Re: [PATCH 2/2] drm/dsi: Implement dcs backlight brightness

[Jani Nikula <jani.nikula@linux.intel.com>]

On Thu, 02 Jun 2016, Vinay Simha BN <simhavcs@gmail.com> wrote:
> Provide a small convenience wrapper that set/get the
> backlight brightness control and creates the backlight
> device for the panel interface

To be pedantic, we should downplay "backlight" in the DSI DCS brightness
control... there need not be a backlight, at all, for brightness control
(see AMOLED).

> Cc: John Stultz <john.stultz@linaro.org>
> Cc: Sumit Semwal <sumit.semwal@linaro.org>
> Cc: Archit Taneja <archit.taneja@gmail.com>
> Cc: Rob Clark <robdclark@gmail.com>
> Signed-off-by: Vinay Simha BN <simhavcs@gmail.com>
>
> --
> v1:
>  *tested in nexus7 2nd gen.
> ---
>  drivers/gpu/drm/drm_mipi_dsi.c | 63 ++++++++++++++++++++++++++++++++++++++++++
>  include/drm/drm_mipi_dsi.h     |  3 ++
>  2 files changed, 66 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_mipi_dsi.c b/drivers/gpu/drm/drm_mipi_dsi.c
> index 2f0b85c..ac4521e 100644
> --- a/drivers/gpu/drm/drm_mipi_dsi.c
> +++ b/drivers/gpu/drm/drm_mipi_dsi.c
> @@ -1026,6 +1026,69 @@ int mipi_dsi_dcs_set_pixel_format(struct mipi_dsi_device *dsi, u8 format)
>  }
>  EXPORT_SYMBOL(mipi_dsi_dcs_set_pixel_format);
>  
> +static int dsi_dcs_bl_get_brightness(struct backlight_device *bl)

I'd rather see convenience helpers that are not tied to struct
backlight_device. You can add a one-line wrapper on top that takes
struct backlight_device pointer.

While at it, please name the helpers according to the DCS command.

> +{
> +	struct mipi_dsi_device *dsi = bl_get_data(bl);
> +	int ret;
> +	u8 brightness;
> +
> +	dsi->mode_flags &= ~MIPI_DSI_MODE_LPM;
> +
> +	ret = mipi_dsi_dcs_read(dsi, MIPI_DCS_GET_DISPLAY_BRIGHTNESS,
> +				&brightness, sizeof(brightness));
> +	if (ret < 0)
> +		return ret;
> +
> +	dsi->mode_flags |= MIPI_DSI_MODE_LPM;
> +
> +	return brightness;
> +}
> +
> +static int dsi_dcs_bl_update_status(struct backlight_device *bl)
> +{
> +	struct mipi_dsi_device *dsi = bl_get_data(bl);
> +	int ret;
> +	u8 brightness = bl->props.brightness;
> +
> +	dsi->mode_flags &= ~MIPI_DSI_MODE_LPM;
> +
> +	ret = mipi_dsi_dcs_write(dsi, MIPI_DCS_SET_DISPLAY_BRIGHTNESS,
> +				 &brightness, sizeof(brightness));
> +	if (ret < 0)
> +		return ret;
> +
> +	dsi->mode_flags |= MIPI_DSI_MODE_LPM;
> +
> +	return 0;
> +}
> +
> +static const struct backlight_ops dsi_bl_ops = {
> +	.update_status = dsi_dcs_bl_update_status,
> +	.get_brightness = dsi_dcs_bl_get_brightness,
> +};
> +
> +/**
> + * drm_panel_create_dsi_backlight() - creates the backlight device for the panel
> + * @dsi: DSI peripheral device
> + *
> + * @return a struct backlight on success, or an ERR_PTR on error
> + */
> +struct backlight_device
> +		*drm_panel_create_dsi_backlight(struct mipi_dsi_device *dsi)
> +{
> +	struct device *dev = &dsi->dev;
> +	struct backlight_properties props;
> +
> +	memset(&props, 0, sizeof(props));
> +	props.type = BACKLIGHT_RAW;
> +	props.brightness = 255;
> +	props.max_brightness = 255;

The DCS spec says the max is either 0xff or 0xffff depending on the
implementation... this obviously affects the helpers as well.

And how about the backlight bits in write_control_display? I just fear
this is a simplistic view of brightness control on DSI, and this will
grow hairy over time. I think I'd rather see generic DSI DCS brightness
*helpers* in this file, and then *perhaps* a separate file for
brightness control in general.

BR,
Jani.

> +
> +	return devm_backlight_device_register(dev, dev_name(dev), dev, dsi,
> +					      &dsi_bl_ops, &props);
> +}
> +EXPORT_SYMBOL(drm_panel_create_dsi_backlight);
> +
>  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 2788dbe..9dd6a97 100644
> --- a/include/drm/drm_mipi_dsi.h
> +++ b/include/drm/drm_mipi_dsi.h
> @@ -12,6 +12,7 @@
>  #ifndef __DRM_MIPI_DSI_H__
>  #define __DRM_MIPI_DSI_H__
>  
> +#include <linux/backlight.h>
>  #include <linux/device.h>
>  
>  struct mipi_dsi_host;
> @@ -269,6 +270,8 @@ int mipi_dsi_dcs_set_tear_off(struct mipi_dsi_device *dsi);
>  int mipi_dsi_dcs_set_tear_on(struct mipi_dsi_device *dsi,
>  			     enum mipi_dsi_dcs_tear_mode mode);
>  int mipi_dsi_dcs_set_pixel_format(struct mipi_dsi_device *dsi, u8 format);
> +struct backlight_device
> +	*drm_panel_create_dsi_backlight(struct mipi_dsi_device *dsi);
>  
>  /**
>   * struct mipi_dsi_driver - DSI driver

-- 
Jani Nikula, Intel Open Source Technology Center