Hi Mauro,
On 09/26/2011 02:10 PM, Mauro Carvalho Chehab wrote:
Em 26-09-2011 05:42, Tomasz Stanislawski escreveu:
On 09/24/2011 05:58 AM, Mauro Carvalho Chehab wrote:
Hi Mauro
Thank you for your comments. Please refer to the answers below.
Em 22-09-2011 12:13, Marek Szyprowski escreveu:
Hello Mauro,
I've collected pending selection API patches together with pending
videobuf2 and Samsung driver fixes to a single git branch. Please pull
them to your media tree.
Marek Szyprowski (1):
staging: dt3155v4l: fix build break
I've applied this one previously, from the patch you sent me.
Tomasz Stanislawski (6):
v4l: add support for selection api
v4l: add documentation for selection API
I need more time to review those two patches. I'll probably do it at the next week.
I generally start analyzing API changes based on the DocBook, so, let me point a few
things I've noticed on a quick read, at the vidioc-g-selection.html DocBook-generated page:
1) "The coordinates are expressed in driver-dependant units"
Why? coordinates should be expressed in pixels, as otherwise there's no way to
use this API on a hardware-independent way.
The documentation for existing cropping API contains following sentence:
"To support a wide range of hardware this specification does not define an origin or units."
I decided to follow the same convention for the selection API.
I thought that the only exception would be images in memory buffers, whose coordinated would be pixels.
However, as Laurent mentioned, some devices are capable of sub-pixel cropping. Moreover, there are image exotic formats that have no well-defined resolution (fractal or vector graphics).
Now I am still not sure if the requirement for resolution in pixels should be used any more. The problem is that this requirement is very intuitive and useful in "let's say" 95% of cases.
How an userspace application is supposed to know the type of scale?
I can't see any way of querying the scale type.
What do you mean by 'scale type'? Do you mean types like 'shrink',
'enlarge', 'no scale'?
2)
0 - driver is free to adjust size, it is recommended to choose the crop/compose rectangle as close as possible to the original one
SEL_SIZE_GE - driver is not allowed to shrink the rectangle. The original rectangle must lay inside the adjusted one
SEL_SIZE_LE - drive is not allowed to grow the rectangle. The adjusted rectangle must lay inside the original one
SEL_SIZE_GE | SEL_SIZE_LE - choose size exactly the same as in desired rectangle.
The macro names above don't match the definition, as they aren't prefixed by V4L2_.
ok.. I will fix it.
3) There was no hyperlink for the struct v4l2_selection, as on other API definitions.
ok.. I will fix it.
4) the language doesn't seem too consistent with the way other ioctl's are defined. For example,
you're using struct::field for a field at the struct. Other parts of the API just say "field foo of struct bar".
ok.. I will fix it.
5) There's not a single mention at the git commit or at the DocBook about why the old crop API
is being deprecated. You need to convince me about such need (ok, I followed a few discussions in
the past, but, my brain patch buffer is shorter than the 7000 patchwork patches I reviewed just on
this week). Besides that: do we really need to obsolete the crop API for TV cards? If so, why? If not,
you need to explain why a developer should opt between one ioctl set of the other.
There are a few reasons to drop support for the cropping ioctls.
First, the selection API covers existing crop API. Therefore there is no need to implement {S/G}_CROP and G_CROPCAP.
The {S/G}_SELECTION is enough to provide the same functionality. We should avoiding duplication inside the api,
therefore S_CROP should be dropped.
Second, there is a patch that adds simulation of old crop API using selection API. Therefore there is no need to
change code of the existing applications.
Both are fine, but you should notice that they aren't arguments why an
userspace application or a driver shouldn't implement/use the crop API.
Third, the selection is much more powerful API, and it could be extended in future. There no more reserved fields inside structures
used by current cropping api.
Ok.
Moreover cropping is very inconsistent.
Why? Please describe its inconsistencies at the DocBook.
Ok .. I will add a new subsection about the deficiencies of old crop API.
For output devices cropping into display actually means composing into display.
Ok.
Moreover it not possible to select only
the part of the image from the buffer to be inserted or filled by the hardware.
You're talking again on output devices, right?
Yes.
The selection API introduced the idea
of the constraints flags, that are used for precise control of the coordinates' rounding policy.
Ok, but I fail to see where a rounding policy would be
needed on an input device.
