Re: [PATCH v2 1/9] media: qcom: camss: Introduce CPU latency QoS during streaming

[Loic Poulain]

On Mon, Nov 2, 2026 at 2:57 PM <bryan.odonoghue@xxxxxxxxxx> wrote:
>
> From: Bryan O'Donoghue <bryan.odonoghue@xxxxxxxxxx>
>
> On a quiescent system - for example the X1 CRD booted to a simple ramdisk
> rootfs during development of V4L2_EVENT_FRAME_SYNC it was observed that
> interrupt for buffer done could be delayed until after the SOF for the next
> frame.
>
> Fiddling about with the interrupt bits, clearing them and changing the
> logic of V4L2_EVENT_FRAME_SYNC to buffer completion didn't work and besides
> "felt wrong".
>
> Tracing it looks like this:
> <idle>-0       [000] d.h1.    36.374380: csid_isr: rdi0 sof=1 done=0
> <idle>-0       [000] d.h1.    36.406601: csid_isr: rdi0 sof=1 done=1
> <idle>-0       [000] d.h1.    36.439910: csid_isr: rdi0 sof=1 done=1
> <idle>-0       [000] d.h1.    36.473245: csid_isr: rdi0 sof=1 done=1
> <idle>-0       [000] d.h1.    36.504786: csid_isr: rdi0 sof=0 done=1
> <idle>-0       [000] d.h1.    36.506350: csid_isr: rdi0 sof=1 done=0
>
> Tracing with /dev/cpu_dma_latency = 0 looks like this:
> <idle>-0       [000] d.h1.   556.149115: csid_isr: rdi0 sof=1 done=0
> <idle>-0       [000] d.h1.   556.181338: csid_isr: rdi0 sof=0 done=1
> <idle>-0       [000] d.h1.   556.182880: csid_isr: rdi0 sof=1 done=0
> <idle>-0       [000] d.h1.   556.214643: csid_isr: rdi0 sof=0 done=1
> <idle>-0       [000] d.h1.   556.216185: csid_isr: rdi0 sof=1 done=0
>
> The second is the pattern you expect CSI SOF followed by data.
>
> We will require SOF to track and understand which data-frame is correlated
> to which camera stastics and parameters frame. On an ordinary system with
> several things pinging off at once, WiFi, display, keyboard you will be
> unlikely to hit deep idle and hence unlikely to see the long exit. However
> we should design for the case where the system is otherwise quiescent with
> only the camera streaming.
>
> Choosing a request of 300us - 1000us seemed to work fine but cutting the
> margin finer, similar to the value on IPU6.
>
> Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@xxxxxxxxxx>

Reviewed-by: Loic Poulain <loic.poulain@xxxxxxxxxxxxxxxx>

> ---
>  drivers/media/platform/qcom/camss/camss.c | 11 ++++++++++-
>  drivers/media/platform/qcom/camss/camss.h |  2 ++
>  2 files changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/media/platform/qcom/camss/camss.c b/drivers/media/platform/qcom/camss/camss.c
> index c54185c93a8a9..52bb19f5fbf3b 100644
> --- a/drivers/media/platform/qcom/camss/camss.c
> +++ b/drivers/media/platform/qcom/camss/camss.c
> @@ -5834,6 +5834,8 @@ static const struct of_device_id camss_dt_match[] = {
>
>  MODULE_DEVICE_TABLE(of, camss_dt_match);
>
> +#define CAMSS_CPU_LATENCY_US   300
> +
>  static int __maybe_unused camss_runtime_suspend(struct device *dev)
>  {
>         struct camss *camss = dev_get_drvdata(dev);
> @@ -5846,6 +5848,8 @@ static int __maybe_unused camss_runtime_suspend(struct device *dev)
>                         return ret;
>         }
>
> +       cpu_latency_qos_remove_request(&camss->cpu_latency);
> +
>  #if IS_ENABLED(CONFIG_PM_CLK)
>         return pm_clk_suspend(dev);
>  #else
> @@ -5866,12 +5870,17 @@ static int __maybe_unused camss_runtime_resume(struct device *dev)
>                 return ret;
>  #endif
>
> +       cpu_latency_qos_add_request(&camss->cpu_latency,
> +                                   CAMSS_CPU_LATENCY_US);
> +
>         for (i = 0; i < camss->res->icc_path_num; i++) {
>                 ret = icc_set_bw(camss->icc_path[i],
>                                  icc_res[i].icc_bw_tbl.avg,
>                                  icc_res[i].icc_bw_tbl.peak);
> -               if (ret)
> +               if (ret) {
> +                       cpu_latency_qos_remove_request(&camss->cpu_latency);
>                         return ret;
> +               }
>         }
>
>         return 0;
> diff --git a/drivers/media/platform/qcom/camss/camss.h b/drivers/media/platform/qcom/camss/camss.h
> index 7dc2fd9f03e0d..7564a7c46fa24 100644
> --- a/drivers/media/platform/qcom/camss/camss.h
> +++ b/drivers/media/platform/qcom/camss/camss.h
> @@ -11,6 +11,7 @@
>  #define QC_MSM_CAMSS_H
>
>  #include <linux/device.h>
> +#include <linux/pm_qos.h>
>  #include <linux/types.h>
>  #include <media/v4l2-async.h>
>  #include <media/v4l2-device.h>
> @@ -141,6 +142,7 @@ struct camss {
>         struct icc_path *icc_path[ICC_SM8250_COUNT];
>         const struct camss_resources *res;
>         bool legacy_phy;
> +       struct pm_qos_request cpu_latency;
>  };
>
>  struct camss_camera_interface {
>
> --
> 2.56.0
>