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Date:   Mon, 9 Oct 2023 10:21:03 +0100
From:   Daniel Thompson <daniel.thompson@linaro.org>
To:     Duje =?utf-8?Q?Mihanovi=C4=87?= <duje.mihanovic@skole.hr>
Cc:     Lee Jones <lee@kernel.org>, Jingoo Han <jingoohan1@gmail.com>,
        Pavel Machek <pavel@ucw.cz>, Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Conor Dooley <conor+dt@kernel.org>,
        Helge Deller <deller@gmx.de>, Karel Balej <balejk@matfyz.cz>,
        dri-devel@lists.freedesktop.org, linux-leds@vger.kernel.org,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-fbdev@vger.kernel.org, ~postmarketos/upstreaming@lists.sr.ht
Subject: Re: [PATCH 2/2] backlight: Add Kinetic KTD2801 driver
Message-ID: <20231009092103.GB96854@aspen.lan>
References: <20231005-ktd2801-v1-0-43cd85b0629a@skole.hr>
 <20231005-ktd2801-v1-2-43cd85b0629a@skole.hr>
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In-Reply-To: <20231005-ktd2801-v1-2-43cd85b0629a@skole.hr>
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On Thu, Oct 05, 2023 at 08:49:09PM +0200, Duje Mihanović wrote:
> Add driver for the Kinetic KTD2801 backlight driver.
>
> Signed-off-by: Duje Mihanović <duje.mihanovic@skole.hr>
> ---
>  MAINTAINERS                                 |   6 ++
>  drivers/video/backlight/Kconfig             |   7 ++
>  drivers/video/backlight/Makefile            |   1 +
>  drivers/video/backlight/ktd2801-backlight.c | 151 ++++++++++++++++++++++++++++
>  4 files changed, 165 insertions(+)
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 35977b269d5e..7da78f06a65d 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -11777,6 +11777,12 @@ S:	Maintained
>  F:	Documentation/devicetree/bindings/leds/backlight/kinetic,ktd253.yaml
>  F:	drivers/video/backlight/ktd253-backlight.c
>
> +KTD2801 BACKLIGHT DRIVER
> +M:	Duje Mihanović <duje.mihanovic@skole.hr>
> +S:	Maintained
> +F:	Documentation/devicetree/bindings/leds/backlight/kinetic,ktd2801.yaml
> +F:	drivers/video/backlight/ktd2801-backlight.c
> +
>  KTEST
>  M:	Steven Rostedt <rostedt@goodmis.org>
>  M:	John Hawley <warthog9@eaglescrag.net>
> diff --git a/drivers/video/backlight/Kconfig b/drivers/video/backlight/Kconfig
> index 51387b1ef012..a2b268293345 100644
> --- a/drivers/video/backlight/Kconfig
> +++ b/drivers/video/backlight/Kconfig
> @@ -183,6 +183,13 @@ config BACKLIGHT_KTD253
>  	  which is a 1-wire GPIO-controlled backlight found in some mobile
>  	  phones.
>
> +config BACKLIGHT_KTD2801
> +	tristate "Backlight Driver for Kinetic KTD2801"
> +	depends on GPIOLIB || COMPILE_TEST
> +	help
> +	  Say Y to enable the backlight driver for the Kinetic KTD2801 1-wire
> +	  GPIO-controlled backlight found in Samsung Galaxy Core Prime VE LTE.
> +
>  config BACKLIGHT_KTZ8866
>  	tristate "Backlight Driver for Kinetic KTZ8866"
>  	depends on I2C
> diff --git a/drivers/video/backlight/Makefile b/drivers/video/backlight/Makefile
> index f72e1c3c59e9..b33b647f31ca 100644
> --- a/drivers/video/backlight/Makefile
> +++ b/drivers/video/backlight/Makefile
> @@ -35,6 +35,7 @@ obj-$(CONFIG_BACKLIGHT_HP680)		+= hp680_bl.o
>  obj-$(CONFIG_BACKLIGHT_HP700)		+= jornada720_bl.o
>  obj-$(CONFIG_BACKLIGHT_IPAQ_MICRO)	+= ipaq_micro_bl.o
>  obj-$(CONFIG_BACKLIGHT_KTD253)		+= ktd253-backlight.o
> +obj-$(CONFIG_BACKLIGHT_KTD2801)		+= ktd2801-backlight.o
>  obj-$(CONFIG_BACKLIGHT_KTZ8866)		+= ktz8866.o
>  obj-$(CONFIG_BACKLIGHT_LM3533)		+= lm3533_bl.o
>  obj-$(CONFIG_BACKLIGHT_LM3630A)		+= lm3630a_bl.o
> diff --git a/drivers/video/backlight/ktd2801-backlight.c b/drivers/video/backlight/ktd2801-backlight.c
> new file mode 100644
> index 000000000000..24a5f9e5d606
> --- /dev/null
> +++ b/drivers/video/backlight/ktd2801-backlight.c
> @@ -0,0 +1,151 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +#include <linux/backlight.h>
> +#include <linux/delay.h>
> +#include <linux/gpio/consumer.h>
> +#include <linux/of.h>
> +#include <linux/platform_device.h>
> +#include <linux/property.h>
> +
> +#define EW_DELAY	150
> +#define EW_DET		270
> +#define LOW_BIT_HIGH	5
> +#define LOW_BIT_LOW	(4 * HIGH_BIT_LOW)
> +#define HIGH_BIT_LOW	5
> +#define HIGH_BIT_HIGH	(4 * HIGH_BIT_LOW)

