[v5,0/5] drm: Support basic DPCD backlight in panel-simple and add a new panel ATNA33XC20

This series adds the support for the eDP panel that needs the backlight
controlling over the DP AUX channel using DPCD registers of the panel
as per the VESA's standard.

This series also adds support for the Samsung eDP AMOLED panel that
needs DP AUX to control the backlight, and introduces new delays in the
@panel_desc.delay to support this panel.

This patch series depends on the following two series:
- Doug's series [1], exposed the DP AUX channel to the panel-simple.
- Lyude's series [2], introduced new drm helper functions for DPCD
  backlight.

This series is the logical successor to the series [3].

Changes in v1:
- Created dpcd backlight helper with very basic functionality, added
  backlight registration in the ti-sn65dsi86 bridge driver.

Changes in v2:
- Created a new DisplayPort aux backlight driver and moved the code from
  drm_dp_aux_backlight.c (v1) to the new driver.

Changes in v3:
- Fixed module compilation (kernel test bot).

Changes in v4:
- Added basic DPCD backlight support in panel-simple.
- Added support for a new Samsung panel ATNA33XC20 that needs DPCD
  backlight controlling and has a requirement of delays between enable
  GPIO and regulator.

Changes in v5:
Addressed review suggestions from Douglas:
- Created a new API drm_panel_dp_aux_backlight() in drm_panel.c
- Moved DP AUX backlight functions from panel-simple.c to drm_panel.c
- panel-simple probe() calls drm_panel_dp_aux_backlight() to create
  backlight when the backlight phandle is not specified in panel DT
  and DP AUX channel is present.
- Added check for drm_edp_backlight_supported() before registering.
- Removed the @uses_dpcd_backlight flag from panel_desc as this
  should be auto-detected.
- Updated comments/descriptions.  

[1] https://lore.kernel.org/dri-devel/20210525000159.3384921-1-dianders@chromium.org/
[2] https://lore.kernel.org/dri-devel/20210514181504.565252-1-lyude@redhat.com/
[3] https://lore.kernel.org/dri-devel/1619416756-3533-1-git-send-email-rajeevny@codeaurora.org/

Rajeev Nandan (5):
  drm/panel: add basic DP AUX backlight support
  drm/panel-simple: Support DP AUX backlight
  drm/panel-simple: Support for delays between GPIO & regulator
  dt-bindings: display: simple: Add Samsung ATNA33XC20
  drm/panel-simple: Add Samsung ATNA33XC20

 .../bindings/display/panel/panel-simple.yaml       |   2 +
 drivers/gpu/drm/drm_panel.c                        | 108 +++++++++++++++++++++
 drivers/gpu/drm/panel/panel-simple.c               |  67 +++++++++++++
 include/drm/drm_panel.h                            |  15 ++-
 4 files changed, 188 insertions(+), 4 deletions(-)

On Sun, 30 May 2021 21:26:11 +0530, Rajeev Nandan wrote:
> Add Samsung 13.3" FHD eDP AMOLED panel.
> 
> Signed-off-by: Rajeev Nandan <rajeevny@codeaurora.org>
> Reviewed-by: Douglas Anderson <dianders@chromium.org>
> ---
> 
> (no changes since v4)
> 
> Changes in v4:
> - New
> 
>  Documentation/devicetree/bindings/display/panel/panel-simple.yaml | 2 ++
>  1 file changed, 2 insertions(+)
> 

Acked-by: Rob Herring <robh@kernel.org>

Hi,

On Sun, May 30, 2021 at 8:57 AM Rajeev Nandan <rajeevny@codeaurora.org> wrote:
>
> +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);
> +       int ret = 0;
> +
> +       if (brightness > 0) {
> +               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;
> +               }
> +       }

I was trying to figure out if there are any races / locking problems /
problems trying to tweak the backlight when the panel is off. I don't
_think_ there are. Specifically:

1. Before turning the panel off, drm_panel will call
backlight_disable(). That will set BL_CORE_FBBLANK which is not set by
any other calls. Then it will call your
dp_aux_backlight_update_status().

2. Once BL_CORE_FBBLANK is set then backlight_get_brightness() will
always return 0.

This means that a transition from 0 -> non-zero (and enable) will
always only happen when the panel is on, which is good. It also means
that we'll always transition to 0 (disable the backlight) when the
panel turns off.

In terms of other races, it looks like the AUX transfer code handles
grabbing a mutex around transfers so we should be safe there.

So I guess the above is just a long-winded way of saying that this
looks right to me. :-)

BTW: we should probably make sure that the full set of people
identified by `./scripts/get_maintainer.pl -f
./drivers/video/backlight` are CCed on your series. I see Daniel
already and I've added the rest.


