Re: [PATCH 20/25] media: ov5647: Program mode only if it has changed

[Jacopo Mondi <jacopo@jmondi.org>]

Hello,

On Wed, Jul 01, 2020 at 10:25:54AM +0300, Laurent Pinchart wrote:
> Hi Dafna,
>
> On Tue, Jun 30, 2020 at 03:01:21PM +0200, Dafna Hirschfeld wrote:
> > On 30.06.20 14:05, Jacopo Mondi wrote:
> > > On Tue, Jun 30, 2020 at 12:56:44PM +0200, Dafna Hirschfeld wrote:
> > >> On 30.06.20 12:06, Jacopo Mondi wrote:
> > >>> On Tue, Jun 30, 2020 at 11:32:12AM +0200, Dafna Hirschfeld wrote:
> > >>>> On 30.06.20 09:43, Jacopo Mondi wrote:
> > >>>>> On Mon, Jun 29, 2020 at 07:48:16PM +0200, Dafna Hirschfeld wrote:
> > >>>>>> On 23.06.20 18:55, Jacopo Mondi wrote:
> > >>>>>>> Store in the driver structure a pointer to the currently applied mode
> > >>>>>>> and program a new one at s_stream(1) time only if it has changed.
> > >>>>>>
> > >>>>>> Hi,
> > >>>>>> I think this can be more readably implemented with a 'is_streaming' boolean
> > >>>>>> field.
> > >>>>>
> > >>>>> How would you like to use an 'is_streaming' flag to decide if the
> > >>>>> sensor mode has to be updated ?
> > >>>>
> > >>>> since 'current_mode' is set to NULL upon `ov5647_stream_off`
> > >>>> and you return from 'ov5647_set_stream' immediately if 'mode == current_mode'
> > >>>> it seem very similar to returning immediately from 'ov5647_set_stream' if the
> > >>>> device is currently streaming.
> > >>>
> > >>> No, the code returns immediately from ov5647_set_mode() if mode ==
> > >>> current_mode. The modes comparison makes sure the sensor is not
> > >>> reprogrammed if the mode hasn't changed. The remaning part of
> > >>> s_stream() is executed regardless of the mode configuration. Am I
> > >>> missing some part of the picture ?
> > >>>
> > >>>> But actually in this patch it seems to be possible to change the mode
> > >>>> while streaming, if the callbacks are executed:
> > >>>>
> > >>>> s_stream(1)
> > >>>> s_fmt
> > >>>> s_stream(1)
> > >>>>
> > >>>> which is maybe a bug?
> > >>>
> > >>> The new format is stored in sensor->mode, and applied at the next
> > >>> s_stream(1) operation if it differs from the already programmed one,
> > >>> at least, this is how it is intended to work, have you found any
> > >>> failing s_stream/set_fmt/s_stream which could be caused by a bug ?
> > >>
> > >> What I meant is that there could be valid sequence of calls
> > >>
> > >> s_stream(enable=1)
> > >> s_fmt
> > >> s_stream(enable=1)
> > >>
> > >> For example if two video devices are connected to the sensor and they
> > >> stream simultaneously. There was a discussion about adding a code to the core
> > >
> > > I'm not sure it is possible, given that the subdev has a single source
> > > pad
> >
> > Video devices should not be connected directly to the sensor, they can also
> > be connected to the sensor through an isp entity that is connected to the sensor
> > from one side and to two video devices from the other side.
>
> I don't think it should be the job of the sensor driver to handle this.
> A sensor can be streaming or not streaming, and a .s_stream(1) call
> while already streaming shouldn't happen. It's the job of the ISP driver
> (with help from core code) to ensure this won't happen. Otherwise we
> would have to protect against that case in all sensor drivers,
> duplicating code in many places and opening the door to bugs. Subdev
> drivers should be as simple as possible.
>

Most of the sensor driver I've briefly looked at implement a simple
check to avoid double stream(1) but they do not implement any form of
refcounting. I think that does more harm than good to be honest, as it
would hide  potential problematic start stream sequences, but would
stop the sensor at the first stream(0), leaving one of the multiple
receivers stuck.

I would prefer avoid doing this here.

However the driver already refcounting on s_power(), which if I'm not
mistaken could be avoided, as bridges should use
v4l2_pipeline_pm_get(), which already does refcounting, if I'm not
mistaken.