These names are pretty cryptic (they don't even mention that they
are time values and that the unit is microseconds). They also look
like they were derived by tuning so comments would be nice explaining
where they come from (or, failing that, why they are correct).


> +#define DS		5
> +#define EOD_L		10
> +#define EOD_H		350
> +#define PWR_DOWN_DELAY	2600
> +
> +#define KTD2801_DEFAULT_BRIGHTNESS	100
> +#define KTD2801_MAX_BRIGHTNESS		255
> +
> +struct ktd2801_backlight {
> +	struct device *dev;
> +	struct backlight_device *bd;
> +	struct gpio_desc *desc;
> +	bool was_on;
> +};
> +
> +static int ktd2801_update_status(struct backlight_device *bd)
> +{
> +	struct ktd2801_backlight *ktd2801 = bl_get_data(bd);
> +	u8 brightness = (u8) backlight_get_brightness(bd);
> +
> +	if (backlight_is_blank(bd)) {
> +		gpiod_set_value(ktd2801->desc, 1);
> +		udelay(PWR_DOWN_DELAY);
> +		ktd2801->was_on = false;
> +		return 0;
> +	}
> +
> +	if (!ktd2801->was_on) {
> +		gpiod_set_value(ktd2801->desc, 0);
> +		udelay(EW_DELAY);
> +		gpiod_set_value(ktd2801->desc, 1);
> +		udelay(EW_DET);
> +		gpiod_set_value(ktd2801->desc, 0);
> +		ktd2801->was_on = true;
> +	}

Isn't this implementing the same single GPIO line protocol used by
drivers/leds/flash/leds-ktd2692.c?

If so, it would be good to pull the expresswire handling into a library
so it can be shared between drivers. leds-ktd2692.c does a pretty
good job of decomposing the expresswire management into functions (e.g.
separating data framing from setting of control values). Expresswire is
a data framing protocol rather than a bus so I think just implementing
it as library code is probably sufficient.

Also, can the expresswire code have protocol-violation watchdogs that
trigger a re-transmit of the message if we get pre-empted in the middle
of sending a message to the backlight (see calls to ktime_get_ns() in
ktd253-backlight.c ).


> +
> +	gpiod_set_value(ktd2801->desc, 0);
> +	udelay(DS);
> +
> +	for (int i = 0; i < 8; i++) {
> +		u8 next_bit = (brightness & 0x80) >> 7;
> +
> +		if (!next_bit) {
> +			gpiod_set_value(ktd2801->desc, 1);
> +			udelay(LOW_BIT_LOW);
> +			gpiod_set_value(ktd2801->desc, 0);
> +			udelay(LOW_BIT_HIGH);
> +		} else {
> +			gpiod_set_value(ktd2801->desc, 1);
> +			udelay(HIGH_BIT_LOW);
> +			gpiod_set_value(ktd2801->desc, 0);
> +			udelay(HIGH_BIT_HIGH);
> +		}
> +		brightness <<= 1;
> +	}
> +	gpiod_set_value(ktd2801->desc, 1);
> +	udelay(EOD_L);
> +	gpiod_set_value(ktd2801->desc, 0);
> +	udelay(EOD_H);
> +	return 0;
> +}
> +
> +static const struct backlight_ops ktd2801_backlight_ops = {
> +	.update_status = ktd2801_update_status,
> +};
> +
> +static int ktd2801_backlight_probe(struct platform_device *pdev)
> +{
> +	struct device *dev = &pdev->dev;
> +	struct backlight_device *bd;
> +	struct ktd2801_backlight *ktd2801;
> +	u32 brightness, max_brightness;
> +	int ret;
> +
> +	ktd2801 = devm_kzalloc(dev, sizeof(*ktd2801), GFP_KERNEL);
> +	if (!ktd2801)
> +		return -ENOMEM;
> +	ktd2801->dev = dev;

This appears to be a write-only variable.

Overall the driver looks good but it would be nice to figure out how to
share expresswire framing between drivers (don't worry about ktd253, it
uses a different protocol).


Daniel.