Re: [PATCH v4 24/29] media: v4l2-subdev: Introduce v4l2_subdev_get_frame_desc()

Sakari Ailus
2026-04-21T12:18:35Z

Hi Laurent,

On Thu, Apr 16, 2026 at 07:16:54PM +0300, Laurent Pinchart wrote:
> On Mon, Apr 13, 2026 at 11:07:07AM +0300, Sakari Ailus wrote:
> > On Fri, Apr 10, 2026 at 12:53:42PM +0200, Jacopo Mondi wrote:
> > > On Wed, Apr 08, 2026 at 06:39:33PM +0300, Sakari Ailus wrote:
> > > > Introduce v4l2_subdev_get_frame_desc() in order to facilitate implementing
> > > > drivers that need frame descriptors. If the remote sub-device does not
> > > > support frame descriptors, v4l2_subdev_get_frame_desc() creates one (with
> > > > a single entry) opportunistically, thus avoiding the need to add frame
> > > > descriptor support to sensor drivers the device for which only generates a
>
> s/the device for which/whose device/

I guess that'd work, too.

>
> I think all sensor drivers should implement .get_frame_desc(), possibly
> with a helper.

Is there a need to? It's largely just extra churn for more than 80 drivers
-- this can also be extended to cover routes so few of these drivers would
in the end need anything driver-specific. But the option for that needs to
remain.

>
> > > > single stream, or managing the situation on the caller side.
>
> That part I agree with, we currently need to manage the situation on the
> caller side because not all subdevs implement the operation. That will
> still be the case for a while, so this helper is useful.
>
> Can you also replace manual calls to .get_frame_desc() with the helper ?
> Some of the existing callers implement fallbacks, I fonud them at least
> in
>
> drivers/media/platform/broadcom/bcm2835-unicam.c
> drivers/media/platform/raspberrypi/rp1-cfe/cfe.c
> drivers/media/platform/renesas/rzg2l-cru/rzg2l-video.c
> drivers/media/platform/ti/cal/cal.c
>
> (directly in the function that calls .get_frame_desc(), or in its
> caller)
>
> Replacing those will ensure the helper works as expected.

Sure. I'll add more patches for that.

>
> > > > Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> > > > ---
> > > > drivers/media/v4l2-core/v4l2-subdev.c | 96 +++++++++++++++++++++++++++
> > > > include/media/v4l2-subdev.h | 20 ++++++
> > > > 2 files changed, 116 insertions(+)
> > > >
> > > > diff --git a/drivers/media/v4l2-core/v4l2-subdev.c b/drivers/media/v4l2-core/v4l2-subdev.c
> > > > index 647587c0499a..40b28e070726 100644
> > > > --- a/drivers/media/v4l2-core/v4l2-subdev.c
> > > > +++ b/drivers/media/v4l2-core/v4l2-subdev.c
> > > > @@ -20,6 +20,7 @@
> > > > #include <linux/version.h>
> > > > #include <linux/videodev2.h>
> > > >
> > > > +#include <media/mipi-csi2.h>
> > > > #include <media/v4l2-ctrls.h>
> > > > #include <media/v4l2-device.h>
> > > > #include <media/v4l2-event.h>
> > > > @@ -2758,3 +2759,98 @@ void v4l2_subdev_put_privacy_led(struct v4l2_subdev *sd)
> > > > #endif
> > > > }
> > > > EXPORT_SYMBOL_GPL(v4l2_subdev_put_privacy_led);
> > > > +
> > > > +static int get_mipi_dt_for_mbus(u32 code)
>
> v4l2_subdev_get_mipi_dt_for_mbus()
>
> Or move the function to v4l2-common.c, name it mipi_csi2_dt_for_mbus()
> and declare it in include/media/mipi-csi2.h.

How about calling it mipi_csi2_dt_for_mbus()? I think it can be located in
v4l2-common.c for now, no need to add a separate file just for this; all
users would effectively be related to V4L2 anyway.

