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 <[email protected]>
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
