[PATCH v2] drm/panel: Set max rate for Ilitek ILI9881C

[Andrzej Hajda]

On 18.11.2018 22:31, Linus Walleij wrote:
> After adding the hs_rate and lp_rate fields to the DSI device
> we need to populate these accordingly so display drivers can
> respect them.
>
> This figure for HS rate comes from the ILI9881C manual, the
> calculation is explained in the comment.
>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: Andrzej Hajda <a.hajda at samsung.com>
> Acked-by: Maxime Ripard <maxime.ripard at bootlin.com>
> Signed-off-by: Linus Walleij <linus.walleij at linaro.org>
> ---
> ChangeLog v1->v2:
> - Add LP speed after Andrzej's observation.
> - Collect Maxime's ACK (hope it's fine also with the LP speed)
> ---
>  drivers/gpu/drm/panel/panel-ilitek-ili9881c.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/drivers/gpu/drm/panel/panel-ilitek-ili9881c.c b/drivers/gpu/drm/panel/panel-ilitek-ili9881c.c
> index 3ad4a46c4e94..bd276c666318 100644
> --- a/drivers/gpu/drm/panel/panel-ilitek-ili9881c.c
> +++ b/drivers/gpu/drm/panel/panel-ilitek-ili9881c.c
> @@ -465,6 +465,19 @@ static int ili9881c_dsi_probe(struct mipi_dsi_device *dsi)
>  	dsi->mode_flags = MIPI_DSI_MODE_VIDEO_SYNC_PULSE;
>  	dsi->format = MIPI_DSI_FMT_RGB888;
>  	dsi->lanes = 4;
> +	/*
> +	 * The datasheet (table 39) specifies "limited clock channel speed"
> +	 * for 4 lanes as 550 Mbps for RGB888. As this is 4 bits at the time,
> +	 * the maximum HS frequency is 550/4 = 137.5 MHz.
> +	 */

Are you sure it is OK?

550Mbps is per lane, so since DSI is DoubleDataRate clock should be the
half of the speed - 275MHz.

Anyway more I think about it more doubts I have. hs_rate can be helpful
for command mode panels - panel refresh rate (provided by timings)
imposes only lower limit on the speed, max hs rate will impose upper limit.

In case of video mode, timings determines exactly value of the clock, so
max hs rate is unnecessary, am I right?


Regards

Andrzej



> +	dsi->hs_rate = 137500000;
> +	/*
> +	 * Table 42 says: "Length of LP-00, LP-01, LP-10 or LP-11 periods":
> +	 * min 50ns, max 75ns. 1/50ns = max LPM rate = 20.000.000Hz.
> +	 * Since LP transmissions are to periods per bit, this should be
> +	 * 10 MHz.
> +	 */
> +	dsi->lp_rate = 10000000;
>  
>  	return mipi_dsi_attach(dsi);
>  }