drm/msm/mdss: Add a TODO for better managing the MDSS clock power state

[PATCH] drm/msm/mdss: Add a TODO for better managing the MDSS clock power state

Posted by Konrad Dybcio 4 weeks, 1 day ago

From: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>

There's a small window where the MDP clock could be set to a high rate
(say, from the bootloader) without a corresponding RPM(H)PD vote to
back it up. This is normally not an issue, but could be, if rmmod fails
to shut down the display driver cleanly, and the module is inserted
again, or when the providers' .sync_state has timed out.

Mark a TODO to fix it one day. Linking the relevant discussion below.

Link: https://lore.kernel.org/linux-arm-msm/d5c4eed5-bd87-4156-b178-2d78140ec8a9@oss.qualcomm.com/
Signed-off-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
---
 drivers/gpu/drm/msm/msm_mdss.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/gpu/drm/msm/msm_mdss.c b/drivers/gpu/drm/msm/msm_mdss.c
index 9047e8d9ee89..b783dfec83b8 100644
--- a/drivers/gpu/drm/msm/msm_mdss.c
+++ b/drivers/gpu/drm/msm/msm_mdss.c
@@ -262,6 +262,14 @@ static int msm_mdss_enable(struct msm_mdss *msm_mdss)
 	icc_set_bw(msm_mdss->reg_bus_path, 0,
 		   msm_mdss->reg_bus_bw);
 
+	/*
+	 * TODO:
+	 * Previous users (e.g. the bootloader) may have left this clock at a high rate, which
+	 * would remain set, as prepare_enable() doesn't reprogram it. This theoretically poses a
+	 * risk of brownout, but realistically this path is almost exclusively excercised after the
+	 * correct OPP has been set in one of the MDPn or DPU drivers, or during initial probe,
+	 * before the RPM(H)PD sync_state is done.
+	 */
 	ret = clk_bulk_prepare_enable(msm_mdss->num_clocks, msm_mdss->clocks);
 	if (ret) {
 		dev_err(msm_mdss->dev, "clock enable failed, ret:%d\n", ret);

---
base-commit: fc7b1a72c6cd5cbbd989c6c32a6486e3e4e3594d
change-id: 20260310-topic-mdss_power_todo-4a19cf5f5183

Best regards,
-- 
Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>

Re: [PATCH] drm/msm/mdss: Add a TODO for better managing the MDSS clock power state

Posted by Dmitry Baryshkov 1 week, 5 days ago

On Tue, 10 Mar 2026 14:20:25 +0100, Konrad Dybcio wrote:
> There's a small window where the MDP clock could be set to a high rate
> (say, from the bootloader) without a corresponding RPM(H)PD vote to
> back it up. This is normally not an issue, but could be, if rmmod fails
> to shut down the display driver cleanly, and the module is inserted
> again, or when the providers' .sync_state has timed out.
> 
> Mark a TODO to fix it one day. Linking the relevant discussion below.
> 
> [...]

Applied to msm-next, thanks!

[1/1] drm/msm/mdss: Add a TODO for better managing the MDSS clock power state
      https://gitlab.freedesktop.org/lumag/msm/-/commit/d19faa0dcc6a

Best regards,
-- 
With best wishes
Dmitry

Re: [PATCH] drm/msm/mdss: Add a TODO for better managing the MDSS clock power state

Posted by Dmitry Baryshkov 4 weeks, 1 day ago

On Tue, Mar 10, 2026 at 02:20:25PM +0100, Konrad Dybcio wrote:
> From: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
> 
> There's a small window where the MDP clock could be set to a high rate
> (say, from the bootloader) without a corresponding RPM(H)PD vote to
> back it up. This is normally not an issue, but could be, if rmmod fails
> to shut down the display driver cleanly, and the module is inserted
> again, or when the providers' .sync_state has timed out.
> 
> Mark a TODO to fix it one day. Linking the relevant discussion below.
> 
> Link: https://lore.kernel.org/linux-arm-msm/d5c4eed5-bd87-4156-b178-2d78140ec8a9@oss.qualcomm.com/
> Signed-off-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
> ---
>  drivers/gpu/drm/msm/msm_mdss.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/gpu/drm/msm/msm_mdss.c b/drivers/gpu/drm/msm/msm_mdss.c
> index 9047e8d9ee89..b783dfec83b8 100644
> --- a/drivers/gpu/drm/msm/msm_mdss.c
> +++ b/drivers/gpu/drm/msm/msm_mdss.c
> @@ -262,6 +262,14 @@ static int msm_mdss_enable(struct msm_mdss *msm_mdss)
>  	icc_set_bw(msm_mdss->reg_bus_path, 0,
>  		   msm_mdss->reg_bus_bw);
>  
> +	/*
> +	 * TODO:
> +	 * Previous users (e.g. the bootloader) may have left this clock at a high rate, which
> +	 * would remain set, as prepare_enable() doesn't reprogram it. This theoretically poses a
> +	 * risk of brownout, but realistically this path is almost exclusively excercised after the
> +	 * correct OPP has been set in one of the MDPn or DPU drivers, or during initial probe,
> +	 * before the RPM(H)PD sync_state is done.
> +	 */

I'd have preferred if it was not exercising 100-char limit, but there is
no reason to enforce that.


Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>

>  	ret = clk_bulk_prepare_enable(msm_mdss->num_clocks, msm_mdss->clocks);
>  	if (ret) {
>  		dev_err(msm_mdss->dev, "clock enable failed, ret:%d\n", ret);
> 
> ---
> base-commit: fc7b1a72c6cd5cbbd989c6c32a6486e3e4e3594d
> change-id: 20260310-topic-mdss_power_todo-4a19cf5f5183
> 
> Best regards,
> -- 
> Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
> 

-- 
With best wishes
Dmitry