To me, grasping how s_stream() and s_start() work for real is still hard,
as those are the only two operation propagated along the pipeline by
briges, even for MC platforms, and it seems looking at the existing
driver, the confusion is big, as all of them handle things slightly
differently :/


> > >> to follow the s_stream callback and call it only if the subdev is not streaming
> > >> but currently subdevs should support it themselves.
> > >
> > > Oh, so you're concerned about the fact userspace can call s_stream(1)
> > > twice consecutively! it's indipendent from s_ftm if I got your point.
> > >
> > > In this case a flag to control if the device is streaming already should
> > > help, yes, I overlooked that indeed.
> > >
> > >>>>>>> Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
> > >>>>>>> ---
> > >>>>>>>      drivers/media/i2c/ov5647.c | 16 +++++++++++++++-
> > >>>>>>>      1 file changed, 15 insertions(+), 1 deletion(-)
> > >>>>>>>
> > >>>>>>> diff --git a/drivers/media/i2c/ov5647.c b/drivers/media/i2c/ov5647.c
> > >>>>>>> index 39e320f321bd8..ac114269e1c73 100644
> > >>>>>>> --- a/drivers/media/i2c/ov5647.c
> > >>>>>>> +++ b/drivers/media/i2c/ov5647.c
> > >>>>>>> @@ -96,6 +96,7 @@ struct ov5647 {
> > >>>>>>>      	bool				clock_ncont;
> > >>>>>>>      	struct v4l2_ctrl_handler	ctrls;
> > >>>>>>>      	struct ov5647_mode		*mode;
> > >>>>>>> +	struct ov5647_mode		*current_mode;
> > >>>>>>>      };
> > >>>>>>>      static inline struct ov5647 *to_sensor(struct v4l2_subdev *sd)
> > >>>>>>> @@ -750,9 +751,13 @@ static int ov5647_set_virtual_channel(struct v4l2_subdev *sd, int channel)
> > >>>>>>>      static int ov5647_set_mode(struct v4l2_subdev *sd)
> > >>>>>>>      {
> > >>>>>>>      	struct i2c_client *client = v4l2_get_subdevdata(sd);
> > >>>>>>> +	struct ov5647 *sensor = to_sensor(sd);
> > >>>>>>>      	u8 resetval, rdval;
> > >>>>>>>      	int ret;
> > >>>>>>> +	if (sensor->mode == sensor->current_mode)
> > >>>>>>> +		return 0;
> > >>>>>>> +
> > >>>>>>>      	ret = ov5647_read(sd, OV5647_SW_STANDBY, &rdval);
> > >>>>>>>      	if (ret < 0)
> > >>>>>>>      		return ret;
> > >>>>>>> @@ -778,6 +783,8 @@ static int ov5647_set_mode(struct v4l2_subdev *sd)
> > >>>>>>>      			return ret;
> > >>>>>>>      	}
> > >>>>>>> +	sensor->current_mode = sensor->mode;
> > >>>>>>> +
> > >>>>>>>      	return 0;
> > >>>>>>>      }
> > >>>>>>> @@ -816,6 +823,7 @@ static int ov5647_stream_on(struct v4l2_subdev *sd)
> > >>>>>>>      static int ov5647_stream_off(struct v4l2_subdev *sd)
> > >>>>>>>      {
> > >>>>>>> +	struct ov5647 *sensor = to_sensor(sd);
> > >>>>>>>      	int ret;
> > >>>>>>>      	ret = ov5647_write(sd, OV5647_REG_MIPI_CTRL00, MIPI_CTRL00_CLOCK_LANE_GATE |
> > >>>>>>> @@ -827,7 +835,13 @@ static int ov5647_stream_off(struct v4l2_subdev *sd)
> > >>>>>>>      	if (ret < 0)
> > >>>>>>>      		return ret;
> > >>>>>>> -	return ov5647_write(sd, OV5640_REG_PAD_OUT, 0x01);
> > >>>>>>> +	ret = ov5647_write(sd, OV5640_REG_PAD_OUT, 0x01);
> > >>>>>>> +	if (ret < 0)
> > >>>>>>> +		return ret;
> > >>>>>>> +
> > >>>>>>> +	sensor->current_mode = NULL;
> > >>>>>>> +
> > >>>>>>> +	return 0;
> > >>>>>>>      }
> > >>>>>>>      static int set_sw_standby(struct v4l2_subdev *sd, bool standby)
> > >>>>>>>
>
> --
> Regards,
>
> Laurent Pinchart