Re: [PATCH] drm: xlnx: zynqmp: release reset to DP controller before accessing DP registers

[Laurent Pinchart]

Hi Quanyang,

Thank you for the patch.

On Sat, Mar 20, 2021 at 04:37:39PM +0800, quanyang.wang@xxxxxxxxxxxxx wrote:
> From: Quanyang Wang <quanyang.wang@xxxxxxxxxxxxx>
> 
> When insmod zynqmp-dpsub.ko after rmmod it, system will hang with the
> error log as below:
> 
> root@xilinx-zynqmp:~# insmod zynqmp-dpsub.ko
> [   88.391289] [drm] Initialized zynqmp-dpsub 1.0.0 20130509 for fd4a0000.display on minor 0
> [   88.529906] Console: switching to colour frame buffer device 128x48
> [   88.549402] zynqmp-dpsub fd4a0000.display: [drm] fb0: zynqmp-dpsubdrm frame buffer device
> [   88.571624] zynqmp-dpsub fd4a0000.display: ZynqMP DisplayPort Subsystem driver probed
> root@xilinx-zynqmp:~# rmmod zynqmp_dpsub
> [   94.023404] Console: switching to colour dummy device 80x25
> root@xilinx-zynqmp:~# insmod zynqmp-dpsub.ko
> 	<hang here>
> 
> This is because that in zynqmp_dp_probe it tries to access some DP
> registers while the DP controller is still in the reset state. When
> running "rmmod zynqmp_dpsub", zynqmp_dp_reset(dp, true) in
> zynqmp_dp_phy_exit is called to force the DP controller into the reset
> state. Then insmod will call zynqmp_dp_probe to write to the DP registers,
> but at this moment the DP controller isn't brought out of the reset state
> since the function zynqmp_dp_reset(dp, false) is called later and this
> will result the system hang.
> 
> Releasing the reset to DP controller before any read/write operation to it
> will fix this issue.
> 
> Signed-off-by: Quanyang Wang <quanyang.wang@xxxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/xlnx/zynqmp_dp.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xlnx/zynqmp_dp.c b/drivers/gpu/drm/xlnx/zynqmp_dp.c
> index 99158ee67d02..bb45b3663d6b 100644
> --- a/drivers/gpu/drm/xlnx/zynqmp_dp.c
> +++ b/drivers/gpu/drm/xlnx/zynqmp_dp.c
> @@ -402,10 +402,6 @@ static int zynqmp_dp_phy_init(struct zynqmp_dp *dp)
>  		}
>  	}
>  
> -	ret = zynqmp_dp_reset(dp, false);
> -	if (ret < 0)
> -		return ret;
> -
>  	zynqmp_dp_clr(dp, ZYNQMP_DP_PHY_RESET, ZYNQMP_DP_PHY_RESET_ALL_RESET);
>  
>  	/*
> @@ -1682,6 +1678,10 @@ int zynqmp_dp_probe(struct zynqmp_dpsub *dpsub, struct drm_device *drm)
>  		return PTR_ERR(dp->reset);
>  	}
>  
> +	ret = zynqmp_dp_reset(dp, false);
> +	if (ret < 0)
> +		return ret;
> +

This change looks good to me.

>  	ret = zynqmp_dp_phy_probe(dp);
>  	if (ret)
>  		return ret;

But shouldn't we call zynqmp_dp_reset(dp, true) here ? Or rather, call
it in the error path at the end of the function, with a goto label.

For symmetry, should we also move the zynqmp_dp_reset() call from
zynqmp_dp_phy_exit() to zynqmp_dp_remove() ?

-- 
Regards,

Laurent Pinchart