Re: [PATCH RFC 7/8] media: qcom: camss: Account for C-PHY when calculating link frequency

[Casey Connolly <casey.connolly@linaro.org>]

Hi David,

On 11/9/25 10:39, David Heidelberg via B4 Relay wrote:
> From: David Heidelberg <david@ixit.cz>
> 
> Ensure that the link frequency divider correctly accounts for C-PHY
> operation. The divider differs between D-PHY and C-PHY, as described
> in the MIPI CSI-2 specification.
> 
> For more details, see:
> https://docs.kernel.org/driver-api/media/tx-rx.html#pixel-rate
> 
> Suggested-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> Signed-off-by: David Heidelberg <david@ixit.cz>
> ---
>   drivers/media/platform/qcom/camss/camss-csid.c   | 2 +-
>   drivers/media/platform/qcom/camss/camss-csiphy.c | 6 ++++--
>   drivers/media/platform/qcom/camss/camss.c        | 7 ++++---
>   drivers/media/platform/qcom/camss/camss.h        | 2 +-
>   4 files changed, 10 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/media/platform/qcom/camss/camss-csid.c b/drivers/media/platform/qcom/camss/camss-csid.c
> index 68adea33cc719..9fb5834b28e2b 100644
> --- a/drivers/media/platform/qcom/camss/camss-csid.c
> +++ b/drivers/media/platform/qcom/camss/camss-csid.c
> @@ -545,7 +545,7 @@ static int csid_set_clock_rates(struct csid_device *csid)
>   	fmt = csid_get_fmt_entry(csid->res->formats->formats, csid->res->formats->nformats,
>   				 csid->fmt[MSM_CSIPHY_PAD_SINK].code);
>   	link_freq = camss_get_link_freq(&csid->subdev.entity, fmt->bpp,
> -					csid->phy.lane_cnt);
> +					csid->phy.lane_cnt, csid->phy.cphy);
>   	if (link_freq < 0)
>   		link_freq = 0;
>   
> diff --git a/drivers/media/platform/qcom/camss/camss-csiphy.c b/drivers/media/platform/qcom/camss/camss-csiphy.c
> index a734fb7dde0a4..61f2b2ac3f159 100644
> --- a/drivers/media/platform/qcom/camss/camss-csiphy.c
> +++ b/drivers/media/platform/qcom/camss/camss-csiphy.c
> @@ -144,8 +144,9 @@ static int csiphy_set_clock_rates(struct csiphy_device *csiphy)
>   	u8 bpp = csiphy_get_bpp(csiphy->res->formats->formats, csiphy->res->formats->nformats,
>   				csiphy->fmt[MSM_CSIPHY_PAD_SINK].code);
>   	u8 num_lanes = csiphy->cfg.csi2->lane_cfg.num_data;
> +	bool cphy = csiphy->cfg.csi2->lane_cfg.cphy;
>   
> -	link_freq = camss_get_link_freq(&csiphy->subdev.entity, bpp, num_lanes);
> +	link_freq = camss_get_link_freq(&csiphy->subdev.entity, bpp, num_lanes, cphy);
>   	if (link_freq < 0)
>   		link_freq  = 0;
>   
> @@ -270,9 +271,10 @@ static int csiphy_stream_on(struct csiphy_device *csiphy)
>   	u8 bpp = csiphy_get_bpp(csiphy->res->formats->formats, csiphy->res->formats->nformats,
>   				csiphy->fmt[MSM_CSIPHY_PAD_SINK].code);
>   	u8 num_lanes = csiphy->cfg.csi2->lane_cfg.num_data;
> +	bool cphy = csiphy->cfg.csi2->lane_cfg.cphy;
>   	u8 val;
>   
> -	link_freq = camss_get_link_freq(&csiphy->subdev.entity, bpp, num_lanes);
> +	link_freq = camss_get_link_freq(&csiphy->subdev.entity, bpp, num_lanes, cphy);
>   
>   	if (link_freq < 0) {
>   		dev_err(csiphy->camss->dev,
> diff --git a/drivers/media/platform/qcom/camss/camss.c b/drivers/media/platform/qcom/camss/camss.c
> index 549780f3f948b..248aa6b21b5ad 100644
> --- a/drivers/media/platform/qcom/camss/camss.c
> +++ b/drivers/media/platform/qcom/camss/camss.c
> @@ -3912,20 +3912,21 @@ struct media_pad *camss_find_sensor_pad(struct media_entity *entity)
>    * camss_get_link_freq - Get link frequency from sensor
>    * @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
> + * @nr_of_lanes: Number of lanes in the link to the sensor

Missing cphy doc comment.>    *
>    * 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)
> +			unsigned int nr_of_lanes, bool cphy)
>   {
>   	struct media_pad *sensor_pad;
> +	unsigned int div = nr_of_lanes * 2 * (cphy ? 7 : 16);

What do the magic numbers 7 and 16 mean? It would be nice to describe these.

Kind regards,
Casey (she/they)

>   
>   	sensor_pad = camss_find_sensor_pad(entity);
>   	if (!sensor_pad)
>   		return -ENODEV;
>   
> -	return v4l2_get_link_freq(sensor_pad, bpp, 2 * lanes);
> +	return v4l2_get_link_freq(sensor_pad, 16 * bpp, div);
>   }
>   
>   /*
> diff --git a/drivers/media/platform/qcom/camss/camss.h b/drivers/media/platform/qcom/camss/camss.h
> index 9d9a62640e25d..0ab908b0c037f 100644
> --- a/drivers/media/platform/qcom/camss/camss.h
> +++ b/drivers/media/platform/qcom/camss/camss.h
> @@ -162,7 +162,7 @@ int camss_enable_clocks(int nclocks, struct camss_clock *clock,
>   void camss_disable_clocks(int nclocks, struct camss_clock *clock);
>   struct media_pad *camss_find_sensor_pad(struct media_entity *entity);
>   s64 camss_get_link_freq(struct media_entity *entity, unsigned int bpp,
> -			unsigned int lanes);
> +			unsigned int lanes, bool cphy);
>   int camss_get_pixel_clock(struct media_entity *entity, u64 *pixel_clock);
>   int camss_pm_domain_on(struct camss *camss, int id);
>   void camss_pm_domain_off(struct camss *camss, int id);
>