>
> > > > +{
> > > > + switch (code) {
> > > > + case MEDIA_BUS_FMT_BGR888_1X24:
> > > > + return MIPI_CSI2_DT_RGB888;
> > > > + case MEDIA_BUS_FMT_Y8_1X8:
> > > > + case MEDIA_BUS_FMT_SBGGR8_1X8:
> > > > + case MEDIA_BUS_FMT_SGBRG8_1X8:
> > > > + case MEDIA_BUS_FMT_SGRBG8_1X8:
> > > > + case MEDIA_BUS_FMT_SRGGB8_1X8:
> > > > + return MIPI_CSI2_DT_RAW8;
> > > > + case MEDIA_BUS_FMT_Y10_1X10:
> > > > + case MEDIA_BUS_FMT_SBGGR10_1X10:
> > > > + case MEDIA_BUS_FMT_SGBRG10_1X10:
> > > > + case MEDIA_BUS_FMT_SGRBG10_1X10:
> > > > + case MEDIA_BUS_FMT_SRGGB10_1X10:
> > > > + return MIPI_CSI2_DT_RAW10;
> > > > + case MEDIA_BUS_FMT_Y12_1X12:
> > > > + case MEDIA_BUS_FMT_SBGGR12_1X12:
> > > > + case MEDIA_BUS_FMT_SGBRG12_1X12:
> > > > + case MEDIA_BUS_FMT_SGRBG12_1X12:
> > > > + case MEDIA_BUS_FMT_SRGGB12_1X12:
> > > > + return MIPI_CSI2_DT_RAW12;
> > > > + case MEDIA_BUS_FMT_Y14_1X14:
> > > > + case MEDIA_BUS_FMT_SBGGR14_1X14:
> > > > + case MEDIA_BUS_FMT_SGBRG14_1X14:
> > > > + case MEDIA_BUS_FMT_SGRBG14_1X14:
> > > > + case MEDIA_BUS_FMT_SRGGB14_1X14:
> > > > + return MIPI_CSI2_DT_RAW14;
> > > > + case MEDIA_BUS_FMT_Y16_1X16:
> > > > + case MEDIA_BUS_FMT_SBGGR16_1X16:
> > > > + case MEDIA_BUS_FMT_SGBRG16_1X16:
> > > > + case MEDIA_BUS_FMT_SGRBG16_1X16:
> > > > + case MEDIA_BUS_FMT_SRGGB16_1X16:
> > > > + return MIPI_CSI2_DT_RAW16;
> > > > + case MEDIA_BUS_FMT_SBGGR20_1X20:
> > > > + case MEDIA_BUS_FMT_SGBRG20_1X20:
> > > > + case MEDIA_BUS_FMT_SGRBG20_1X20:
> > > > + case MEDIA_BUS_FMT_SRGGB20_1X20:
> > > > + return MIPI_CSI2_DT_RAW20;
> > > > + default:
> > > > + return -EINVAL;
> > > > + }
> > > > +}
> > > > +
> > > > +int v4l2_subdev_get_frame_desc(struct v4l2_subdev *sd, unsigned int pad,
> > > > + struct v4l2_mbus_frame_desc *desc)
> > > > +{
> > > > + struct v4l2_subdev_format subdev_fmt = {
> > > > + .which = V4L2_SUBDEV_FORMAT_ACTIVE,
> > > > + .pad = pad,
> > > > + };
> > > > + int ret;
> > > > +
> > > > + if (v4l2_subdev_has_op(sd, pad, get_frame_desc)) {
> > > > + unsigned int type = desc->type;
> > > > +
> > > > + ret = v4l2_subdev_call(sd, pad, get_frame_desc, pad, desc);
> > > > +
> > > > + if (desc->type != type)
> > > > + return -EINVAL;
>
> I'd add a dev_err() here. There are .get_frame_desc() callers that check
> if the returned type matches what they expect and log an error
> otherwise. When using this helper the check can't be performed in the
> callera any more, leading to possibly hard to debug issues if no message
> is printed.

dev_err_once()?

>
> > > > +
> > > > + return ret;
> > > > + }
> > > > +
> > > > + if (desc->type != V4L2_MBUS_FRAME_DESC_TYPE_PARALLEL &&
> > > > + desc->type != V4L2_MBUS_FRAME_DESC_TYPE_CSI2)
> > > > + return -EINVAL;
> > > > +
> > > > + ret = v4l2_subdev_call(sd, pad, get_fmt, NULL, NULL,
> > > > + &subdev_fmt);
> > > > + if (ret)
> > > > + return ret;
> > > > +
> > > > + struct v4l2_mbus_frame_desc_entry entry = {
> > > > + .pixelcode = subdev_fmt.format.code,
> > > > + };
> > > > +
> > > > + if (desc->type == V4L2_MBUS_FRAME_DESC_TYPE_CSI2) {
> > > > + int dt;
> > > > +
> > > > + dt = get_mipi_dt_for_mbus(subdev_fmt.format.code);
> > > > + if (dt < 0)
> > > > + return dt;
> > > > +
> > > > + entry.bus.csi2.dt = dt;
> > > > + }
> > > > +
> > > > + desc->entry[0] = entry;
> > > > + desc->num_entries = 1;
> > > > +
> > > > + return 0;
> > > > +}
> > > > +EXPORT_SYMBOL_GPL(v4l2_subdev_get_frame_desc);
> > > > diff --git a/include/media/v4l2-subdev.h b/include/media/v4l2-subdev.h
> > > > index 4588992b4417..93b672edd08e 100644
> > > > --- a/include/media/v4l2-subdev.h
> > > > +++ b/include/media/v4l2-subdev.h
> > > > @@ -2058,4 +2058,24 @@ void v4l2_subdev_notify_event(struct v4l2_subdev *sd,
> > > > */
> > > > bool v4l2_subdev_is_streaming(struct v4l2_subdev *sd);
> > > >
> > > > +/**
> > > > + * v4l2_subdev_get_frame_desc() - Get a pad's frame descriptor
> > > > + * @sd: The sub-device
> > > > + * @pad: The number of the pad in @sd from which to obtain the frame descriptor
> > > > + * @desc: A pointer to a frame descriptor, with its type field set
> > > > + *
> > > > + * Obtain a frame descriptor from a sub-device. If the sub-device supports the
> > > > + * get_frame_desc pad operation, its result is returned, just like calling it
> > > > + * directly using v4l2_subdev_call(). If the sub-device driver does not support
> > > > + * it, then one containing a single entry is created using the information from
>
> s/one /a frame descriptor/

Yes.

>
> > > > + * the sub-device active state, which this function locks for the duration of
> > > > + * the call to obtain it.
> > >
> > > This doesn't seem to apply anymore...
> >
> > I'll rephrase it.
> >
> > > > + *
> > > > + * The caller is required to set @desc->type to the expected bus type.
> > >
> > > Is it worth mentioning that only V4L2_MBUS_FRAME_DESC_TYPE_PARALLEL
> > > and V4L2_MBUS_FRAME_DESC_TYPE_CSI2 are supported ?
> >
> > I'll add that.
> >
> > > > + *
> > > > + * Return: %0 on success or negative error code on failure.
> > > > + */
> > > > +int v4l2_subdev_get_frame_desc(struct v4l2_subdev *sd, unsigned int pad,
> > > > + struct v4l2_mbus_frame_desc *desc);
> > >
> > > With the documentation addressed
> > > Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
>

--
Kind regards,

Sakari Ailus