Re: [PATCH 3/3] drm/bridge: ti-sn65dsi83: add test pattern generation support

["Luca Ceresoli" <luca.ceresoli@bootlin.com>]

Hello,

On Thu Feb 26, 2026 at 5:16 PM CET, Luca Ceresoli wrote:
> Generation of a test pattern output is a useful tool for panel bringup and
> debugging, and very simple to support with this chip.
>
> The value of REG_VID_CHA_ACTIVE_LINE_LENGTH_LOW needs to be divided by two
> for the test pattern to work in dual LVDS mode. While not clearly stated in
> the datasheet, this is needed according to the DSI Tuner [0] output. And
> some dual-LVDS panels refuse to show any picture without this division by
> two.
>
> [0] https://www.ti.com/tool/DSI-TUNER
>
> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>

I just noticed a small glitch in the implementation.

> +static bool sn65dsi83_test_pattern;
> +module_param_named(test_pattern, sn65dsi83_test_pattern, bool, 0644);
> +
>  enum sn65dsi83_channel {
>  	CHANNEL_A,
>  	CHANNEL_B
> @@ -645,7 +649,11 @@ static void sn65dsi83_atomic_pre_enable(struct drm_bridge *bridge,
>  			  REG_LVDS_LANE_CHB_LVDS_TERM : 0));
>  	regmap_write(ctx->regmap, REG_LVDS_CM, 0x00);
>
> -	le16val = cpu_to_le16(mode->hdisplay);
> +	/*
> +	 * Active line length needs to be halved for test pattern
> +	 * generation in dual LVDS output.
> +	 */
> +	le16val = cpu_to_le16(mode->hdisplay / (sn65dsi83_test_pattern ? 2 : 1));

In case sn65dsi83_test_pattern is changed from user space after this
cpu_to_le16()...

>  	regmap_bulk_write(ctx->regmap, REG_VID_CHA_ACTIVE_LINE_LENGTH_LOW,
>  			  &le16val, 2);
>  	le16val = cpu_to_le16(mode->vdisplay);
> @@ -668,7 +676,8 @@ static void sn65dsi83_atomic_pre_enable(struct drm_bridge *bridge,
>  		     (mode->hsync_start - mode->hdisplay) / dual_factor);
>  	regmap_write(ctx->regmap, REG_VID_CHA_VERTICAL_FRONT_PORCH,
>  		     mode->vsync_start - mode->vdisplay);
> -	regmap_write(ctx->regmap, REG_VID_CHA_TEST_PATTERN, 0x00);
> +	regmap_write(ctx->regmap, REG_VID_CHA_TEST_PATTERN,
> +		     sn65dsi83_test_pattern ? REG_VID_CHA_TEST_PATTERN_EN : 0);

...but before this regmap_write(), the two registers affected by
sn65dsi83_test_pattern would be written with inconsistent values.

I'm resending with that fixed.

Luca

--
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com