On Mon, 17 Nov 2025, Imre Deak <[email protected]> wrote: > On Mon, Nov 17, 2025 at 07:09:38AM +0200, Suraj Kandpal wrote: >> > -----Original Message----- >> > From: Jani Nikula <[email protected]> >> > Sent: Thursday, November 13, 2025 9:55 PM >> > To: Deak, Imre <[email protected]>; Kandpal, Suraj >> > <[email protected]> >> > Cc: [email protected]; [email protected]; intel- >> > [email protected]; Nautiyal, Ankit K <[email protected]>; >> > Murthy, Arun R <[email protected]> >> > Subject: Re: [PATCH] drm/display/dp_mst: Add protection against 0 vcpi >> > >> > On Thu, 13 Nov 2025, Imre Deak <[email protected]> wrote: >> > > On Thu, Nov 13, 2025 at 10:09:19AM +0530, Suraj Kandpal wrote: >> > >> When releasing a timeslot there is a slight chance we may end up with >> > >> the wrong payload mask due to overflow if the delayed_destroy_work >> > >> ends up coming into play after a DP 2.1 monitor gets disconnected >> > >> which causes vcpi to become 0 then we try to make the payload = >> > >> ~BIT(vcpi - 1) which is a negative shift. >> > >> >> > >> Signed-off-by: Suraj Kandpal <[email protected]> >> > >> --- >> > >> drivers/gpu/drm/display/drm_dp_mst_topology.c | 4 +++- >> > >> 1 file changed, 3 insertions(+), 1 deletion(-) >> > >> >> > >> diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c >> > >> b/drivers/gpu/drm/display/drm_dp_mst_topology.c >> > >> index 64e5c176d5cc..3cf1eafcfcb5 100644 >> > >> --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c >> > >> +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c >> > >> @@ -4531,6 +4531,7 @@ int drm_dp_atomic_release_time_slots(struct >> > drm_atomic_state *state, >> > >> struct drm_dp_mst_atomic_payload *payload; >> > >> struct drm_connector_state *old_conn_state, *new_conn_state; >> > >> bool update_payload = true; >> > >> + int bit; >> > >> >> > >> old_conn_state = drm_atomic_get_old_connector_state(state, port- >> > >connector); >> > >> if (!old_conn_state->crtc) >> > >> @@ -4572,7 +4573,8 @@ int drm_dp_atomic_release_time_slots(struct >> > drm_atomic_state *state, >> > >> if (!payload->delete) { >> > >> payload->pbn = 0; >> > >> payload->delete = true; >> > >> - topology_state->payload_mask &= ~BIT(payload->vcpi - 1); >> > >> + bit = payload->vcpi ? payload->vcpi - 1 : 0; >> > >> + topology_state->payload_mask &= ~BIT(bit); >> > > >> > > This looks wrong, clearing the bit for an unrelated payload. >> > >> > Agreed. >> > >> > The logs have, among other things, >> > >> > <7> [515.138211] xe 0000:03:00.0: [drm:intel_dp_sink_set_dsc_decompression >> > [xe]] Failed to enable sink decompression state >> > >> > <7> [515.193484] xe 0000:03:00.0: [drm:drm_dp_add_payload_part1 >> > [drm_display_helper]] VCPI 0 for port ffff888126ce9000 not in topology, not >> > creating a payload to remote >> > >> > <7> [515.194671] xe 0000:03:00.0: [drm:drm_dp_add_payload_part2 >> > [drm_display_helper]] Part 1 of payload creation for DP-5 failed, skipping >> > part 2 >> > >> > <7> [515.347331] xe 0000:03:00.0: [drm:drm_dp_remove_payload_part1 >> > [drm_display_helper]] Payload for VCPI 0 not in topology, not sending >> > remove >> > >> > So it's no wonder the port's not in topology and everything fails. We >> > obviously >> > need to skip payload_mask updates when the VCPI is 0, but that's just a >> > symptom of other stuff going wrong first. Perhaps we could do with some >> > earlier error handling too? >> >> Yes I agree the question is how high will the error handling needs to be >> added. >> A lot of weird things going on here. >> >> 1st one is how is it finding a payload which we do not create while we >> call destroy function >> >> 2nd how is VCPI with id 0 possible from what I see VCPI are 1 at least >> that's what I gather from drm_dp_mst_atomic_check_payload_alloc_limits.So >> what >> are we missing when we create a payload? >> >> Imre, Jani any idea still new to how payload creation work so am I >> missing something. > > A VCPI ID will be assigned to a payload during an atomic commit only if > the corresponding MST connector is still connected. If the MST connector > gets disconnected by the time of the atomic commit - as in the above > case - no VCPI ID will assigned and the allocation table in the branch > device cannot be updated either for the payload, as indicated by the > above payload creation/removal failed messages. > > I think the fix should be not to clear the VCPI ID if it's 0. Valid VCPI > IDs start from 1.
Agreed. As I said above, "We obviously need to skip payload_mask updates when the VCPI is 0". But there are *also* a bunch of other things going wrong before that, but we plunge on. Should we do something about that? BR, Jani. > >> Regards >> Suraj Kandpal >> >> > BR, >> > Jani. >> > >> > >> > > >> > >> } >> > >> >> > >> return 0; >> > >> -- >> > >> 2.34.1 >> > >> >> > >> > -- >> > Jani Nikula, Intel -- Jani Nikula, Intel
