Re: [PATCH v3 2/2] leds: Add driver for NCP5623 3-channel I2C LED driver

[Florian Vaussard]

Hi Jacek,

Thank you for your comments!

Le 18. 09. 16 à 20:20, Jacek Anaszewski a écrit :
> Hi Florian,
> 
> Thanks for the updated patch set. I have few comments below.
> 
> On 09/16/2016 01:34 PM, Florian Vaussard wrote:
>> The NCP5623 is a 3-channel LED driver from On Semiconductor controlled
>> through I2C. The PWM of each channel can be independently set with 32
>> distinct levels. In addition, the intensity of the current source can be
>> globally set using an external bias resistor fixing the reference
>> current (Iref) and a dedicated register (ILED), following the
>> relationship:
>>
>> I = 2400*Iref/(31-ILED)
>>
>> with Iref = Vref/Rbias, and Vref = 0.6V.
>>
>> Signed-off-by: Florian Vaussard <florian.vaussard@xxxxxxxxxx>
>> ---
>>  drivers/leds/Kconfig        |  11 +++
>>  drivers/leds/Makefile       |   1 +
>>  drivers/leds/leds-ncp5623.c | 234 ++++++++++++++++++++++++++++++++++++++++++++
>>  3 files changed, 246 insertions(+)
>>  create mode 100644 drivers/leds/leds-ncp5623.c
>>

[...]

>> diff --git a/drivers/leds/leds-ncp5623.c b/drivers/leds/leds-ncp5623.c
>> new file mode 100644
>> index 0000000..875785a
>> --- /dev/null
>> +++ b/drivers/leds/leds-ncp5623.c
>> @@ -0,0 +1,234 @@
>> +/*
>> + * Copyright 2016 Florian Vaussard <florian.vaussard@xxxxxxxxxx>
>> + *
>> + * Based on leds-tlc591xx.c
> 
> I think that besides LED class facilities the only
> common thing is led_no. I'd skip this statement.
> 
>> + *
>> + * This program is free software; you can redistribute it and/or modify
>> + * it under the terms of the GNU General Public License as published by
>> + * the Free Software Foundation; version 2 of the License.
>> + */
>> +
>> +#include <linux/bitops.h>
>> +#include <linux/i2c.h>
>> +#include <linux/kernel.h>
>> +#include <linux/leds.h>
>> +#include <linux/module.h>
>> +#include <linux/of.h>
>> +#include <linux/of_device.h>
>> +#include <linux/slab.h>
>> +
>> +#define NCP5623_MAX_LEDS    3
>> +#define NCP5623_MAX_STEPS    31
>> +#define NCP5623_MAX_CURRENT    31
> 
> Both values refer in fact to the same thing.
> And actually the name is not accurate since max current level
> is 30.
> 

They do not refer to the same thing:

* NCP5623_MAX_STEPS is the maximum number of PWM steps (0x1F -> 100% duty cycle)
* NCP5623_MAX_CURRENT is related to the current delivered by the current source
(0x1F -> ILED = ILEDmax = 2400*Iref). In this case, 30 and 31 give the same ILED
according to the remark in Eq. 2 and according to Fig. 5 in the datasheet, but
31 is still a valid value.

