[PATCH 11/13] drm/amd/display: Refuse DP AUX while GMUX keeps the GPU off

[Atharva Tiwari <atharvatiwarilinuxdev@gmail.com>]

From: Andre Eikmeyer <dev@deq.rocks>

fwupd reads /dev/drm_dp_aux* while the GPU is runtime suspended. The AUX
device holds no runtime PM reference, so the transfer touched registers of
the powered off GPU and caused a CPU CATTER.

Signed-off-by: Andre Eikmeyer <dev@deq.rocks>
---
 .../display/amdgpu_dm/amdgpu_dm_mst_types.c   | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c
index 7261317d4..b099bf520 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c
@@ -24,6 +24,7 @@
  *
  */
 
+#include <linux/pm_runtime.h>
 #include <linux/vmalloc.h>
 #include <drm/display/drm_dp_helper.h>
 #include <drm/display/drm_dp_mst_helper.h>
@@ -111,6 +112,20 @@ STATIC_IFN_KUNIT ssize_t dm_dp_aux_transfer(struct drm_dp_aux *aux,
 	if (WARN_ON(msg->size > 16))
 		return -E2BIG;
 
+	ddc = TO_DM_AUX(aux)->ddc_service;
+	adev = ddc->ctx->driver_context;
+
+	/*
+	 * User space reaches this through the drm_dp_aux_dev character device
+	 * without a runtime PM reference. Refuse the transfer while GMUX keeps
+	 * the GPU powered off, a register access then hangs the PEG link.
+	 */
+	pm_runtime_get_noresume(adev->dev);
+	if (READ_ONCE(adev->gmux_off)) {
+		pm_runtime_put_noidle(adev->dev);
+		return -EBUSY;
+	}
+
 	payload.address = msg->address;
 	payload.data = msg->buffer;
 	payload.length = msg->size;
@@ -126,13 +141,13 @@ STATIC_IFN_KUNIT ssize_t dm_dp_aux_transfer(struct drm_dp_aux *aux,
 	result = dc_link_aux_transfer_raw(TO_DM_AUX(aux)->ddc_service, &payload,
 				      &operation_result);
 
+	pm_runtime_put_autosuspend(adev->dev);
+
 	/*
 	 * w/a on certain intel platform where hpd is unexpected to pull low during
 	 * 1st sideband message transaction by return AUX_RET_ERROR_HPD_DISCON
 	 * aux transaction is succuess in such case, therefore bypass the error
 	 */
-	ddc = TO_DM_AUX(aux)->ddc_service;
-	adev = ddc->ctx->driver_context;
 	if (adev->dm.aux_hpd_discon_quirk) {
 		if (msg->address == DP_SIDEBAND_MSG_DOWN_REQ_BASE &&
 			operation_result == AUX_RET_ERROR_HPD_DISCON) {
-- 
2.55.0