[PATCH v3] thunderbolt: Fix Thunderbolt 3 display flickering issue on 2nd hot plug onwards

[PATCH v3] thunderbolt: Fix Thunderbolt 3 display flickering issue on 2nd hot plug onwards

[Sanjay R Mehta]

Previously, on unplug events, the TMU mode was disabled first
followed by the Time Synchronization Handshake, irrespective of
whether the tb_switch_tmu_rate_write() API was successful or not.

However, this caused a problem with Thunderbolt 3 (TBT3)
devices, as the TSPacketInterval bits were always enabled by default,
leading the host router to assume that the device router's TMU was
already enabled and preventing it from initiating the Time
Synchronization Handshake. As a result, TBT3 monitors experienced
display flickering from the second hot plug onwards.

To address this issue, we have modified the code to only disable the
Time Synchronization Handshake during TMU disable if the
tb_switch_tmu_rate_write() function is successful. This ensures that
the TBT3 devices function correctly and eliminates the display
flickering issue.

Co-developed-by: Sanath S <Sanath.S@amd.com>
Signed-off-by: Sanath S <Sanath.S@amd.com>
Signed-off-by: Sanjay R Mehta <sanju.mehta@amd.com>
---

v2->v3:
-------
	- Rebase to 6.5.0-rc4.
---
 drivers/thunderbolt/tmu.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/thunderbolt/tmu.c b/drivers/thunderbolt/tmu.c
index 1269f41..0dfd1e0 100644
--- a/drivers/thunderbolt/tmu.c
+++ b/drivers/thunderbolt/tmu.c
@@ -579,7 +579,9 @@ int tb_switch_tmu_disable(struct tb_switch *sw)
 		 * uni-directional mode and we don't want to change it's TMU
 		 * mode.
 		 */
-		tb_switch_tmu_rate_write(sw, tmu_rates[TB_SWITCH_TMU_MODE_OFF]);
+		ret = tb_switch_tmu_rate_write(sw, tmu_rates[TB_SWITCH_TMU_MODE_OFF]);
+		if (ret)
+			return ret;
 
 		tb_port_tmu_time_sync_disable(up);
 		ret = tb_port_tmu_time_sync_disable(down);
-- 
2.7.4

Re: [PATCH v3] thunderbolt: Fix Thunderbolt 3 display flickering issue on 2nd hot plug onwards

[Mika Westerberg]

On Wed, Aug 02, 2023 at 06:11:49AM -0500, Sanjay R Mehta wrote:
> Previously, on unplug events, the TMU mode was disabled first
> followed by the Time Synchronization Handshake, irrespective of
> whether the tb_switch_tmu_rate_write() API was successful or not.
> 
> However, this caused a problem with Thunderbolt 3 (TBT3)
> devices, as the TSPacketInterval bits were always enabled by default,
> leading the host router to assume that the device router's TMU was
> already enabled and preventing it from initiating the Time
> Synchronization Handshake. As a result, TBT3 monitors experienced
> display flickering from the second hot plug onwards.
> 
> To address this issue, we have modified the code to only disable the
> Time Synchronization Handshake during TMU disable if the
> tb_switch_tmu_rate_write() function is successful. This ensures that
> the TBT3 devices function correctly and eliminates the display
> flickering issue.
> 
> Co-developed-by: Sanath S <Sanath.S@amd.com>
> Signed-off-by: Sanath S <Sanath.S@amd.com>
> Signed-off-by: Sanjay R Mehta <sanju.mehta@amd.com>

Marked for stable and applied to thunderbolt.git/fixes, thanks!