Date:	Thu, 12 Nov 2015 10:04:20 +0100
From:	Jacek Anaszewski <jacek.anaszewski@...il.com>
To:	"Kim, Milo" <milo.kim@...com>,
	Jacek Anaszewski <j.anaszewski@...sung.com>
CC:	devicetree@...r.kernel.org, lee.jones@...aro.org,
	linux-kernel@...r.kernel.org, linux-leds@...r.kernel.org
Subject: Re: [PATCH RESEND 15/16] leds: add LM3633 driver

Hi Milo,

On 11/11/2015 03:16 AM, Kim, Milo wrote:
> Hi Jacek,
>
> On 11/10/2015 10:44 PM, Jacek Anaszewski wrote:
>>>>> +static ssize_t lm3633_led_store_pattern_levels(struct device *dev,
>>>>> >>>+                           struct device_attribute *attr,
>>>>> >>>+                           const char *buf, size_t len)
>>>>> >>>+{
>>>>> >>>+    struct ti_lmu_led *lmu_led = to_ti_lmu_led(dev);
>>>>> >>>+    struct ti_lmu_led_chip *chip = lmu_led->chip;
>>>>> >>>+    unsigned int low, high;
>>>>> >>>+    u8 reg, offset, val;
>>>>> >>>+    int ret;
>>>>> >>>+
>>>>> >>>+    /*
>>>>> >>>+     * Sequence
>>>>> >>>+     *
>>>>> >>>+     * 1) Read pattern level data
>>>>> >>>+     * 2) Disable a bank before programming a pattern
>>>>> >>>+     * 3) Update LOW BRIGHTNESS register
>>>>> >>>+     * 4) Update HIGH BRIGHTNESS register
>>>>> >>>+     *
>>>>> >>>+     * Level register addresses have offset number based on the
>>>>> LED
>>>>> >>>bank.
>>>>> >>>+     */
>>>>> >>>+
>>>>> >>>+    ret = sscanf(buf, "%u %u", &low, &high);
>>>>> >>>+    if (ret != 2)
>>>>> >>>+        return -EINVAL;
>>>>> >>>+
>>>>> >>>+    low = min_t(unsigned int, low, LM3633_LED_MAX_BRIGHTNESS);
>>>> >>
>>>> >>Why don't you take into account the value defined by led-max-microamp
>>>> >>DT property here?
>>> >
>>> >'low' and 'high' are brightness value. The range is from 0 to 255.
>>> It is
>>> >mostly related with LED sysfs -/sys/class/led/<led>/brightness.
>>> >On the other hand, led-max-microamp is current limit. It is from 5mA to
>>> >30mA. It's different configuration in this device.
>> Doesn't the current has direct influence on the LED brightness?
>> Are there other means for adjusting brightness than current setting?
>> I see in your enum ti_lmu_max_current, that there are 26 possible
>> current levels. I think that they should reflect 26 possible
>> brightness levels, so max_brightness can be at most 26.
>
> Let me describe LM3633 in more details. I'd like to have your opinion
> about led-max-microamp and max_brightness usages. Datasheet would be
> better to figure out characteristics.
>
>      http://www.ti.com/lit/ds/symlink/lm3633.pdf
>
> Max current level is not same as brightness level in LM3633.
> LM3633 device has two parameters for output brightness control.
> One is brightness code(base register address is 0x44). The other is
> current limit(base register address is 0x22). It also provides hardware
> protection like excessive current.
>
>      0 <= brightness_code <= 0xff
>      0 <= current_limit_code <= 0x1f, it means
>      5mA <= current_limit <= 30mA.
>
> LM3633 device calculates the output current below.
>
>      (1) Linear brightness mode
>      Iout = current_limit * brightness_code/255
>
>      (2) Exponential brightness mode
>      Iout = current_limit * 0.85 ^ (44 - (brightness_code + 1)/5.18)
>
> So output current(Iout) can not be in excess of current_limit.
>
>>
>> led-max-microamp was designed for defining max brightness limit.
>> It should be converted into max brightness level in the driver and
>> assigned to max_brightness property of struct led_classdev.
>> This attribute overrides legacy 0-255 brightness scale.
>>
>
> It could be applied when brightness is determined by only one parameter
> - current level. Flash/torch device would be a good example. In this
> device, current setting is directly scaled to the output brightness.
>
> However, LM3633 has two parameters for the brightness control - current
> limit and brightness level. Max current setting is one of brightness
> control parameters. In this patch, 'led-max-microamp' from DT is
> converted to 'current_limit_code'. Then, this value is written once when
> the driver is initialized. On the other hand, 'brightness_code' can be
> changed at run time. And 'max_brightness' of led_classdev is set to max
> brightness register value, 0xff.
>
> It sounds 'led-max-microamp' property might not be a general usage in
> LM3633. Do you have some idea?

There are defined formulas for calculating the current for given
combination of current_limit and brightness_code. If you made one
of this parameters constant, then you'd be able to control the current
with the other one. I propose to set current limit to max, i.e.
"Full-Scale Current Setting" (base addr 0x22 for LVLEDn) should
be set to 0x1F (29.8 mA). Then, you can control the output current
with the registers "Control N Brightness" (base addr 0x44 for LVLEDn).

Taking above into account, you can set the led-max-microamp to the
exact max current value you want for given LED. The driver will be
responsible for clamping this to the greatest possible value of
"Control N Brightness", which will be assigned to max_brightness
property.

You can refer to drivers/leds/leds-aat1290.c, which presents
the same approach.

Apart from the above considerations - in the beginning of the
next week I will apply LED core optimizations to the for-next
branch of LED git tree, which will remove the need for using
work queues internally in LED class drivers. Please rebase your
code on those changes, remove work queue use from your driver,
and implement brightness_set_blocking op instead of brightness_set.
You can refer to the patches on devel branch, which adjust
flash drivers to those modifications.

git://git.kernel.org/pub/scm/linux/kernel/git/j.anaszewski/linux-leds.git
branch: devel
base commit:
26eb03b leds: core: Add two new LED_BLINK_ flags

-- 
Best Regards,
Jacek Anaszewski
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