Please refer to following use case:
- there is a video capture hardware and a grabber application
- a face-detection is implemented in capture hardware
- the application obtains the position of the face using extended control
- application would like to grab only the face, avoiding any extra content
- the application configures cropping rectangle with V4L2_SEL_SIZE_GE
flag to assure that no part of the face is lost
In other words, I think we should split the issues with
the crop api into two groups:
1) for input devices
2) for output devices.
No. I strongly prefer to keep consistent API for both capture and output
devices. We should avoid adding extra ioctls and structures.
Moreover, there are mem2mem devices approaching from horizon.
They combine features of both types of queues. It would be much simpler
for developers and application to use the same API.
Are your idea to deprecate the usage of the crop API for both
input and output devices?
I've given up idea of deprecating old crop API. Hopefully, the selection
will substitute the old API in time. For now, I suggest to make the
selection an experimental API until it becomes accepted by at least few
drivers.
Please correct me if I'm wrong, but the problems you've mentioned are all
about (2), right?
The crop API were designed originally for input devices, and is currently
used on several TV and USB webcam devices for input.
Grepping for vidioc_s_crap:
s/crap/crop :)
drivers/media/video/bt8xx/bttv-driver.c
drivers/media/video/et61x251/et61x251_core.c
drivers/media/video/saa7134/saa7134-video.c
drivers/media/video/sn9c102/sn9c102_core.c
drivers/media/video/vino.c
drivers/media/video/zoran/zoran_driver.c
drivers/media/video/cx18/cx18-ioctl.c
Porting them to the selection API means that userspace applications
and kernel drivers will need changes, and this will take a long time
to happen. Ok, this can be done, if there will be large gains on it,
but I fail to see what will be such gains.
No changes to applications have to applied. The old crop ioctl are
simulated by selection API inside V4L2 framework. The simulation is done
only if s_crop or s_cropcap are NULL.
So, please explain us why the above drivers would need to be ported to
the selection API, or why new input drivers/applications would need
the new API instead of the old one.
Some of presented hardware might be capable of composing into memory
buffers. This functionality is not available by the current API.
The support for composing operation may justify porting the drivers to
the selection API.
With respect to (2), the only TV device that (ab)used the crop API to
control its output node is
drivers/media/video/ivtv/ivtv-ioctl.c
To complete the drivers list, currently, the only SoC device currently
implementing vidioc_s_crop for input/output is the davinci driver:
drivers/media/video/davinci/vpif_capture.c
drivers/media/video/davinci/vpif_display.c
IMO, what it seems to be happening with the crop API is similar to what
happened in the past with the control API: the existing API works fine
on simple cases, but fails on more complex scenarios. In the case of
the control API, several controls need to be grouped when selecting an
mpeg compression parameters. So, the VIDIOC_[G|S]_EXT_CTRLS were added
without deprecating the old ioctl's. This way, applications that were
only supporting controls like bright, volume, etc won't need to be changed.
Refer to thread:
http://thread.gmane.org/gmane.linux.drivers.video-input-infrastructure/33799/focus=34727
The concept of the simple pipeline was described there. The selection is
an important part of this proposition. If this concept is accepted then
'simple pipeline' becomes a new V4L2 primitive. All other more complex
cases would be covered by the multimedia device API. The selection is
cropping/composing API dedicated for such a pipelines because old one
was not good enough.
I agree that most of the selection configuration could be moved to
extended control API. By applying the same logic a few further one could
state that the whole configuration (like VIDIOC_S_FMT, VIDIOC_S_STD,
etc) could be moved to the extended controls. Maybe only the memory and
streaming control ioctl would survive. I think that it would be a good
start for V4L3 project.
Internally, it made sense to implement a core way of converting a legacy
ioctl call into the new callbacks for the _EXT_ ioctl's, and to convert
the existing drivers.
Maybe libv4l should deal with support for deprecated APIs.
I could split commit 'v4l: add documentation for selection API' into two commits. One that deprecates S_CROP, and another one that introduces selection.
Actually, It is too soon to deprecate S_CROP while the selection API is tagged
as experimental, but if this is the idea, it is better to add a hint at the V4L2
DocBook.
agreed
6) You should add a note about it at hist-v4l2.html page, stating what happened, and why a new crop
ioctl set is needed.
ok.. I missed it. Sorry.
7) You didn't update the Experimental API Elements or the Obsolete API Elements at the hist-v4l2.html
ok.. I missed it. Sorry.
Thanks,
Mauro
Thank again for your comments.
I hope that my answers will convince you to the selection API.
Best regards,
Tomasz Stanislawski
Regards,
Mauro
Best regards,
Tomasz Stanislawski
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