Re: [v3 2/2] backlight: Add DisplayPort aux backlight driver
From: Lyude Paul
Date: Fri Apr 30 2021 - 14:49:09 EST
JFYI for anyone who is interested, I will be respinning my patches for adding
backlight helpers very soon since we've got pretty much all of the prep work
for it upstream now
On Mon, 2021-04-26 at 12:49 +0300, Jani Nikula wrote:
> On Mon, 26 Apr 2021, Rajeev Nandan <rajeevny@xxxxxxxxxxxxxx> wrote:
> > Add backlight driver for the panels supporting backlight control
> > using DPCD registers on the DisplayPort aux channel.
>
> No, please don't do this.
>
> I wrote you last week in reply to v1 why I thought merging this would
> not be a good idea [1]. Why have you sent two versions since then
> without replying to me, or Cc'ing me or Lyude?
>
> I think it's an even worse idea to merge this to
> drivers/video/backlight. With DP AUX backlight you can't pretend it's
> just an independent aux interface for backlight without everything else
> around it. It's not independent of eDP, and exposing it as a direct
> backlight sysfs interface bypasses the encoder.
>
> And it still remains that there is existing DP AUX backlight code in
> use, in the tree, with more features than this, with plans and
> previously submitted patches to lift from one driver to drm core, and
> with patches to add support to another driver.
>
> I don't say this lightly, or very often at all, but,
>
> NAK.
>
>
> BR,
> Jani.
>
>
> [1] https://lore.kernel.org/dri-devel/871rb5bcf9.fsf@xxxxxxxxx/
>
> >
> > Changes in v2:
> > - New (most of the code reused from drm_dp_aux_backlight.c of v1)
> >
> > Changes in v3:
> > - Add missing ';' to fix module compilation (kernel test bot)
> >
> > Signed-off-by: Rajeev Nandan <rajeevny@xxxxxxxxxxxxxx>
> > ---
> > drivers/video/backlight/Kconfig | 7 +
> > drivers/video/backlight/Makefile | 1 +
> > drivers/video/backlight/dp_aux_backlight.c | 245
> > +++++++++++++++++++++++++++++
> > 3 files changed, 253 insertions(+)
> > create mode 100644 drivers/video/backlight/dp_aux_backlight.c
> >
> > diff --git a/drivers/video/backlight/Kconfig
> > b/drivers/video/backlight/Kconfig
> > index d83c87b..82c88f0 100644
> > --- a/drivers/video/backlight/Kconfig
> > +++ b/drivers/video/backlight/Kconfig
> > @@ -456,6 +456,13 @@ config BACKLIGHT_LED
> > If you have a LCD backlight adjustable by LED class driver, say
> > Y
> > to enable this driver.
> >
> > +config BACKLIGHT_DP_AUX
> > + tristate "DisplayPort aux backlight driver"
> > + depends on DRM && DRM_KMS_HELPER
> > + help
> > + If you have a panel backlight controlled by DPCD registers
> > + on the DisplayPort aux channel, say Y to enable this driver.
> > +
> > endif # BACKLIGHT_CLASS_DEVICE
> >
> > endmenu
> > diff --git a/drivers/video/backlight/Makefile
> > b/drivers/video/backlight/Makefile
> > index 685f3f1..ba23c7c 100644
> > --- a/drivers/video/backlight/Makefile
> > +++ b/drivers/video/backlight/Makefile
> > @@ -57,3 +57,4 @@ obj-$(CONFIG_BACKLIGHT_WM831X) +=
> > wm831x_bl.o
> > obj-$(CONFIG_BACKLIGHT_ARCXCNN) += arcxcnn_bl.o
> > obj-$(CONFIG_BACKLIGHT_RAVE_SP) += rave-sp-backlight.o
> > obj-$(CONFIG_BACKLIGHT_LED) += led_bl.o
> > +obj-$(CONFIG_BACKLIGHT_DP_AUX) += dp_aux_backlight.o
> > diff --git a/drivers/video/backlight/dp_aux_backlight.c
> > b/drivers/video/backlight/dp_aux_backlight.c
> > new file mode 100644
> > index 00000000..3398383
> > --- /dev/null
> > +++ b/drivers/video/backlight/dp_aux_backlight.c
> > @@ -0,0 +1,245 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * Backlight driver to control the brightness over DisplayPort aux
> > channel.
> > + */
> > +
> > +#include <linux/backlight.h>
> > +#include <linux/err.h>
> > +#include <linux/gpio/consumer.h>
> > +#include <linux/kernel.h>
> > +#include <linux/module.h>
> > +#include <linux/platform_device.h>
> > +#include <drm/drm_dp_helper.h>
> > +
> > +#define DP_AUX_MAX_BRIGHTNESS 0xffff
> > +
> > +/**
> > + * struct dp_aux_backlight - DisplayPort aux backlight data
> > + * @dev: pointer to our device.
