[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Message-ID: <Y9fj/72Tl5FN+HSf@orome>
Date: Mon, 30 Jan 2023 16:36:31 +0100
From: Thierry Reding <thierry.reding@...il.com>
To: Emil Renner Berthing <emil.renner.berthing@...onical.com>
Cc: Uwe Kleine-König
<u.kleine-koenig@...gutronix.de>,
Palmer Dabbelt <palmer@...belt.com>,
Paul Walmsley <paul.walmsley@...ive.com>,
Atish Patra <atishp@...shpatra.org>,
"Wesley W. Terpstra" <wesley@...ive.com>,
linux-pwm@...r.kernel.org, linux-riscv@...ts.infradead.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] pwm: sifive: Always let the first pwm_apply_state
succeed
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@...onical.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
Download attachment "signature.asc" of type "application/pgp-signature" (834 bytes)
Powered by blists - more mailing lists