9QHqxoM007951@bytesex.masq.in-berlin.de>
To: volodya@mindspring.com, video4linux-list@redhat.com,
        livid-gatos@linuxvideo.org, linux-kernel@vger.kernel.org
Subject: Re: [V4L] Re: [RFC] alternative kernel multimedia API
In-Reply-To: <Pine.LNX.4.20.0110261037080.11540-100000@node2.localnet.net>
In-Reply-To: <slrn9tiq3v.69j.kraxel@bytesex.org> <Pine.LNX.4.20.0110261037080.11540-100000@node2.localnet.net>
Sender: linux-kernel-owner@vger.kernel.org

> > what exactly is bad with the v4l2 controls?
>  
>  a) no way to use them to set non-integer data (like gamma tables)

Yes, they aren't designed for complex stuff.  They can do boolean,
multiple choice and integer options.  These ones an application can
easily present a reasonable GUI to the user without knowing what the
control exactly is good for.  For more complex stuff like gamma tables
(list of integers?) it is IMHO nearly impossible to build a reasonable
GUI without knowing what these values are actually good for.  Or even
know whenever it is useful to build a GUI for it or not.

>  b) labels are limited to 32 chars, no way to provide "comment" fields.
>    (I would like the driver to provide a short label and longer,
>     user-readable and understable comment field).

IMHO this isn't the job of the device driver.  Think about translations
of these descriptions for example ...

>  c) no way to present control dependencies.

I don't see how your approach handles this.

>    For example in ATI cards there are several ways you can control gamma:
>    gamma table, gamma overlay setting, gamma setting in the decoder chip.
>    same goes for brightness. I would rather expose all the controls and
>    show their relationship then bundle them up in one control

... and thus trying to make the driver tell the applications how the GUI
should present the controls to the user?

> > Ok, you replace the '#define V4L2_CID_HUE 0x???' with the magic string
> > "HUE" then.  The point is?
>  
>  ioctl are kernel specific. My scheme proposes a set of mmaped buffers and
>  character stream. The plus is that I can make Xmultimedia extension that
>  also presents a set of shared buffers and character stream. The
>  application code will be exactly the same.

That's your main point I assume?  You are trying to kill the ioctls
because you want to use the same protocol between application + driver
and application + other application (say a X-Server)?

>  Additionally nonone will need to include kernel headers to compile the
>  application.

Why do you want to avoid this?

>  Also magic string HUE is a bigger namespace then an
>  integer number. 

I doubt we will ever run out of space with 2^32 integer numbers.  I have
yet to see a v4l2 driver which has more than 2^8 controls ...

> > >         * be compatible with a wide range of applications
> > 
> > ???
> > Sorry, but I don't see why your approach handles this better than v4l2.
>  
>  Since the application accesses the driver symbolically they only have to
>  worry about agreeing on semantics. For example the driver can layout its
>  control structs any way it likes (say, to mirror hardware registers) and 
>  the application will bind to them. 

I don't see any difference to v4l2 here ...

>  Also, mmaped buffers + character device fits a larger category then just
>  the devices mentioned in v4l2 specifications. Granted the specification
>  can be extended, but the new driver will have to distribute not only its
>  source but a patch to the kernel headers. The scheme will avoid this.

And why this is better?  You likely still have to teach the applications
about the new features of the driver if you actually want to use them
(unless there are just a few new controls, but this case can easily
handled with v4l2 too).

> > >         * introduce support for new features without the need to modify
> > >           kernel interfaces
> > 
> > Hmm.  I don't like the idea to add new stuff that easily.  People tend
> > to do quick&dirty hacks + crazy stuff with it, leading to more
> > incompatibilies between drivers and applications.
>  
>  And people will always be able to write a driver outside video4linux
>  framework. In fact, plenty of drivers and applications are incompatible as
>  is. The same is true about audio drivers. 

Yes, these incompatibility issues are bad.  But I doubt the situation
will become better if every driver can easily add new stuff.

>  The compatibility is achived best by specifications that are clear and
>  easy to implement. This proposal helps it by providing a well defined
>  interface and by separating semantic meaning from actual interface. 

