Re: [PATCH v4 29/36] media: imx: mipi-csi2: enable setting and getting of frame rates

[Steve Longerbeam <slongerbeam@gmail.com>]

On 02/21/2017 02:21 PM, Steve Longerbeam wrote:
>
>
> On 02/21/2017 04:15 AM, Sakari Ailus wrote:
>> Hi Steve,
>>
>> On Mon, Feb 20, 2017 at 02:56:15PM -0800, Steve Longerbeam wrote:
>>>
>>>
>>> On 02/20/2017 02:04 PM, Sakari Ailus wrote:
>>>> Hi Steve,
>>>>
>>>> On Wed, Feb 15, 2017 at 06:19:31PM -0800, Steve Longerbeam wrote:
>>>>> From: Russell King <rmk+kernel@armlinux.org.uk>
>>>>>
>>>>> Setting and getting frame rates is part of the negotiation mechanism
>>>>> between subdevs.  The lack of support means that a frame rate at the
>>>>> sensor can't be negotiated through the subdev path.
>>>>
>>>> Just wondering --- what do you need this for?
>>>
>>>
>>> Hi Sakari,
>>>
>>> i.MX does need the ability to negotiate the frame rates in the
>>> pipelines. The CSI has the ability to skip frames at the output,
>>> which is something Philipp added to the CSI subdev. That affects
>>> frame interval at the CSI output.
>>>
>>> But as Russell pointed out, the lack of [gs]_frame_interval op
>>> causes media-ctl to fail:
>>>
>>> media-ctl -v -d /dev/media1 --set-v4l2
>>> '"imx6-mipi-csi2":1[fmt:SGBRG8/512x512@1/30]'
>>>
>>> Opening media device /dev/media1
>>> Enumerating entities
>>> Found 29 entities
>>> Enumerating pads and links
>>> Setting up format SGBRG8 512x512 on pad imx6-mipi-csi2/1
>>> Format set: SGBRG8 512x512
>>> Setting up frame interval 1/30 on entity imx6-mipi-csi2
>>> Unable to set frame interval: Inappropriate ioctl for device
>>> (-25)Unable to
>>> setup formats: Inappropriate ioctl for device (25)
>>>
>>>
>>> So i.MX needs to implement this op in every subdev in the
>>> pipeline, otherwise it's not possible to configure the
>>> pipeline with media-ctl.
>>
>> The frame rate is only set on the sub-device which you explicitly set it.
>> I.e. setting the frame rate fails if it's not supported on a pad.
>>
>> Philipp recently posted patches that add frame rate propagation to
>> media-ctl.
>>
>> Frame rate is typically settable (and gettable) only on sensor
>> sub-device's
>> source pad,  which means it normally would not be propagated by the
>> kernel
>> but with Philipp's patches, on the sink pad of the bus receiver.
>> Receivers
>> don't have a way to control it nor they implement the IOCTLs, so that
>> would
>> indeed result in an error.
>>
>
> Frame rate is really an essential piece of information. The spatial
> dimensions and data type provided by set_fmt are really only half the
> equation, the other is temporal, i.e. the data rate.
>
> It's true that subdevices have no control over the frame rate at their
> sink pads, but the same argument applies to set_fmt. Even if it has
> no control over the data format it receives, it still needs that
> information in order to determine the correct format at the source.
> The same argument applies to frame rate.
>
> So in my opinion, the behavior of [gs]_frame_interval should be, if a
> subdevice is capable of modifying the frame rate, then it should
> implement [gs]_frame_interval at _all_ of its pads, similar to set_fmt.
> And frame rate should really be part of link validation the same as
> set_fmt is.
>

Actually, if frame rate were added to link validation then
[gs]_frame_interval would have to be mandatory, even if the
subdev has no control over frame rate, again this is like
set_fmt. Otherwise, if a subdev has not implemented
[gs]_frame_interval, then frame rate validation across
the whole pipeline is broken. Because, if we have

A -> B -> C

and B has not implemented [gs]_frame_interval, and C is expecting
30 fps, then pipeline validation would succeed even though A is 
outputting 60 fps.

Steve