Help requested: Linux, PWMs, and LCD Backlights

cverges · **Joined:** Tue May 12, 2009 7:21 pm **Posts:** 16

Hi all,

I'm trying to use the AT91SAM9263's PWM for an LCD backlight control. Under the U-boot bootloader, I was able to get this working at a 75% duty cycle by writing directly to the AT91's registers. Under Linux, I was hoping to use the existing atmel-pwm-bl module; however, I'm running into issues and need your help.

Here's the code in my BSP:

Code:

/*
 * Atmel Backlight PWM
 */
#if defined(CONFIG_BACKLIGHT_ATMEL_PWM) || defined(CONFIG_BACKLIGHT_ATMEL_PWM_MODULE)
static struct atmel_pwm_bl_platform_data cs_bl_data = {
        .pwm_channel            = AT91_PWM0,
        .pwm_frequency          = 10000,
        .pwm_compare_max        = 100,
        .pwm_duty_max           = 100,
        .pwm_duty_min           = 0,
        .pwm_active_low         = 1,
        .gpio_on                = -1,
        .on_active_low          = 0,
};

static struct platform_device cs_bl_dev = {
        .name           = "atmel-pwm-bl",
        .id             = 0,
        .dev = {
                .platform_data = &cs_bl_data,
        },
};

static void __init cs_add_device_backlight_pwm(void) {
        at91_add_device_pwm(1 << cs_bl_data.pwm_channel);
        platform_device_register(&cs_bl_dev);

#if 0
        at91_set_B_periph(AT91_PIN_PB7, 0);     /* PWM0 */
        at91_sys_write(AT91_PMC_PCER, 1 << AT91SAM9263_ID_PWMC);
#endif
}
#else
static void __init cs_add_device_backlight_pwm(void) {}
#endif

I then call cs_add_device_backlight_pwm() inside the board_init() function. I've also ensured that CONFIG_ATMEL_PWM and CONFIG_BACKLIGHT_ATMEL_PWM are set to 'y' in the .config file.

The PWM signal does not toggle, and no entry appears in /sys/class/backlight.

What am I missing? I'm sure it's painfully obvious, so I appreciate your help in slapping me over the head. :-)

Thanks in advance,
Chris

cverges · **Joined:** Tue May 12, 2009 7:21 pm **Posts:** 16

A little more info ...

I added debug messages to drivers/video/backlight/atmel-pwm-bl.c and discovered that the call to pwm_channel_alloc() inside atmel_pwm_bl_probe() is returning -ENODEV. This response is given by the following check:

Code:

if (!pwm || !(pwm->mask & 1 << index))
        return -ENODEV;

Still working to understand what is missing.

Chris

cverges · **Joined:** Tue May 12, 2009 7:21 pm **Posts:** 16

Some more updates based on testing:

I noticed in the dmesg output that atmel-pwm-bl was being loaded before atmel_pwm. That prompted me to try restructuring the code to the following:

Code:

/*
 * Atmel Backlight PWM
 */
#if defined(CONFIG_ATMEL_PWM) || defined(CONFIG_ATMEL_PWM_MODULE)
static void __init cs_add_device_pwm(void)
{
        at91_add_device_pwm(1 << AT91_PWM0);
}
#else
static void __init cs_add_device_pwm(void)
#endif

#if defined(CONFIG_BACKLIGHT_ATMEL_PWM) || defined(CONFIG_BACKLIGHT_ATMEL_PWM_MODULE)
static struct atmel_pwm_bl_platform_data cs_bl_data = {
        .pwm_channel            = AT91_PWM0,
        .pwm_frequency          = 10000,
        .pwm_compare_max        = 100,
        .pwm_duty_max           = 100,
        .pwm_duty_min           = 50,
        .pwm_active_low         = 0,
        .gpio_on                = -1,
        .on_active_low          = 0,
};

/* Defined in at91sam9263_devices.c */
extern struct platform_device at91sam9263_pwm0_device;

static struct platform_device cs_bl_dev = {
        .name           = "atmel-pwm-bl",
        .id             = -1,
        .dev = {
                .parent         = &at91sam9263_pwm0_device.dev,
                .platform_data  = &cs_bl_data,
        },
};

static void __init cs_add_device_backlight_pwm(void)
{
        platform_device_register(&cs_bl_dev);
}
#else
static void __init cs_add_device_backlight_pwm(void) {}
#endif

Especially note the parent dependency in the cs_bl_dev definition. My hope was that this would cause atmel_pwm to load before atmel-pwm-bl. Unfortunately, that hope was dashed.

I was able to get things working with the following:

[list=][*]CONFIG_ATMEL_PWM=y
[*]CONFIG_BACKLIGHT_ATMEL_PWM=m
[*]Load the kernel, atmel_pwm loads properly
[*]modprobe atmel-pwm-bl[/list]

Using this, I get the following dmesg output:

Code:

# dmesg | grep -i pwm
[    0.220000] registering platform atmel_pwm device
[    0.730000] atmel_pwm atmel_pwm: pwm registered, mask=0x00000001
[    0.730000] atmel_pwm driver loaded
[  277.450000] atmel-pwm-bl atmel-pwm-bl: atmel_pwm_bl_probe
[  277.450000] atmel-pwm-bl atmel-pwm-bl: Atmel PWM backlight driver (7760 Hz)
[  277.520000] backlight atmel-pwm-bl: backlight pwm registered

(I added these messages to the two drivers to help figure out the sequencing issue.)

So that leaves me with two questions:

[list=][*]Why is atmel-pwm-bl being loaded before atmel_pwm in the monolithic kernel, even with the parent dependency is setup?
[*]What can be done about it?[/list]

Chris

cverges · **Joined:** Tue May 12, 2009 7:21 pm **Posts:** 16

It appears as though kbuild's Makefile structure may play a role in why atmel-pwm-bl is being loaded before atmel_pwm:

http://lxr.linux.no/#linux+v2.6.34/Docu ... efiles.txt#L147

Not sure how to fix it yet.

Chris

najay · **Posted:** Tue Jun 07, 2011 8:39 am

I had the same issue on a recent build. I was able to fix it by changing module_init() to subsys_initcall() in atmel_pwm.c

hvantil · **Joined:** Tue Oct 25, 2011 8:28 pm **Posts:** 1

Using najay's suggestion worked for me. Thanks a bunch for that. What is the difference between module_init() and sysinit_call()?

Help requested: Linux, PWMs, and LCD Backlights

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