Re: [PATCH v3 4/5] media: qcom: camss: Take the link frequency from the CSI-2 transmitter

[Loic Poulain]

On Mon, Sep 28, 2026 at 8:44 AM Hitesh Patel <hitesh@xxxxxxxxxxxxxx> wrote:
>
> camss_get_link_freq() walks the pipeline up to an entity with the
> MEDIA_ENT_F_CAM_SENSOR function and reads the link frequency there.
> The CSIPHY settle count and the CSID clock are then derived from it.
>
> The frequency the receiver has to be programmed for is the one on the
> CSI-2 bus, which belongs to whatever drives that bus. When the sensor
> is wired straight to the CSIPHY that is the sensor, and the walk gives
> the right answer. When a CSI-2 to CSI-2 bridge sits in between, such
> as a GMSL or FPD-Link deserializer, the bridge re-times the data onto
> its own output at its own rate: it may aggregate several sensors onto
> one link, forward one sensor at a different rate, or generate a test
> pattern with no sensor at all. The sensor's rate is then not what
> arrives at the SoC, and the CSIPHY does not lock.
>
> The walk can also fail before reaching a sensor. A deserializer has
> one sink pad per serial link and the walk always follows pad 0; a
> sensor attached to any other link is never found and streaming is
> refused with "Cannot get CSI2 transmitter's link frequency".
>
> Stop the walk at the first entity that is not a CAMSS receiver, i.e.
> at the external subdev feeding the CSIPHY, and ask that pad with
> v4l2_get_link_freq(). The helper queries the transmitter through
> .get_mbus_config first and falls back to its V4L2_CID_LINK_FREQ and
> V4L2_CID_PIXEL_RATE controls, so a bridge and a bare sensor are both
> handled by the standard mechanism.
>
> For a sensor connected directly to a CSIPHY the transmitter is the
> sensor, so the pad found and the value returned do not change.
> camss_find_sensor_pad() is kept for camss_get_pixel_clock() and the
> frame skip query, which do want the sensor.
>
> Signed-off-by: Hitesh Patel <hitesh@xxxxxxxxxxxxxx>
> ---
>  drivers/media/platform/qcom/camss/camss.c | 70 +++++++++++++++++++++--
>  1 file changed, 64 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/media/platform/qcom/camss/camss.c b/drivers/media/platform/qcom/camss/camss.c
> index 23f3cc30a1..327ac7b280 100644
> --- a/drivers/media/platform/qcom/camss/camss.c
> +++ b/drivers/media/platform/qcom/camss/camss.c
> @@ -4619,24 +4619,82 @@ struct media_pad *camss_find_sensor_pad(struct media_entity *entity)
>         }
>  }
>
> +/*
> + * camss_is_receiver_subdev - Test whether a subdev is a CAMSS CSI-2 receiver
> + * @camss: CAMSS device
> + * @sd: Subdevice to test
> + *
> + * Return true for a CSIPHY or CSID belonging to @camss, false for anything
> + * else, in particular for the external subdev transmitting to them.
> + */
> +static bool camss_is_receiver_subdev(struct camss *camss,
> +                                    struct v4l2_subdev *sd)
> +{
> +       unsigned int i;
> +
> +       for (i = 0; i < camss->res->csiphy_num; i++)
> +               if (sd == &camss->csiphy[i].subdev)
> +                       return true;
> +
> +       for (i = 0; i < camss->res->csid_num; i++)
> +               if (sd == &camss->csid[i].subdev)
> +                       return true;
> +
> +       return false;
> +}
> +
> +/*
> + * camss_find_transmitter_pad - Find the pad of the CSI-2 transmitter
> + * @entity: Media entity in the current pipeline
> + *
> + * Walk the pipeline upstream through the CAMSS receiver subdevs and return the
> + * source pad of the first entity that is not one of them: the CSI-2
> + * transmitter driving the SoC.
> + *
> + * Return a pointer to the transmitter media pad or NULL if not found
> + */
> +static struct media_pad *camss_find_transmitter_pad(struct media_entity *entity)
> +{
> +       struct camss *camss = container_of(entity->graph_obj.mdev,
> +                                          struct camss, media_dev);
> +       struct v4l2_subdev *sd;
> +       struct media_pad *pad;
> +
> +       while (1) {
> +               pad = &entity->pads[0];
> +               if (!(pad->flags & MEDIA_PAD_FL_SINK))
> +                       return NULL;
> +
> +               pad = media_pad_remote_pad_first(pad);
> +               if (!pad || !is_media_entity_v4l2_subdev(pad->entity))
> +                       return NULL;
> +
> +               entity = pad->entity;
> +               sd = media_entity_to_v4l2_subdev(entity);
> +
> +               if (!camss_is_receiver_subdev(camss, sd))
> +                       return pad;
> +       }
> +}
> +
>  /**
> - * camss_get_link_freq - Get link frequency from sensor
> + * camss_get_link_freq - Get link frequency from the CSI-2 transmitter
>   * @entity: Media entity in the current pipeline
>   * @bpp: Number of bits per pixel for the current format
> - * @lanes: Number of lanes in the link to the sensor
> + * @lanes: Number of lanes in the link to the transmitter
>   *
>   * Return link frequency on success or a negative error code otherwise
>   */
>  s64 camss_get_link_freq(struct media_entity *entity, unsigned int bpp,
>                         unsigned int lanes)
>  {
> -       struct media_pad *sensor_pad;
> +       struct media_pad *tx_pad;
>
> -       sensor_pad = camss_find_sensor_pad(entity);
> -       if (!sensor_pad)
> +       tx_pad = camss_find_transmitter_pad(entity);
> +       if (!tx_pad)
>                 return -ENODEV;
>
> -       return v4l2_get_link_freq(sensor_pad, bpp, 2 * lanes);
> +       return v4l2_get_link_freq(tx_pad, bpp, 2 * lanes);
>  }
>
>  /*
> --

I think we could even simply replacing/refactoring
camss_find_sensor_pad() with camss_find_transmitter_pad()?