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

[rajeevny]

Hi Sam,

On 20-06-2021 15:01, Sam Ravnborg wrote:
> 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@xxxxxxxxxxxxxx>
> > Reviewed-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> > Reviewed-by: Lyude Paul <lyude@xxxxxxxxxx>
> > ---
> >
> > (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.

The backlight_get_brightness() used above has the backlight_is_blank() 
check and returns brightness 0 when the backlight_is_blank(bd) is true.
So, instead of calling backlight_is_blank(bd), we are checking 
brightness value here.
I took the reference from pwm_backlight_update_status() of the PWM 
backlight driver (drivers/video/backlight/pwm_bl.c)

Yes, we can change this _if_ condition to use backlight_is_blank(bd), as 
this is an inline function, and is more meaningful.
With this, there would be one change in the behavior of 
_backlight_update_status function in the following case:

- Setting brightness=0 when the backlight is not blank:
In the current case setting brightness=0 is disabling the backlight.
In the new case, setting brightness=0 will set the brightness to 0 and 
will do nothing to backlight disable.

I think that should not be a problem?

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

This extra check on bl->enabled flag is added to avoid 
enabling/disabling backlight again if it is already enabled/disabled.
Using this flag way can know the transition between backlight blank and 
un-blank, and decide when to enable/disable the backlight.

> > +		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

Thanks,
Rajeev