Re: [PATCH] drm: xlnx: call pm_runtime_get_sync before setting pixel clock

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Quanyang,

Thank you for the patch.

On Wed, Mar 10, 2021 at 12:59:45PM +0800, quanyang.wang@windriver.com wrote:
> From: Quanyang Wang <quanyang.wang@windriver.com>
> 
> The Runtime PM subsystem will force the device "fd4a0000.zynqmp-display"
> to enter suspend state while booting if the following conditions are met:
> - the usage counter is zero (pm_runtime_get_sync hasn't been called yet)
> - no 'active' children (no zynqmp-dp-snd-xx node under dpsub node)
> - no other device in the same power domain (dpdma node has no
> 		"power-domains = <&zynqmp_firmware PD_DP>" property)
> 
> So there is a scenario as below:
> 1) DP device enters suspend state   <- call zynqmp_gpd_power_off
> 2) zynqmp_disp_crtc_setup_clock	    <- configurate register VPLL_FRAC_CFG
> 3) pm_runtime_get_sync		    <- call zynqmp_gpd_power_on and clear previous
> 				       VPLL_FRAC_CFG configuration
> 4) clk_prepare_enable(disp->pclk)   <- enable failed since VPLL_FRAC_CFG
> 				       configuration is corrupted
> 
> From above, we can see that pm_runtime_get_sync may clear register
> VPLL_FRAC_CFG configuration and result the failure of clk enabling.
> Putting pm_runtime_get_sync at the very beginning of the function
> zynqmp_disp_crtc_atomic_enable can resolve this issue.

Isn't this an issue in the firmware though, which shouldn't clear the
previous VPLLF_FRAC_CFG ?

> Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>

Nonetheless, this change looks good to me, I actually had the same patch
in my tree while investigation issues related to the clock rate, so

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

I was hoping it would solve the issue I'm experiencing with the DP
clock, but that's not the case :-( In a nutshell, when the DP is first
started, the clock frequency is incorrect. The following quick & dirty
patch fixes the problem:

diff --git a/drivers/gpu/drm/xlnx/zynqmp_disp.c b/drivers/gpu/drm/xlnx/zynqmp_disp.c
index 74ac0a064eb5..fdbe1b0640aa 100644
--- a/drivers/gpu/drm/xlnx/zynqmp_disp.c
+++ b/drivers/gpu/drm/xlnx/zynqmp_disp.c
@@ -1439,6 +1439,10 @@ zynqmp_disp_crtc_atomic_enable(struct drm_crtc *crtc,

 	pm_runtime_get_sync(disp->dev);

+	ret = clk_prepare_enable(disp->pclk);
+	if (!ret)
+		clk_disable_unprepare(disp->pclk);
+
 	zynqmp_disp_crtc_setup_clock(crtc, adjusted_mode);

 	ret = clk_prepare_enable(disp->pclk);

The problem doesn't seem to be in the kernel, but on the TF-A or PMU
firmware side. Have you experienced this by any chance ?

> ---
>  drivers/gpu/drm/xlnx/zynqmp_disp.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/xlnx/zynqmp_disp.c b/drivers/gpu/drm/xlnx/zynqmp_disp.c
> index 148add0ca1d6..909e6c265406 100644
> --- a/drivers/gpu/drm/xlnx/zynqmp_disp.c
> +++ b/drivers/gpu/drm/xlnx/zynqmp_disp.c
> @@ -1445,9 +1445,10 @@ zynqmp_disp_crtc_atomic_enable(struct drm_crtc *crtc,
>  	struct drm_display_mode *adjusted_mode = &crtc->state->adjusted_mode;
>  	int ret, vrefresh;
>  
> +	pm_runtime_get_sync(disp->dev);
> +
>  	zynqmp_disp_crtc_setup_clock(crtc, adjusted_mode);
>  
> -	pm_runtime_get_sync(disp->dev);
>  	ret = clk_prepare_enable(disp->pclk);
>  	if (ret) {
>  		dev_err(disp->dev, "failed to enable a pixel clock\n");

-- 
Regards,

Laurent Pinchart