Hi Andrey,
Thanks for your patch.
On 2021-03-03 21:08:14 +0300, Andrey Konovalov wrote:
> To get the link frequency value, or to calculate a parameter depending on
> it the receiver driver should use V4L2_CID_LINK_FREQ. If V4L2_CID_LINK_FREQ
> control is not implemented in the remote subdevice, the link frequency
> can be calculated from V4L2_CID_PIXEL_RATE control value. But the latter
> may not give the correct link frequency, and should only be used as the
> last resort. v4l2_get_link_freq() does exactly that, so use it instead
> of reading V4L2_CID_PIXEL_RATE directly.
I like the direction this patch is taking, but I'm a bit concerned about
that V4L2_CID_LINK_FREQ is not able to replace V4L2_CID_PIXEL_RATE as it
is designed today. Maybe my concern is unfounded and only reflects my
own misunderstanding of the API.
When I wrote this code I tried to first do it using V4L2_CID_LINK_FREQ
but I found no way to be able to express the wide rang of values needed
for my use-case given that V4L2_CID_LINK_FREQ is a menu control. I had
to use V4L2_CID_PIXEL_RATE as it allowed me to at runtime calculate and
report the link speed based on input formats. The Use-cases I need to
address are where CSI-2 transmitter themself are a bridge in the video
pipeline, for example
* Case 1 - HDMI video source
HDMI source -> ADV748x (HDMI-to-CSI-2) ->[CSI-2 bus]-> R-Car CSI-2 receiver
The R-Car CSI-2 receiver needs to know the CSI-2 link frequency and
queries the ADV748x using V4L2_CID_PIXEL_RATE. The ADV748x reports the
pixel rate based on the HDMI format detected on its sink pad.
This could be done using V4L2_CID_LINK_FREQ, but as it's a menu control
it becomes rather tricky to populate it with all possible values, but I
guess it could be doable?
* Case 2 - Multiple video streams over a CSI-2 bus (GMSL)
Camera 1 -|
Camera 2 -|
Camera 3 -|---> MAX9286 (GMSL-to CSI-2) ->[CSI-2 bus]-> R-Car CSI-2 receiver
Camera 4 -|
The MAX9286 has 4 sink pads each connected to an independent camera and
a single CSI-2 source pad. When streaming starts the MAX9286 computes
the total CSI-2 link speed as V4L2_CID_PIXEL_RATE based on the format on
each of it's 4 sink pads.
As in case 1 this could be reported by V4L2_CID_LINK_FREQ but I don't
see it as feasible to populate the menu control with all possible
frequencies before hand.
Hopefully this is all easily solvable and I have only misunderstood how
menu controls work. If not I think this needs to be considered as part
of this series as otherwise it could leave the CSI-2 bridge drivers
without a path forward.
>
> Signed-off-by: Andrey Konovalov <andrey.konovalov@xxxxxxxxxx>
I tested this and it works as expected. Also as expected it prints lots
of warnings about the usage of V4L2_CID_PIXEL_RATE :-) Once I understand
how I can fix the CSI-2 transmitters used as bridges in the R-Car boards
I will be happy to add my tag to this series as well as fix the bridge
drivers.
> ---
> drivers/media/platform/rcar-vin/rcar-csi2.c | 18 +++++++-----------
> 1 file changed, 7 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/media/platform/rcar-vin/rcar-csi2.c b/drivers/media/platform/rcar-vin/rcar-csi2.c
> index e06cd512aba2..eec8f9dd9060 100644
> --- a/drivers/media/platform/rcar-vin/rcar-csi2.c
> +++ b/drivers/media/platform/rcar-vin/rcar-csi2.c
> @@ -455,29 +455,25 @@ static int rcsi2_calc_mbps(struct rcar_csi2 *priv, unsigned int bpp,
> unsigned int lanes)
> {
> struct v4l2_subdev *source;
> - struct v4l2_ctrl *ctrl;
> - u64 mbps;
> + s64 mbps;
>
> if (!priv->remote)
> return -ENODEV;
>
> source = priv->remote;
>
> - /* Read the pixel rate control from remote. */
> - ctrl = v4l2_ctrl_find(source->ctrl_handler, V4L2_CID_PIXEL_RATE);
> - if (!ctrl) {
> - dev_err(priv->dev, "no pixel rate control in subdev %s\n",
> + /* Read the link frequency from the remote subdev. */
> + mbps = v4l2_get_link_freq(source->ctrl_handler, bpp, 2 * lanes);
> + if (mbps < 0) {
> + dev_err(priv->dev, "failed to get link rate from subdev %s\n",
> source->name);
> - return -EINVAL;
> + return mbps;
> }
> -
> /*
> * Calculate the phypll in mbps.
> - * link_freq = (pixel_rate * bits_per_sample) / (2 * nr_of_lanes)
> * bps = link_freq * 2
> */
> - mbps = v4l2_ctrl_g_ctrl_int64(ctrl) * bpp;
> - do_div(mbps, lanes * 1000000);
> + do_div(mbps, 1000000 / 2);
>
> return mbps;
> }
> --
> 2.17.1
>
--
Regards,
Niklas Söderlund
