drm/bridge: ti-sn65dsi83: ignore PLL_UNLOCK errors

On hardware based on Toradex Verdin AM62 the recovery mechanism added by
commit ad5c6ec ("drm: bridge: ti-sn65dsi83: Add error recovery
mechanism") has been reported [0] to make the display turn on and off and
and the kernel logging "Unexpected link status 0x01".

According to the report, the error recovery mechanism is triggered by the
PLL_UNLOCK error going active. Analysis suggested the board is unable to
provide the correct DSI clock neede by the SN65DSI84, to which the TI
SN65DSI84 reacts by raising the PLL_UNLOCK, while the display still works
apparently without issues.

On other hardware, where all the clocks are within the components
specifications, the PLL_UNLOCK bit does not trigger while the display is in
normal use. It can trigger for e.g. electromagnetic interference, which is
a transient event and exactly the reason why the error recovery mechanism
has been implemented.

Idelly the PLL_UNLOCK bit could be ignored when working out of
specification, but this requires to detect in software whether it triggers
because the device is working out of specification but visually correctly
for the user or for good reasons (e.g. EMI, or even because working out of
specifications but compromising the visual output).

The ongoing analysis as of this writing [1][2] has not yet found a way for
the driver to discriminate among the two cases. So as a temporary measure
mask the PLL_UNLOCK error bit unconditionally.

[0] https://lore.kernel.org/r/bhkn6hley4xrol5o3ytn343h4unkwsr26p6s6ltcwexnrsjsdx@mgkdf6ztow42
[1] https://lore.kernel.org/all/b71e941c-fc8a-4ac1-9407-0fe7df73b412@gmail.com/
[2] https://lore.kernel.org/all/20251125103900.31750-1-francesco@dolcini.it/

Fixes: ad5c6ec ("drm: bridge: ti-sn65dsi83: Add error recovery mechanism")
Closes: https://lore.kernel.org/r/bhkn6hley4xrol5o3ytn343h4unkwsr26p6s6ltcwexnrsjsdx@mgkdf6ztow42
Cc: stable@vger.kernel.org # 6.15+
Reported-by: João Paulo Gonçalves <joao.goncalves@toradex.com>
Tested-by: Emanuele Ghidoli <emanuele.ghidoli@toradex.com>
Co-developed-by: Hervé Codina <herve.codina@bootlin.com>
Signed-off-by: Hervé Codina <herve.codina@bootlin.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Link: https://patch.msgid.link/20251127-drm-ti-sn65dsi83-ignore-pll-unlock-v1-1-8a03fdf562e9@bootlin.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>

Author: lucaceresoli

drivers/gpu/drm/bridge/ti-sn65dsi83.c

@@ -429,7 +429,14 @@ static void sn65dsi83_handle_errors(struct sn65dsi83 *ctx)
 	 */
 
 	ret = regmap_read(ctx->regmap, REG_IRQ_STAT, &irq_stat);
-	if (ret || irq_stat) {
+
+	/*
+	 * Some hardware (Toradex Verdin AM62) is known to report the
+	 * PLL_UNLOCK error interrupt while working without visible
+	 * problems. In lack of a reliable way to discriminate such cases
+	 * from user-visible PLL_UNLOCK cases, ignore that bit entirely.
+	 */
+	if (ret || irq_stat & ~REG_IRQ_STAT_CHA_PLL_UNLOCK) {
 		/*
 		 * IRQ acknowledged is not always possible (the bridge can be in
 		 * a state where it doesn't answer anymore). To prevent an
@@ -654,7 +661,7 @@ static void sn65dsi83_atomic_enable(struct drm_bridge *bridge,
 	if (ctx->irq) {
 		/* Enable irq to detect errors */
 		regmap_write(ctx->regmap, REG_IRQ_GLOBAL, REG_IRQ_GLOBAL_IRQ_EN);
-		regmap_write(ctx->regmap, REG_IRQ_EN, 0xff);
+		regmap_write(ctx->regmap, REG_IRQ_EN, 0xff & ~REG_IRQ_EN_CHA_PLL_UNLOCK_EN);
 	} else {
 		/* Use the polling task */
 		sn65dsi83_monitor_start(ctx);