Re: [PATCH] drm/bridge: ti-sn65dsi83: ignore PLL_UNLOCK errors

[Emanuele Ghidoli <ghidoliemanuele@gmail.com>]

On 27/11/2025 09:42, Luca Ceresoli wrote:
> On hardware based on Toradex Verdin AM62 the recovery mechanism added by
> commit ad5c6ecef27e ("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/
> 
> Closes: https://lore.kernel.org/r/bhkn6hley4xrol5o3ytn343h4unkwsr26p6s6ltcwexnrsjsdx@mgkdf6ztow42
> Cc: stable@vger.kernel.org # 6.15+
> 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>
> ---
> Francesco, Emanuele, João: can you please apply this patch and report
> whether the display on the affected boards gets back to working as before?
> 
> Cc: João Paulo Gonçalves <jpaulo.silvagoncalves@gmail.com>
> Cc: Francesco Dolcini <francesco@dolcini.it>
> Cc: Emanuele Ghidoli <ghidoliemanuele@gmail.com>
> ---
>  drivers/gpu/drm/bridge/ti-sn65dsi83.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi83.c b/drivers/gpu/drm/bridge/ti-sn65dsi83.c
> index 033c44326552..fffb47b62f43 100644
> --- a/drivers/gpu/drm/bridge/ti-sn65dsi83.c
> +++ b/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);
> 
> ---
> base-commit: c884ee70b15a8d63184d7c1e02eba99676a6fcf7
> change-id: 20251126-drm-ti-sn65dsi83-ignore-pll-unlock-4a28aa29eb5c
> 
> Best regards,

Hi Luca,
the display works correctly with this patch, thanks!

Kind regards.

Tested-by: Emanuele Ghidoli <emanuele.ghidoli@toradex.com>