Hi Doug

On 11/9/2021 10:04 AM, 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@chromium.org

Reviewed-by: Abhinav Kumar quic_abhinavk@quicinc.com

---

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)