> +/**
> + * drm_panel_dp_aux_backlight - create and use DP AUX backlight
> + * @panel: DRM panel
> + * @aux: The DP AUX channel to use
> + *
> + * Use this function to create and handle backlight if your panel
> + * supports backlight control over DP AUX channel using DPCD
> + * registers as per VESA's standard backlight control interface.
> + *
> + * When the panel is enabled backlight will be enabled after a
> + * successful call to &drm_panel_funcs.enable()
> + *
> + * When the panel is disabled backlight will be disabled before the
> + * call to &drm_panel_funcs.disable().
> + *
> + * A typical implementation for a panel driver supporting backlight
> + * control over DP AUX will call this function at probe time.
> + * Backlight will then be handled transparently without requiring
> + * any intervention from the driver.
> + *
> + * drm_panel_dp_aux_backlight() must be called after the call to drm_panel_init().
> + *
> + * Return: 0 on success or a negative error code on failure.
> + */
> +int drm_panel_dp_aux_backlight(struct drm_panel *panel, struct drm_dp_aux *aux)
> +{
> +       struct dp_aux_backlight *bl;
> +       struct backlight_properties props = { 0 };
> +       u16 current_level;
> +       u8 current_mode;
> +       u8 edp_dpcd[EDP_DISPLAY_CTL_CAP_SIZE];
> +       int ret;
> +
> +       if (!panel || !panel->dev || !aux)
> +               return -EINVAL;
> +
> +       bl = devm_kzalloc(panel->dev, sizeof(*bl), GFP_KERNEL);
> +       if (!bl)
> +               return -ENOMEM;
> +
> +       bl->aux = aux;
> +
> +       ret = drm_dp_dpcd_read(aux, DP_EDP_DPCD_REV, edp_dpcd,
> +                              EDP_DISPLAY_CTL_CAP_SIZE);
> +       if (ret < 0)
> +               return ret;
> +
> +       if (!drm_edp_backlight_supported(edp_dpcd)) {
> +               DRM_DEV_INFO(panel->dev, "DP AUX backlight is not supported\n");
> +               return 0;
> +       }

nit: move this part up above the memory allocation. There's no reason
to allocate memory for "bl" if the backlight isn't supported.


> @@ -64,8 +65,8 @@ enum drm_panel_orientation;
>   * the panel. This is the job of the .unprepare() function.
>   *
>   * Backlight can be handled automatically if configured using
> - * drm_panel_of_backlight(). Then the driver does not need to implement the
> - * functionality to enable/disable backlight.
> + * drm_panel_of_backlight() or drm_panel_dp_aux_backlight(). Then the driver
> + * does not need to implement the functionality to enable/disable backlight.
>   */
>  struct drm_panel_funcs {
>         /**
> @@ -144,8 +145,8 @@ struct drm_panel {
>          * Backlight device, used to turn on backlight after the call
>          * to enable(), and to turn off backlight before the call to
>          * disable().
> -        * backlight is set by drm_panel_of_backlight() and drivers
> -        * shall not assign it.
> +        * backlight is set by drm_panel_of_backlight()/drm_panel_dp_aux_backlight()
> +        * and drivers shall not assign it.

Slight nit that I would have wrapped the drm_panel_dp_aux_backlight()
to the next line just to avoid one really long line followed by a
short one.

Other than the two nits (ordering of memory allocation and word
wrapping in a comment), this looks good to me. Feel free to add my
Reviewed-by tag when you fix the nits.

NOTE: Even though I have commit access to drm-misc now, I wouldn't
feel comfortable merging this to drm-misc myself without review
feedback from someone more senior. Obviously we're still blocked on my
and Lyude's series landing first, but even assuming those just land
as-is we'll need some more adult supervision before this can land. ;-)
That being said, I personally think this looks pretty nice now.


-Doug



>          */
>         struct backlight_device *backlight;
>
> @@ -208,11 +209,17 @@ static inline int of_drm_get_panel_orientation(const struct device_node *np,
>  #if IS_ENABLED(CONFIG_DRM_PANEL) && (IS_BUILTIN(CONFIG_BACKLIGHT_CLASS_DEVICE) || \
>         (IS_MODULE(CONFIG_DRM) && IS_MODULE(CONFIG_BACKLIGHT_CLASS_DEVICE)))
>  int drm_panel_of_backlight(struct drm_panel *panel);
> +int drm_panel_dp_aux_backlight(struct drm_panel *panel, struct drm_dp_aux *aux);
>  #else
>  static inline int drm_panel_of_backlight(struct drm_panel *panel)
>  {
>         return 0;
>  }
> +static inline int drm_panel_dp_aux_backlight(struct drm_panel *panel,
> +                                            struct drm_dp_aux *aux)
> +{
> +       return 0;
> +}
>  #endif
>
>  #endif
> --
> 2.7.4
>