>> +#define NCP5623_MAX_CURRENT_UA    30000
>> +
>> +#define NCP5623_CMD_SHIFT    5
>> +#define CMD_SHUTDOWN        (0x00 << NCP5623_CMD_SHIFT)
>> +#define CMD_ILED        (0x01 << NCP5623_CMD_SHIFT)
>> +#define CMD_PWM1        (0x02 << NCP5623_CMD_SHIFT)
>> +#define CMD_PWM2        (0x03 << NCP5623_CMD_SHIFT)
>> +#define CMD_PWM3        (0x04 << NCP5623_CMD_SHIFT)
>> +#define CMD_UPWARD_DIM        (0x05 << NCP5623_CMD_SHIFT)
>> +#define CMD_DOWNWARD_DIM    (0x06 << NCP5623_CMD_SHIFT)
>> +#define CMD_DIM_STEP        (0x07 << NCP5623_CMD_SHIFT)
>> +
>> +#define LED_TO_PWM_CMD(led)    ((0x02 + led) << NCP5623_CMD_SHIFT)
>> +
>> +#define NCP5623_DATA_MASK    GENMASK(NCP5623_CMD_SHIFT - 1, 0)
>> +#define NCP5623_CMD(cmd, data)    (cmd | (data & NCP5623_DATA_MASK))
>> +
>> +struct ncp5623_led {
>> +    int led_no;
>> +    u32 led_max_current;
>> +    struct led_classdev ldev;
>> +    struct ncp5623_priv *priv;
>> +};
>> +
>> +struct ncp5623_priv {
>> +    struct ncp5623_led leds[NCP5623_MAX_LEDS];
>> +    u32 led_iref;
>> +    u32 leds_max_current;
>> +    struct i2c_client *client;
>> +};
>> +
>> +static struct ncp5623_led *ldev_to_led(struct led_classdev *ldev)
>> +{
>> +    return container_of(ldev, struct ncp5623_led, ldev);
>> +}
>> +
>> +static int ncp5623_send_cmd(struct ncp5623_priv *priv, u8 cmd, u8 data)
>> +{
>> +    char cmd_data[1] = { NCP5623_CMD(cmd, data) };
>> +    int err;
>> +
>> +    err = i2c_master_send(priv->client, cmd_data, ARRAY_SIZE(cmd_data));
>> +
>> +    return (err < 0 ? err : 0);
>> +}
>> +
>> +static int ncp5623_brightness_set(struct led_classdev *led_cdev,
>> +                  enum led_brightness brightness)
>> +{
>> +    struct ncp5623_led *led = ldev_to_led(led_cdev);
>> +
>> +    return ncp5623_send_cmd(led->priv, LED_TO_PWM_CMD(led->led_no),
>> +                brightness);
>> +}
>> +
>> +static int ncp5623_configure(struct device *dev,
>> +                 struct ncp5623_priv *priv)
>> +{
>> +    unsigned int i;
>> +    unsigned int n;
>> +    struct ncp5623_led *led;
>> +    int effective_current;
>> +    int err;
> 
> Below way of calculating max_brightness is not clear to me.
> Let's analyze it below, using values from your DT example.
> 
>> +
>> +    /* Setup the internal current source, round down */
>> +    n = 2400 * priv->led_iref / priv->leds_max_current + 1;
> 
> n = 2400 * 10 / 20000 + 1 = 2
> 
>> +    if (n > NCP5623_MAX_CURRENT)
>> +        n = NCP5623_MAX_CURRENT;
>> +
>> +    effective_current = 2400 * priv->led_iref / n;
> 
> effective_current = 2400 * 10 / 2 = 12000
> 
>> +    dev_dbg(dev, "setting maximum current to %u uA\n", effective_current);
>> +
>> +    err = ncp5623_send_cmd(priv, CMD_ILED, NCP5623_MAX_CURRENT - n);
>> +    if (err < 0) {
>> +        dev_err(dev, "cannot set the current\n");
>> +        return err;
>> +    }
>> +
>> +    /* Setup each individual LED */
>> +    for (i = 0; i < NCP5623_MAX_LEDS; i++) {
>> +        led = &priv->leds[i];
>> +
>> +        if (led->led_no < 0)
>> +            continue;
>> +
>> +        led->priv = priv;
>> +        led->ldev.brightness_set_blocking = ncp5623_brightness_set;
>> +
>> +        led->ldev.max_brightness = led->led_max_current *
>> +            NCP5623_MAX_STEPS / effective_current;
> 
> led->ldev.max_brightness = 20000 * 31 / 12000 = 51
> 
> This is not intuitive, and I'm not even sure if the result is in line
> with what you intended.
> 

There is indeed a problem in the case the allowed current on the LED is greater
than the effective current provided by the current source, as in your example.
Here I should put something like:

	led->ldev.max_brightness =
		min(NCP5623_MAX_STEPS, x * NCP5623_MAX_STEPS / y);

> Instead I propose the following:
> 
> n_iled_max =
>      31 - (priv->led_iref * 2400 / priv->leds_max_current +
>            !!(priv->led_iref * 2400 % priv->leds_max_current))
> 
> (n_iled_max =
>      31 - (24000 / 20000 + !!(24000 % 20000)) = 31 - (1 + 1) = 29)
> 
> ncp5623_send_cmd(priv, CMD_ILED, n_iled_max)
> 

This is a good proposition, especially with the DIV_ROUND_UP proposed by Pavel.
I simulated both and I noticed a problem in both cases for very low currents, as
we would have negative values for the register setting (see attached figure). I
will fix this in the next version.

> and then below for each LED:
> 
> led->ldev.max_brightness =
>      31 - (priv->led_iref * 2400 / led->led_max_current +
>            !!(priv->led_iref * 2400 % led->led_max_current))
> 
> (for led-max-microamp = 5000
>  led->ldev.max_brightness =
>  31 - (24000 / 5000 + !!(24000 % 5000)) = 31 - (4 + 1) = 26,
>  which reflects quite well the logarithmic relation shown
>  on Figure 5 in the documentation)
> 

Here you are mixing the current source and the PWM settings together. For
example, if my current source was set to 20mA at the previous stage, but my LED
can only sustain 10mA, I must limit the PWM duty cycle to 50%. Thus:

max_brightness = 10mA * 31 / 20mA = 15

(0 => 0% duty cycle, 31 => 100% duty cycle)

Best regards,
Florian

Attachment: current-comparison.pdf
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