Re: [BUG] drm: zynqmp_dp: Lockup in zynqmp_dp_bridge_detect when device is unbound

[Laurent Pinchart]

Hi Sean,

On Fri, May 03, 2024 at 05:54:32PM -0400, Sean Anderson wrote:
> Hi,
> 
> I have discovered a bug in the displayport driver on drm-misc-next. To
> trigger it, run
> 
> echo fd4a0000.display > /sys/bus/platform/drivers/zynqmp-dpsub/unbind
> 
> The system will become unresponsive and (after a bit) splat with a hard
> LOCKUP. One core will be unresponsive at the first zynqmp_dp_read in
> zynqmp_dp_bridge_detect.
> 
> I believe the issue is due the registers being unmapped and the block
> put into reset in zynqmp_dp_remove instead of zynqmp_dpsub_release.

That is on purpose. Drivers are not allowed to access the device at all
after .remove() returns.

> This
> could be resolved by deferring things until zynqmp_dpsub_release
> (requiring us to skip devm_*), or by adding a flag to struct zynqmp_dp
> and checking it before each callback. A subsystem-level implementation
> might be better for the latter.
> 
> For a better traceback, try applying the below patch and running the
> following commands before triggering the lockup:
> 
> echo 4 > /sys/module/drm/parameters/debug
> echo 8 > /proc/sys/kernel/printk
> 
> diff --git a/drivers/gpu/drm/xlnx/zynqmp_dp.c b/drivers/gpu/drm/xlnx/zynqmp_dp.c
> index 9df068a413f3..17b477b14ab5 100644
> --- a/drivers/gpu/drm/xlnx/zynqmp_dp.c
> +++ b/drivers/gpu/drm/xlnx/zynqmp_dp.c
> @@ -296,6 +296,7 @@ struct zynqmp_dp_config {
>   * @train_set: set of training data
>   */
>  struct zynqmp_dp {
> +       unsigned long long magic;
>         struct device *dev;
>         struct zynqmp_dpsub *dpsub;
>         void __iomem *iomem;
> @@ -1533,6 +1534,8 @@ static enum drm_connector_status zynqmp_dp_bridge_detect(struct drm_bridge *brid
>         u32 state, i;
>         int ret;
>  
> +       WARN_ON(dp->magic != 0x0123456789abcdefULL);
> +
>         /*
>          * This is from heuristic. It takes some delay (ex, 100 ~ 500 msec) to
>          * get the HPD signal with some monitors.
> @@ -1723,6 +1726,7 @@ int zynqmp_dp_probe(struct zynqmp_dpsub *dpsub)
>         if (!dp)
>                 return -ENOMEM;
>  
> +       dp->magic = 0x0123456789abcdefULL;
>         dp->dev = &pdev->dev;
>         dp->dpsub = dpsub;
>         dp->status = connector_status_disconnected;
> @@ -1839,4 +1843,5 @@ void zynqmp_dp_remove(struct zynqmp_dpsub *dpsub)
>  
>         zynqmp_dp_phy_exit(dp);
>         zynqmp_dp_reset(dp, true);
> +       dp->magic = 0xdeadbeefdeadbeefULL;
>  }

-- 
Regards,

Laurent Pinchart