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

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:

quoted

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.

quoted

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 <redacted>
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.

quoted

+	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.

quoted

+		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