[PATCH 2/5] drm/nouveau/gsp: don't report failed DP AUX transfers as completed

[David R. Hedges via B4 Relay]

From: "David R. Hedges" <linuxdev844@xxxxxxxxxxxxxxxx>

Commit 9776c0a75a1a ("nouveau/dp: handle retries for AUX CH transfers
with GSP.") put the NV0073_CTRL_CMD_DP_AUXCH_CTRL call in a retry loop
and dropped the check for the call failing.  When RM fails it with
anything other than a retry request, r535_dp_aux_xfer() ignores the
error and returns the reply fields as if a transaction had completed:
replyType becomes the AUX reply, and size and data are copied out.
NVIDIA's own DisplayPort library (EvoAuxBus::transaction() in
open-gpu-kernel-modules) treats a failed AUXCH_CTRL call as a NACK and
ignores the reply.

RM does fail these calls.  On a GA102 with GSP-RM 570.144, replugging
a DP monitor that hasn't come back from DPMS off produces bursts of

  gsp: cli:0xc1d00001 obj:0x00730000 ctrl cmd:0x00731341 failed: 0x0000ffff

(NV_ERR_GENERIC, occasionally NV_ERR_INVALID_DATA), one per AUX
transaction, and the DPCD and EDID reads made during a burst are
judged on whatever the reply fields happen to contain.

Return the error from the control call again, as r535_dp_aux_xfer()
did before the retry loop was added.

Fixes: 9776c0a75a1a ("nouveau/dp: handle retries for AUX CH transfers with GSP.")
Cc: stable@xxxxxxxxxxxxxxx
Link: https://gitlab.freedesktop.org/drm/nouveau/-/work_items/515
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: David R. Hedges <linuxdev844@xxxxxxxxxxxxxxxx>
---
 drivers/gpu/drm/nouveau/nvkm/subdev/gsp/rm/r535/disp.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/nvkm/subdev/gsp/rm/r535/disp.c b/drivers/gpu/drm/nouveau/nvkm/subdev/gsp/rm/r535/disp.c
index f5f22173fc2c..953392504413 100644
--- a/drivers/gpu/drm/nouveau/nvkm/subdev/gsp/rm/r535/disp.c
+++ b/drivers/gpu/drm/nouveau/nvkm/subdev/gsp/rm/r535/disp.c
@@ -1101,9 +1101,12 @@ r535_dp_aux_xfer(struct nvkm_outp *outp, u8 type, u32 addr, u8 *data, u8 *psize)
 			msleep(ctrl->retryTimeMs);
 			nvkm_gsp_rm_ctrl_done(&disp->rm.objcom, ctrl);
 		} else {
-			memcpy(data, ctrl->data, size);
-			*psize = ctrl->size;
-			ret = ctrl->replyType;
+			/* The reply only describes a transaction if RM completed one. */
+			if (ret == 0) {
+				memcpy(data, ctrl->data, size);
+				*psize = ctrl->size;
+				ret = ctrl->replyType;
+			}
 			nvkm_gsp_rm_ctrl_done(&disp->rm.objcom, ctrl);
 			break;
 		}

-- 
2.55.0