Re: [PATCH] drm/msm/dp: Avoid unpowered AUX xfers that caused crashes

[khsieh]

On 2021-11-09 10:04, Douglas Anderson wrote:
> If you happened to try to access `/dev/drm_dp_aux` devices provided by
> the MSM DP AUX driver too early at bootup you could go boom. Let's
> avoid that by only allowing AUX transfers when the controller is
> powered up.
>
> Specifically the crash that was seen (on Chrome OS 5.4 tree with
> relevant backports):
>   Kernel panic - not syncing: Asynchronous SError Interrupt
>   CPU: 0 PID: 3131 Comm: fwupd Not tainted 5.4.144-16620-g28af11b73efb 
> #1
>   Hardware name: Google Lazor (rev3+) with KB Backlight (DT)
>   Call trace:
>    dump_backtrace+0x0/0x14c
>    show_stack+0x20/0x2c
>    dump_stack+0xac/0x124
>    panic+0x150/0x390
>    nmi_panic+0x80/0x94
>    arm64_serror_panic+0x78/0x84
>    do_serror+0x0/0x118
>    do_serror+0xa4/0x118
>    el1_error+0xbc/0x160
>    dp_catalog_aux_write_data+0x1c/0x3c
>    dp_aux_cmd_fifo_tx+0xf0/0x1b0
>    dp_aux_transfer+0x1b0/0x2bc
>    drm_dp_dpcd_access+0x8c/0x11c
>    drm_dp_dpcd_read+0x64/0x10c
>    auxdev_read_iter+0xd4/0x1c4
>
> I did a little bit of tracing and found that:
> * We register the AUX device very early at bootup.
> * Power isn't actually turned on for my system until
>   hpd_event_thread() -> dp_display_host_init() -> dp_power_init()
> * You can see that dp_power_init() calls dp_aux_init() which is where
>   we start allowing AUX channel requests to go through.
>
> In general this patch is a bit of a bandaid but at least it gets us
> out of the current state where userspace acting at the wrong time can
> fully crash the system.
> * I think the more proper fix (which requires quite a bit more
>   changes) is to power stuff on while an AUX transfer is
>   happening. This is like the solution we did for ti-sn65dsi86. This
>   might be required for us to move to populating the panel via the
>   DP-AUX bus.
> * Another fix considered was to dynamically register / unregister. I
>   tried that at <https://crrev.com/c/3169431/3> but it got
>   ugly. Currently there's a bug where the pm_runtime() state isn't
>   tracked properly and that causes us to just keep registering more
>   and more.
>
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
Reviewed-by: Kuogee Hsieh <quic_khsieh@xxxxxxxxxxx>
>  drivers/gpu/drm/msm/dp/dp_aux.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
>
> diff --git a/drivers/gpu/drm/msm/dp/dp_aux.c 
> b/drivers/gpu/drm/msm/dp/dp_aux.c
> index eb40d8413bca..6d36f63c3338 100644
> --- a/drivers/gpu/drm/msm/dp/dp_aux.c
> +++ b/drivers/gpu/drm/msm/dp/dp_aux.c
> @@ -33,6 +33,7 @@ struct dp_aux_private {
>  	bool read;
>  	bool no_send_addr;
>  	bool no_send_stop;
> +	bool initted;
>  	u32 offset;
>  	u32 segment;
>
> @@ -331,6 +332,10 @@ static ssize_t dp_aux_transfer(struct drm_dp_aux 
> *dp_aux,
>  	}
>
>  	mutex_lock(&aux->mutex);
> +	if (!aux->initted) {
> +		ret = -EIO;
> +		goto exit;
> +	}
>
>  	dp_aux_update_offset_and_segment(aux, msg);
>  	dp_aux_transfer_helper(aux, msg, true);
> @@ -380,6 +385,8 @@ static ssize_t dp_aux_transfer(struct drm_dp_aux 
> *dp_aux,
>  	}
>
>  	aux->cmd_busy = false;
> +
> +exit:
>  	mutex_unlock(&aux->mutex);
>
>  	return ret;
> @@ -431,8 +438,13 @@ void dp_aux_init(struct drm_dp_aux *dp_aux)
>
>  	aux = container_of(dp_aux, struct dp_aux_private, dp_aux);
>
> +	mutex_lock(&aux->mutex);
> +
>  	dp_catalog_aux_enable(aux->catalog, true);
>  	aux->retry_cnt = 0;
> +	aux->initted = true;
> +
> +	mutex_unlock(&aux->mutex);
>  }
>
>  void dp_aux_deinit(struct drm_dp_aux *dp_aux)
> @@ -441,7 +453,12 @@ void dp_aux_deinit(struct drm_dp_aux *dp_aux)
>
>  	aux = container_of(dp_aux, struct dp_aux_private, dp_aux);
>
> +	mutex_lock(&aux->mutex);
> +
> +	aux->initted = false;
>  	dp_catalog_aux_enable(aux->catalog, false);
> +
> +	mutex_unlock(&aux->mutex);
>  }
>
>  int dp_aux_register(struct drm_dp_aux *dp_aux)