Re: [v7 1/5] drm/panel: add basic DP AUX backlight support

[Sam Ravnborg <sam@ravnborg.org>]

Hi Rajeev

On Sat, Jun 19, 2021 at 04:10:26PM +0530, Rajeev Nandan wrote:
> Some panels support backlight control over DP AUX channel using
> VESA's standard backlight control interface.
> Using new DRM eDP backlight helpers, add support to create and
> register a backlight for those panels in drm_panel to simplify
> the panel drivers.
> 
> The panel driver with access to "struct drm_dp_aux" can create and
> register a backlight device using following code snippet in its
> probe() function:
> 
> 	err = drm_panel_dp_aux_backlight(panel, aux);
> 	if (err)
> 		return err;

IT very good to have this supported by drm_panel, so we avoid
bolierplate in various drivers.

> 
> Then drm_panel will handle backlight_(enable|disable) calls
> similar to the case when drm_panel_of_backlight() is used.
> 
> Currently, we are not supporting one feature where the source
> device can combine the backlight brightness levels set through
> DP AUX and the BL_PWM_DIM eDP connector pin. Since it's not
> required for the basic backlight controls, it can be added later.
> 
> Signed-off-by: Rajeev Nandan <rajeevny@codeaurora.org>
> Reviewed-by: Douglas Anderson <dianders@chromium.org>
> Reviewed-by: Lyude Paul <lyude@redhat.com>
> ---
> 
> (no changes since v6)
> 
> Changes in v5:
> - New
> 
> Changes in v6:
> - Fixed ordering of memory allocation (Douglas)
> - Updated word wrapping in a comment (Douglas)
> 
>  drivers/gpu/drm/drm_panel.c | 108 ++++++++++++++++++++++++++++++++++++++++++++
>  include/drm/drm_panel.h     |  15 ++++--
>  2 files changed, 119 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_panel.c b/drivers/gpu/drm/drm_panel.c
> index f634371..9e65342 100644
> --- a/drivers/gpu/drm/drm_panel.c
> +++ b/drivers/gpu/drm/drm_panel.c
> @@ -26,12 +26,20 @@
>  #include <linux/module.h>
>  
>  #include <drm/drm_crtc.h>
> +#include <drm/drm_dp_helper.h>
>  #include <drm/drm_panel.h>
>  #include <drm/drm_print.h>
>  
>  static DEFINE_MUTEX(panel_lock);
>  static LIST_HEAD(panel_list);
>  
> +struct dp_aux_backlight {
> +	struct backlight_device *base;
> +	struct drm_dp_aux *aux;
> +	struct drm_edp_backlight_info info;
> +	bool enabled;
> +};
> +
>  /**
>   * DOC: drm panel
>   *
> @@ -342,6 +350,106 @@ int drm_panel_of_backlight(struct drm_panel *panel)
>  	return 0;
>  }
>  EXPORT_SYMBOL(drm_panel_of_backlight);
> +
> +static int dp_aux_backlight_update_status(struct backlight_device *bd)
> +{
> +	struct dp_aux_backlight *bl = bl_get_data(bd);
> +	u16 brightness = backlight_get_brightness(bd);
backlight_get_brightness() returns an int, so using u16 seems wrong.
But then drm_edp_backlight_enable() uses u16 for level - so I guess it
is OK.
We use unsigned long, int, u16 for brightness. Looks like something one
could look at one day, but today is not that day.

> +	int ret = 0;
> +
> +	if (brightness > 0) {
Use backlight_is_blank(bd) here, as this is really what you test for.

I cannot see why you need the extra check on ->enabled?
Would it be sufficient to check backlight_is_blank() only?

> +		if (!bl->enabled) {
> +			drm_edp_backlight_enable(bl->aux, &bl->info, brightness);
> +			bl->enabled = true;
> +			return 0;
> +		}
> +		ret = drm_edp_backlight_set_level(bl->aux, &bl->info, brightness);
> +	} else {
> +		if (bl->enabled) {
> +			drm_edp_backlight_disable(bl->aux, &bl->info);
> +			bl->enabled = false;
> +		}
> +	}
> +
> +	return ret;
> +}

	Sam