> > + * @aux: the DisplayPort aux channel.
> > + * @enable_gpio: the backlight enable gpio.
> > + * @enabled: true if backlight is enabled else false.
> > + */
> > +struct dp_aux_backlight {
> > + struct device *dev;
> > + struct drm_dp_aux *aux;
> > + struct gpio_desc *enable_gpio;
> > + bool enabled;
> > +};
> > +
> > +static struct drm_dp_aux *i2c_to_aux(struct i2c_adapter *i2c)
> > +{
> > + return container_of(i2c, struct drm_dp_aux, ddc);
> > +}
> > +
> > +static int dp_aux_backlight_enable(struct dp_aux_backlight *aux_bl)
> > +{
> > + u8 val = 0;
> > + int ret;
> > +
> > + if (aux_bl->enabled)
> > + return 0;
> > +
> > + /* Set backlight control mode */
> > + ret = drm_dp_dpcd_readb(aux_bl->aux,
> > DP_EDP_BACKLIGHT_MODE_SET_REGISTER,
> > + &val);
> > + if (ret < 0)
> > + return ret;
> > +
> > + val &= ~DP_EDP_BACKLIGHT_CONTROL_MODE_MASK;
> > + val |= DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD;
> > + ret = drm_dp_dpcd_writeb(aux_bl->aux,
> > DP_EDP_BACKLIGHT_MODE_SET_REGISTER,
> > + val);
> > + if (ret < 0)
> > + return ret;
> > +
> > + /* Enable backlight */
> > + ret = drm_dp_dpcd_readb(aux_bl->aux,
> > DP_EDP_DISPLAY_CONTROL_REGISTER,
> > + &val);
> > + if (ret < 0)
> > + return ret;
> > +
> > + val |= DP_EDP_BACKLIGHT_ENABLE;
> > + ret = drm_dp_dpcd_writeb(aux_bl->aux,
> > DP_EDP_DISPLAY_CONTROL_REGISTER,
> > + val);
> > + if (ret < 0)
> > + return ret;
> > +
> > + if (aux_bl->enable_gpio)
> > + gpiod_set_value(aux_bl->enable_gpio, 1);
> > +
> > + aux_bl->enabled = true;
> > +
> > + return 0;
> > +}
> > +
> > +static int dp_aux_backlight_disable(struct dp_aux_backlight *aux_bl)
> > +{
> > + u8 val = 0;
> > + int ret;
> > +
> > + if (!aux_bl->enabled)
> > + return 0;
> > +
> > + if (aux_bl->enable_gpio)
> > + gpiod_set_value(aux_bl->enable_gpio, 0);
> > +
> > + ret = drm_dp_dpcd_readb(aux_bl->aux,
> > DP_EDP_DISPLAY_CONTROL_REGISTER,
> > + &val);
> > + if (ret < 0)
> > + return ret;
> > +
> > + val &= ~DP_EDP_BACKLIGHT_ENABLE;
> > + ret = drm_dp_dpcd_writeb(aux_bl->aux,
> > DP_EDP_DISPLAY_CONTROL_REGISTER,
> > + val);
> > + if (ret < 0)
> > + return ret;
> > +
> > + aux_bl->enabled = false;
> > +
> > + return 0;
> > +}
> > +
> > +static int dp_aux_backlight_update_status(struct backlight_device *bd)
> > +{
> > + struct dp_aux_backlight *aux_bl = bl_get_data(bd);
> > + u16 brightness = backlight_get_brightness(bd);
> > + u8 val[2] = { 0x0 };
> > + int ret = 0;
> > +
> > + if (brightness > 0) {
> > + val[0] = brightness >> 8;
> > + val[1] = brightness & 0xff;
> > + ret = drm_dp_dpcd_write(aux_bl->aux,
> > DP_EDP_BACKLIGHT_BRIGHTNESS_MSB,
> > + val, sizeof(val));
> > + if (ret < 0)
> > + return ret;
> > +
> > + dp_aux_backlight_enable(aux_bl);
> > + } else {
> > + dp_aux_backlight_disable(aux_bl);
> > + }
> > +
> > + return 0;
> > +}
> > +
> > +static int dp_aux_backlight_get_brightness(struct backlight_device *bd)
> > +{
> > + struct dp_aux_backlight *aux_bl = bl_get_data(bd);
> > + u8 val[2] = { 0x0 };
> > + int ret = 0;
> > +
> > + if (backlight_is_blank(bd))
> > + return 0;
> > +
> > + ret = drm_dp_dpcd_read(aux_bl->aux,
> > DP_EDP_BACKLIGHT_BRIGHTNESS_MSB,