The semantic needs to be documented too, otherwise it easily gets worse
than v4l2.  One big plus for v4l2 is that it actually has very good API
specs.

>  First the internal kernel library will insure that the control interface
>  is standard across drivers. Secondly, the drivers are free to use whatever
>  fields they want - no restriction their.  Thirdly, the way the applications
>  know they deal, say, with a grabber device is when the driver says it
>  supports a certain INTERFACE_CLASS. Hence, to make sure the drivers
>  behave, all we have to do is bar any driver which declares itself
>  compliant with a certain INTERFACE_CLASS but isn't from entering the
>  kernel.

I don't see how this is different from the v4l2 ioctl interface.  We
have capability flags, ...

>  And the added advantage is that we can have many INTERFACE_CLASSes
>  which vary in implementation difficulty, so that BASIC_GRABBER-YUV422
>  would be very easy to implement (just expose the buffers) and something
>  more advanced could be added later. This solves the conflict of people
>  wanting to have _some_ driver and the time consumption of writing a driver
>  that supports all features.

I doubt this fixes the problem (driver devel time) you've mentioned.
Most of the time of the driver development is needed for the code which
talks to the hardware.  Building a simple v4l(2) ioctl interface which
just says "I can nothing but capture, and the format list is YUV422" is
easy compared to the other work which needs to be done for a working
driver.  v4l(2) doesn't force you to wait with the release until you
have a full-featured driver ...

> > >  The last point is the very important in my opinion. For example, in
> > >  current specification, v4l2 has no support for TV-out in graphics cards.
> > >  It has no support for setting complex parameters like gamma tables. 
> > >  Using memory-mapped buffers requires device specific ioctls.
> > 
> > Which device specific ioctls?  They are common for _all_ v4l2 devices.
>  
>  Quote from http://www.thedirks.org/v4l2/v4l2dev.htm :
>  
>  A common use for memory-mapped buffers is for streaming data to and from
>  drivers with minimum overhead. Drivers will maintain internal queues of
>  buffers and process them asynchonously. The ioctl commands for doing that
>  are described in the device-specific documents. Memory-mapping can also be
>  used for a variety of other purposes. Drivers can define hardware-specific
>  memory buffer types if needed. Use V4L2_BUF_TYPE_PRIVATE_BASE and greater
>  values for such buffer types. 

Ok, for different device types.  Currently only capture drivers exist
for v4l2 btw.  I doubt that your application will use the _same_ code
path for a _different_ device type.

> > >  The goal is to create an interface that does not rely on structures
> > >  defined in kernel headers for communication. 
> > 
> > Why do you want to do that?
>  
>   * not elegant

"elegant" is hardly a argument because it is subjective.

>   * kernel structures force a specific model onto a driver

The control strings you want to read() and write() to the driver do
exactly the same, because they must have some clear defined semantic to
make the whole model actually workable.

>   * can cause problems with different compilers

Then your compiler is buggy.

>   * confuse applications when a driver does not implement a field

With v4l2 controls this shouldn't be a issue any more.

>  However, this is hard to implement with ioctl's as they rely on fixed
>  length structures. You would have to call first to find out the size of
>  the buffer you need (which the driver would have to compute) and then the
>  second time to get the data. Eeeks.

Wrong.  Look at the specs for the v4l2 controls.

>  Now what is left are the structs describing the format of memory buffers. 
>  But the driver typically has this info in a struct itself. Instead of
>  getting it via ioctl simply mmap the area - you'll be getting the exact
>  same information yourself with less context switches.

read() + write() need context switches too.  And ioctl() can actually
pass informations in both directions, there are cases where you need
_more_ context switches with read()/write() than with ioctl() because
with ioctl one system calls does the job, but with read()/write() you
need two:  one for the request and one to get back the actual data.

>  This gets rid of the structs. 

Using strings instead adds a overhead to both driver and applications
for string parsing.  And the application likely puts the parsed data
into a struct again anyway ...

>  What is left of v4l now are synchronization ioctls. But we can replace
>  them with read/writes on the control device. With the added benifit that
>  we can select on it to wait for an event.

v4l2 expects drivers to support select too, so you can wait for your
capture requests using select.

  Gerd

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