Hi,
Let me chime in as well, based on the discussions I had last week in
Düsseldorf:
On 10/09/2014 12:06 PM, Kamil Debski wrote:
Hi,
From: Nicolas Dufresne [mailto:nicolas.dufresne@xxxxxxxxxxxxx]
Sent: Wednesday, October 08, 2014 4:35 PM
Le 2014-10-08 06:24, Kamil Debski a écrit :
Hi,
This patch seems complicated and I do not understand your motives.
Could you explain what is the problem with the current aligning of
the
values?
Is this a hardware problem? Which MFC version does it affect?
Is it a software problem? If so, maybe the user space application
should
take extra care on what value it passes/receives to try_fmt?
This looks like something I wanted to bring here as an RFC but never
manage to get the time. In an Odroid Integration we have started using
the following simple patch to work around this:
https://github.com/dsd/linux-
odroid/commit/c76b38c1d682b9870ea3b00093ad6500a9c5f5f6
The context is that right now we have decided that alignment in s_fmt
shall be done with a closest rounding. So the format returned may be
bigger, or smaller, that's basically random. I've been digging through
a
lot, and so far I have found no rational that explains this choice
other
that this felt right.
In real life, whenever the resulting format is smaller then request,
there is little we can do other then fail or try again blindly other
sizes. But with bigger raw buffers, we can use zero-copy cropping
techniques to keep going. Here's a example:
image_generator -> hw_converter -> display
As hw_converter is a V4L2 M2M, an ideal use case here would be for
image_generator to use buffers from the hw_converter. For the scenario,
it is likely that a fixed video size is wanted, but this size is also
likely not to match HW requirement. If hw_converter decide to give back
something smaller, there is nothing image_generator can do. It would
have to try again with random size to find out that best match. It's a
bit silly to force that on application, as the hw_converter know the
closest best match, which is simply the next valid bigger size if that
exist.
hope that helps,
Nicolas
Nicolas, thank you for shedding light on this problem. I see that it is
not MFC specific. It seems that the problem applies to all Video4Linux2
devices that use v4l_bound_align_image. I agree with you that this deservers
an RFC and some discussion as this would change the behaviour of quite
a few drivers.
I think the documentation does not specify how TRY_FMT/S_FMT should adjust
the parameters. Maybe it would a good idea to add some flagS that determine
the behaviour?
I think we should add flags here as well. NEAREST (the default), ROUND_DOWN and
ROUND_UP. Existing calls will use NEAREST. I can think of use-cases for all
three of these, and I think the caller should just have to specify what is
needed.
Just replacing the algorithm used seems asking for problems, you want to be
able to select what you want to do.
Regards,
Hans
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