> > + &val, sizeof(val));
> > + if (ret < 0)
> > + return ret;
> > +
> > + return (val[0] << 8 | val[1]);
> > +}
> > +
> > +static const struct backlight_ops aux_bl_ops = {
> > + .update_status = dp_aux_backlight_update_status,
> > + .get_brightness = dp_aux_backlight_get_brightness,
> > +};
> > +
> > +
> > +static int dp_aux_backlight_probe(struct platform_device *pdev)
> > +{
> > + struct dp_aux_backlight *aux_bl;
> > + struct backlight_device *bd;
> > + struct backlight_properties bl_props = { 0 };
> > + struct device_node *np;
> > + struct i2c_adapter *ddc;
> > + int ret = 0;
> > + u32 val;
> > +
> > + aux_bl = devm_kzalloc(&pdev->dev, sizeof(*aux_bl), GFP_KERNEL);
> > + if (!aux_bl)
> > + return -ENOMEM;
> > +
> > + aux_bl->dev = &pdev->dev;
> > +
> > + np = of_parse_phandle(pdev->dev.of_node, "ddc-i2c-bus", 0);
> > + if (!np) {
> > + dev_err(&pdev->dev, "failed to get aux ddc I2C bus\n");
> > + return -ENODEV;
> > + }
> > +
> > + ddc = of_find_i2c_adapter_by_node(np);
> > + of_node_put(np);
> > + if (!ddc)
> > + return -EPROBE_DEFER;
> > +
> > + aux_bl->aux = i2c_to_aux(ddc);
> > + dev_dbg(&pdev->dev, "using dp aux %s\n", aux_bl->aux->name);
> > +
> > + aux_bl->enable_gpio = devm_gpiod_get_optional(&pdev->dev,
> > "enable",
> > + GPIOD_OUT_LOW);
> > + if (IS_ERR(aux_bl->enable_gpio)) {
> > + ret = PTR_ERR(aux_bl->enable_gpio);
> > + goto free_ddc;
> > + }
> > +
> > + val = DP_AUX_MAX_BRIGHTNESS;
> > + of_property_read_u32(pdev->dev.of_node, "max-brightness", &val);
> > + if (val > DP_AUX_MAX_BRIGHTNESS)
> > + val = DP_AUX_MAX_BRIGHTNESS;
> > +
> > + bl_props.max_brightness = val;
> > + bl_props.brightness = val;
> > + bl_props.type = BACKLIGHT_RAW;
> > + bd = devm_backlight_device_register(&pdev->dev, dev_name(&pdev-
> > >dev),
> > + &pdev->dev, aux_bl,
> > + &aux_bl_ops, &bl_props);
> > + if (IS_ERR(bd)) {
> > + ret = PTR_ERR(bd);
> > + dev_err(&pdev->dev,
> > + "failed to register backlight (%d)\n", ret);
> > + goto free_ddc;
> > + }
> > +
> > + platform_set_drvdata(pdev, bd);
> > +
> > + return 0;
> > +
> > +free_ddc:
> > + if (ddc)
> > + put_device(&ddc->dev);
> > +
> > + return ret;
> > +}
> > +
> > +static int dp_aux_backlight_remove(struct platform_device *pdev)
> > +{
> > + struct backlight_device *bd = platform_get_drvdata(pdev);
> > + struct dp_aux_backlight *aux_bl = bl_get_data(bd);
> > + struct i2c_adapter *ddc = &aux_bl->aux->ddc;
> > +
> > + if (ddc)
> > + put_device(&ddc->dev);
> > +
> > + return 0;
> > +}
> > +
> > +static const struct of_device_id dp_aux_bl_of_match_table[] = {
> > + { .compatible = "dp-aux-backlight"},
> > + {},
> > +};
> > +MODULE_DEVICE_TABLE(of, dp_aux_bl_of_match_table);
> > +
> > +static struct platform_driver dp_aux_backlight_driver = {
> > + .driver = {
> > + .name = "dp-aux-backlight",
> > + .of_match_table = dp_aux_bl_of_match_table,
> > + },
> > + .probe = dp_aux_backlight_probe,
> > + .remove = dp_aux_backlight_remove,
> > +
> > +};
> > +module_platform_driver(dp_aux_backlight_driver);
> > +
> > +MODULE_DESCRIPTION("DisplayPort aux backlight driver");
> > +MODULE_LICENSE("GPL v2");
>
--
Cheers,
Lyude Paul (she/her)
Software Engineer at Red Hat