linux-kernel - Re: [PATCH v6 13/13] video: backlight: mt6370: Add MediaTek MT6370 support

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Message-ID: <20220725103128.xtaw2c4y5fobowg7@maple.lan>
Date:   Mon, 25 Jul 2022 11:31:28 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     ChiaEn Wu <peterwu.pub@...il.com>
Cc:     lee.jones@...aro.org, jingoohan1@...il.com, pavel@....cz,
        robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org,
        matthias.bgg@...il.com, sre@...nel.org, chunfeng.yun@...iatek.com,
        gregkh@...uxfoundation.org, jic23@...nel.org, lars@...afoo.de,
        lgirdwood@...il.com, broonie@...nel.org, linux@...ck-us.net,
        heikki.krogerus@...ux.intel.com, deller@....de,
        andy.shevchenko@...il.com, chiaen_wu@...htek.com,
        alice_chen@...htek.com, cy_huang@...htek.com,
        dri-devel@...ts.freedesktop.org, linux-leds@...r.kernel.org,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-iio@...r.kernel.org, linux-fbdev@...r.kernel.org,
        szunichen@...il.com
Subject: Re: [PATCH v6 13/13] video: backlight: mt6370: Add MediaTek MT6370
 support

On Fri, Jul 22, 2022 at 06:24:07PM +0800, ChiaEn Wu wrote:
> diff --git a/drivers/video/backlight/Kconfig b/drivers/video/backlight/Kconfig
> index a003e02..846dbe7 100644
> --- a/drivers/video/backlight/Kconfig
> +++ b/drivers/video/backlight/Kconfig
> @@ -268,6 +268,18 @@ config BACKLIGHT_MAX8925
>  	  If you have a LCD backlight connected to the WLED output of MAX8925
>  	  WLED output, say Y here to enable this driver.
>
> +config BACKLIGHT_MT6370
> +	tristate "MediaTek MT6370 Backlight Driver"
> +	depends on MFD_MT6370
> +	help
> +	  This enables support for Mediatek MT6370 Backlight driver.
> +	  It's commonly used to drive the display WLED. There are 4 channels
> +	  inside, and each channel supports up to 30mA of current capability
> +	  with 2048 current steps in exponential or linear mapping curves.

Does the MT6372 support more steps than this? In other words does it use
a fourteen bit scale or does it use an 11-bit scale at a different
register location?


> +
> +	  This driver can also be built as a module. If so, the module
> +	  will be called "mt6370-backlight".
> +
> [...]
> diff --git a/drivers/video/backlight/mt6370-backlight.c b/drivers/video/backlight/mt6370-backlight.c
> new file mode 100644
> index 0000000..ba00a8f
> --- /dev/null
> +++ b/drivers/video/backlight/mt6370-backlight.c
> [...]
> +static int mt6370_bl_update_status(struct backlight_device *bl_dev)
> +{
> +	struct mt6370_priv *priv = bl_get_data(bl_dev);
> +	int brightness = backlight_get_brightness(bl_dev);
> +	unsigned int enable_val;
> +	u8 brightness_val[2];
> +	int ret;
> +
> +	if (brightness) {
> +		brightness_val[0] = (brightness - 1) & MT6370_BL_DIM2_MASK;
> +		brightness_val[1] = (brightness - 1) >> fls(MT6370_BL_DIM2_MASK);
> +
> +		/*
> +		 * To make MT6372 using 14 bits to control the brightness
> +		 * backward compatible with 11 bits brightness control
> +		 * (like MT6370 and MT6371 do), we left shift the value
> +		 * and pad with 1 to remaining bits. Hence, the MT6372's
> +		 * backlight brightness will be almost the same as MT6370's
> +		 * and MT6371's.
> +		 */
> +		if (priv->vid_type == MT6370_VID_6372) {
> +			brightness_val[0] <<= MT6370_BL_DIM2_6372_SHIFT;
> +			brightness_val[0] |= MT6370_BL_DUMMY_6372_MASK;
> +		}

This somewhat depends on the answer to the first question above, but
what is the point of this shifting? If the range is 14-bit then the
driver should set max_brightness to 16384 and present the full range of
the MT6372 to the user.

Especially when using linear mappings (which are a totally pointless
scale to use for a backlight) the extra steps are useful for backlight
animation.


Daniel.