Hi,

On Sun, May 30, 2021 at 8:57 AM Rajeev Nandan <rajeevny@codeaurora.org> wrote:
>
> Some panels datasheets may specify a delay between the enable GPIO and
> the regulator. Support this in panel-simple.
>
> Signed-off-by: Rajeev Nandan <rajeevny@codeaurora.org>
> ---
>
> Changes in v4:
> - New
>
> Changes in v5:
> - Update description (Douglas)
> - Warn if "power_to_enable" or "disable_to_power_off" is non-zero and panel->enable_gpio
>   is NULL (Douglas)
>
>  drivers/gpu/drm/panel/panel-simple.c | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
>
> diff --git a/drivers/gpu/drm/panel/panel-simple.c b/drivers/gpu/drm/panel/panel-simple.c
> index 047fad5..e3f5b7e 100644
> --- a/drivers/gpu/drm/panel/panel-simple.c
> +++ b/drivers/gpu/drm/panel/panel-simple.c
> @@ -133,6 +133,22 @@ struct panel_desc {
>                 unsigned int prepare_to_enable;
>
>                 /**
> +                * @delay.power_to_enable: Time for the power to enable the display on.
> +                *
> +                * The time (in milliseconds) to wait after powering up the display
> +                * before asserting its enable pin.
> +                */
> +               unsigned int power_to_enable;
> +
> +               /**
> +                * @delay.disable_to_power_off: Time for the disable to power the display off.
> +                *
> +                * The time (in milliseconds) to wait before powering off the display
> +                * after deasserting its enable pin.
> +                */
> +               unsigned int disable_to_power_off;
> +
> +               /**
>                  * @delay.enable: Time for the panel to display a valid frame.
>                  *
>                  * The time (in milliseconds) that it takes for the panel to
> @@ -347,6 +363,10 @@ static int panel_simple_suspend(struct device *dev)
>         struct panel_simple *p = dev_get_drvdata(dev);
>
>         gpiod_set_value_cansleep(p->enable_gpio, 0);
> +
> +       if (p->desc->delay.disable_to_power_off)
> +               msleep(p->desc->delay.disable_to_power_off);
> +
>         regulator_disable(p->supply);
>         p->unprepared_time = ktime_get();
>
> @@ -407,6 +427,9 @@ static int panel_simple_prepare_once(struct panel_simple *p)
>                 return err;
>         }
>
> +       if (p->desc->delay.power_to_enable)
> +               msleep(p->desc->delay.power_to_enable);
> +
>         gpiod_set_value_cansleep(p->enable_gpio, 1);
>
>         delay = p->desc->delay.prepare;
> @@ -782,6 +805,11 @@ static int panel_simple_probe(struct device *dev, const struct panel_desc *desc,
>                 break;
>         }
>
> +       if (!panel->enable_gpio && desc->delay.disable_to_power_off)
> +               dev_warn(dev, "Specify enable_gpio when using disable_to_power_off delay\n");
> +       if (!panel->enable_gpio && desc->delay.power_to_enable)
> +               dev_warn(dev, "Specify enable_gpio when using power_to_enable delay\n");

Last nit is that the warning messages could be a little confusing to
someone reading the logs. I guess the target audience of the error
message is probably someone doing bringup. That person specified a
panel in their device tree and maybe isn't even aware that they're
using "disable_to_power_off" or "power_to_enable". Maybe wording
instead:

Need a delay after disabling panel GPIO, but a GPIO wasn't provided.
Need a delay after enabling panel GPIO, but a GPIO wasn't provided.

That's definitely getting into nittiness, though and I wouldn't be
upset if the patch landed with the existing messages. Thus, with or
without the change to the error message:

Reviewed-by: Douglas Anderson <dianders@chromium.org>

On 03-06-2021 05:35, Doug Anderson wrote:
> Hi,
> 
> On Sun, May 30, 2021 at 8:57 AM Rajeev Nandan <rajeevny@codeaurora.org> 
> wrote:
>> 

> 
> Other than the two nits (ordering of memory allocation and word
> wrapping in a comment), this looks good to me. Feel free to add my
> Reviewed-by tag when you fix the nits.
> 
> NOTE: Even though I have commit access to drm-misc now, I wouldn't
> feel comfortable merging this to drm-misc myself without review
> feedback from someone more senior. Obviously we're still blocked on my
> and Lyude's series landing first, but even assuming those just land
> as-is we'll need some more adult supervision before this can land. ;-)
> That being said, I personally think this looks pretty nice now.
> 
> 
> -Doug

Thank you, Doug.

I'll address the review comments of this patch and another patch (v5 
3/5)
in the next spin. I'll wait for Lyude to check this series, as she 
wanted
to review it in a few days.


Thanks,
Rajeev