Re: [v2 3/4] platform: (TS-5500) add LED support

Vivien Didelot
2011-08-30T21:56:50Z

On Tue, 30 Aug 2011 14:17:15 -0700,
"H. Peter Anvin" <hpa@zytor.com> wrote:

> On 08/30/2011 02:15 PM, Mark Brown wrote:
> > On Tue, Aug 30, 2011 at 05:14:24PM -0400, Vivien Didelot wrote:
> >> "H. Peter Anvin" <hpa@zytor.com> wrote:
> >>> On 08/29/2011 03:16 PM, Mark Brown wrote:
> >
> >>>> Can you not do outb() from atomic context? The reason lots of
> >>>> LED drivers update the hardware in a workqueue is that they
> >>>> communicate with the hardware over buses that can't be used in
> >>>> atomic context like I2C or SPI but if that's not an issue then
> >>>> the workqueue is not required and the code can be simplified.
> >
> >>> outb() can definitely be executed from atomic context.
> >
> >> Good to know, thanks. I removed the work_struct and instead lock a
> >> mutex before setting led->new_brightness and calling outb().
> >
> > You can't take a mutex in atomic context...
>
> OK, so what is the potential race that this mutex is called for? If
> it just means that the brightness can be redundantly set to the same
> value more than once, no atomicity is needed.
>
> -hpa
>
I wrote the led_set function like:

static void ts5500_led_set(struct led_classdev *led_cdev,
enum led_brightness value)
{
struct ts5500_led *led = container_of(led_cdev,
struct ts5500_led,
cdev);
mutex_lock(&led->lock);
led->new_brightness = (value == LED_OFF) ? LED_OFF : LED_FULL;
outb(value, led->ioaddr);
mutex_unlock(&led->lock);
}

I guess the wrong value could be read if we get preempted just before
the outb() call, am I wrong?

Vivien