[RFC PATCH v3 1/3] drm/mipi-dsi: Fix byte order of 16-bit DCS set/get brightness

[Neil Armstrong]

On 16/01/2023 23:49, Richard Acayan wrote:
> From: Daniel Mentz <danielmentz at google.com>
> 
> The MIPI DCS specification demands that brightness values are sent in
> big endian byte order. It also states that one parameter (i.e. one byte)
> shall be sent/received for 8 bit wide values, and two parameters shall
> be used for values that are between 9 and 16 bits wide.
> 
> Add new functions to properly handle 16-bit brightness in big endian,
> since the two 8- and 16-bit cases are distinct from each other.
> 
> Fixes: 1a9d759331b8 ("drm/dsi: Implement DCS set/get display brightness")
> Signed-off-by: Daniel Mentz <danielmentz at google.com>
> Link: https://android.googlesource.com/kernel/msm/+/754affd62d0ee268c686c53169b1dbb7deac8550
> [richard: fix 16-bit brightness_get]
> [richard: use separate functions instead of switch/case]
> [richard: split into 16-bit component]
> Signed-off-by: Richard Acayan <mailingradian at gmail.com>
> Tested-by: Caleb Connolly <caleb at connolly.tech>
> ---
>   drivers/gpu/drm/drm_mipi_dsi.c | 52 ++++++++++++++++++++++++++++++++++
>   include/drm/drm_mipi_dsi.h     |  4 +++
>   2 files changed, 56 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_mipi_dsi.c b/drivers/gpu/drm/drm_mipi_dsi.c
> index 497ef4b6a90a..4bc15fbd009d 100644
> --- a/drivers/gpu/drm/drm_mipi_dsi.c
> +++ b/drivers/gpu/drm/drm_mipi_dsi.c
> @@ -1224,6 +1224,58 @@ int mipi_dsi_dcs_get_display_brightness(struct mipi_dsi_device *dsi,
>   }
>   EXPORT_SYMBOL(mipi_dsi_dcs_get_display_brightness);
>   
> +/**
> + * mipi_dsi_dcs_set_display_brightness_large() - sets the 16-bit brightness value
> + *    of the display
> + * @dsi: DSI peripheral device
> + * @brightness: brightness value
> + *
> + * Return: 0 on success or a negative error code on failure.
> + */
> +int mipi_dsi_dcs_set_display_brightness_large(struct mipi_dsi_device *dsi,
> +					     u16 brightness)
> +{
> +	u8 payload[2] = { brightness >> 8, brightness & 0xff };
> +	ssize_t err;
> +
> +	err = mipi_dsi_dcs_write(dsi, MIPI_DCS_SET_DISPLAY_BRIGHTNESS,
> +				 payload, sizeof(payload));
> +	if (err < 0)
> +		return err;
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(mipi_dsi_dcs_set_display_brightness_large);
> +
> +/**
> + * mipi_dsi_dcs_get_display_brightness_large() - gets the current 16-bit
> + *    brightness value of the display
> + * @dsi: DSI peripheral device
> + * @brightness: brightness value
> + *
> + * Return: 0 on success or a negative error code on failure.
> + */
> +int mipi_dsi_dcs_get_display_brightness_large(struct mipi_dsi_device *dsi,
> +					     u16 *brightness)
> +{
> +	u8 brightness_be[2];
> +	ssize_t err;
> +
> +	err = mipi_dsi_dcs_read(dsi, MIPI_DCS_GET_DISPLAY_BRIGHTNESS,
> +				brightness_be, sizeof(brightness_be));
> +	if (err <= 0) {
> +		if (err == 0)
> +			err = -ENODATA;
> +
> +		return err;
> +	}
> +
> +	*brightness = (brightness_be[0] << 8) | brightness_be[1];
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(mipi_dsi_dcs_get_display_brightness_large);
> +
>   static int mipi_dsi_drv_probe(struct device *dev)
>   {
>   	struct mipi_dsi_driver *drv = to_mipi_dsi_driver(dev->driver);
> diff --git a/include/drm/drm_mipi_dsi.h b/include/drm/drm_mipi_dsi.h
> index 4f503d99f668..16f30975b22b 100644
> --- a/include/drm/drm_mipi_dsi.h
> +++ b/include/drm/drm_mipi_dsi.h
> @@ -296,6 +296,10 @@ int mipi_dsi_dcs_set_display_brightness(struct mipi_dsi_device *dsi,
>   					u16 brightness);
>   int mipi_dsi_dcs_get_display_brightness(struct mipi_dsi_device *dsi,
>   					u16 *brightness);
> +int mipi_dsi_dcs_set_display_brightness_large(struct mipi_dsi_device *dsi,
> +					     u16 brightness);
> +int mipi_dsi_dcs_get_display_brightness_large(struct mipi_dsi_device *dsi,
> +					     u16 *brightness);
>   
>   /**
>    * mipi_dsi_generic_write_seq - transmit data using a generic write packet

Reviewed-by: Neil Armstrong <neil.armstrong at linaro.org>