[PATCH v2] pwm: sifive: Always let the first pwm_apply_state succeed

[Thierry Reding]

On Wed, Nov 09, 2022 at 12:37:24PM +0100, Emil Renner Berthing wrote:
> Commit 2cfe9bbec56ea579135cdd92409fff371841904f added support for the
> RGB and green PWM controlled LEDs on the HiFive Unmatched board
> managed by the leds-pwm-multicolor and leds-pwm drivers respectively.
> All three colours of the RGB LED and the green LED run from different
> lines of the same PWM, but with the same period so this works fine when
> the LED drivers are loaded one after the other.
> 
> Unfortunately it does expose a race in the PWM driver when both LED
> drivers are loaded at roughly the same time. Here is an example:
> 
>   |          Thread A           |          Thread B           |
>   |  led_pwm_mc_probe           |  led_pwm_probe              |
>   |    devm_fwnode_pwm_get      |                             |
>   |      pwm_sifive_request     |                             |
>   |        ddata->user_count++  |                             |
>   |                             |    devm_fwnode_pwm_get      |
>   |                             |      pwm_sifive_request     |
>   |                             |        ddata->user_count++  |
>   |         ...                 |          ...                |
>   |    pwm_state_apply          |    pwm_state_apply          |
>   |      pwm_sifive_apply       |      pwm_sifive_apply       |
> 
> Now both calls to pwm_sifive_apply will see that ddata->approx_period,
> initially 0, is different from the requested period and the clock needs
> to be updated. But since ddata->user_count >= 2 both calls will fail
> with -EBUSY, which will then cause both LED drivers to fail to probe.
> 
> Fix it by letting the first call to pwm_sifive_apply update the clock
> even when ddata->user_count != 1.
> 
> Fixes: 9e37a53eb051 ("pwm: sifive: Add a driver for SiFive SoC PWM")
> Signed-off-by: Emil Renner Berthing <emil.renner.berthing at canonical.com>
> ---
>  drivers/pwm/pwm-sifive.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)

I've applied this as-is for now. What I'm wondering is if perhaps we
want to implement something into the PWM core to deal with this, now
fairly common, situation.

Thierry
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