[PATCH v9 2/3] pwm: rp1: Add RP1 PWM controller driver
Gary Guo
gary at garyguo.net
Wed Sep 23 06:46:02 PDT 2026
On Wed Sep 23, 2026 at 1:51 PM BST, Andrea della Porta wrote:
> Hi Gary,
> thanks for your feedback!
>
> On 20:51 Tue 22 Sep , Gary Guo wrote:
>> On Fri Sep 18, 2026 at 10:59 AM BST, Andrea della Porta wrote:
>> > From: Naushir Patuck <naush at raspberrypi.com>
>> >
>> > The Raspberry Pi RP1 southbridge features an embedded PWM
>> > controller with 4 output channels, alongside an RPM interface
>> > to read the fan speed on the Raspberry Pi 5.
>> >
>> > Add the supporting driver.
>> >
>> > Signed-off-by: Naushir Patuck <naush at raspberrypi.com>
>> > Co-developed-by: Stanimir Varbanov <svarbanov at suse.de>
>> > Signed-off-by: Stanimir Varbanov <svarbanov at suse.de>
>> > Signed-off-by: Andrea della Porta <andrea.porta at suse.com>
>>
>> Put yourself a Co-developed-by perhaps? You made significant changes.
>
> Ack.
>
>>
>> I've tested the device on my Pi 5, and the fan is indeed spinning now :)
>
> Nice! Thanks for testing it :)
>
>>
>> > ---
>> > drivers/pwm/Kconfig | 8 +
>> > drivers/pwm/Makefile | 1 +
>> > drivers/pwm/pwm-rp1.c | 499 ++++++++++++++++++++++
>> > include/soc/bcm2835/raspberrypi-pwm-rp1.h | 10 +
>> > 4 files changed, 518 insertions(+)
>> > create mode 100644 drivers/pwm/pwm-rp1.c
>> > create mode 100644 include/soc/bcm2835/raspberrypi-pwm-rp1.h
>> >
>> > diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
>> > index 7297760868790..e1baec8d294d1 100644
>> > --- a/drivers/pwm/Kconfig
>> > +++ b/drivers/pwm/Kconfig
>> > @@ -637,6 +637,14 @@ config PWM_ROCKCHIP
>> > Generic PWM framework driver for the PWM controller found on
>> > Rockchip SoCs.
>> >
>> > +config PWM_RASPBERRYPI_RP1
>> > + tristate "RP1 PWM support"
>> > + depends on MISC_RP1 || COMPILE_TEST
>> > + depends on HAS_IOMEM
>> > + select REGMAP_MMIO
>> > + help
>> > + PWM framework driver for Raspberry Pi RP1 controller.
>> > +
>> > config PWM_SAMSUNG
>> > tristate "Samsung PWM support"
>> > depends on PLAT_SAMSUNG || ARCH_S5PV210 || ARCH_EXYNOS || COMPILE_TEST
>> > diff --git a/drivers/pwm/Makefile b/drivers/pwm/Makefile
>> > index 5630a521a7cff..c07fd24f69f39 100644
>> > --- a/drivers/pwm/Makefile
>> > +++ b/drivers/pwm/Makefile
>> > @@ -57,6 +57,7 @@ obj-$(CONFIG_PWM_RENESAS_RZG2L_GPT) += pwm-rzg2l-gpt.o
>> > obj-$(CONFIG_PWM_RENESAS_RZ_MTU3) += pwm-rz-mtu3.o
>> > obj-$(CONFIG_PWM_RENESAS_TPU) += pwm-renesas-tpu.o
>> > obj-$(CONFIG_PWM_ROCKCHIP) += pwm-rockchip.o
>> > +obj-$(CONFIG_PWM_RASPBERRYPI_RP1) += pwm-rp1.o
>> > obj-$(CONFIG_PWM_SAMSUNG) += pwm-samsung.o
>> > obj-$(CONFIG_PWM_SIFIVE) += pwm-sifive.o
>> > obj-$(CONFIG_PWM_SL28CPLD) += pwm-sl28cpld.o
>> > diff --git a/drivers/pwm/pwm-rp1.c b/drivers/pwm/pwm-rp1.c
>> > new file mode 100644
>> > index 0000000000000..cfd38e46cc589
>> > --- /dev/null
>> > +++ b/drivers/pwm/pwm-rp1.c
>> > +
>> > +int rp1_pwm_read_tachometer(struct device *dev)
>> > +{
>> > + struct pwm_chip *chip;
>> > + struct rp1_pwm *rp1;
>> > + u32 tach_val;
>> > + int ret;
>> > +
>> > + if (!dev)
>> > + return -EINVAL;
>> > +
>> > + device_lock(dev);
>>
>> You should use the device link mechanism for synchronizing unbind / runtime PM.
>> That can be done in your RP1 fan driver, and it doesn't need locking on this
>> driver.
>
> This does not enforce the locking though.
What locking do you think is needed?
Driver core takes care of ordering so you will never see a depended device
going away while a dependant device is still bound. Your RP1 fan driver just
need to make sure that it never calls this API after it's itself unbound --
which it needs to guarantee anyway.
> Is it enough to just rely on
> caller to create the device link in advance? IOW, just add a documentation
> comment to the exported function prologue stating that the consumer is
> responsible to sync via a device link is acceptable?
If you use pwm_get API then a device link is automatically created for you
already (however, this automatic link does not pass runtime PM flags, so if you
need that you still need to add link explicitly).
How this is supposed to be synchronized or documented is very hard to get right
without seeing the user side driver -- if you already have a working version of
the RP1 fan driver it might benefit to have it attached as a RFC patch in the
series; or an option to to drop the tachometer API and add it as part of the fan
driver series.
> Otherwise the only way I see to protect it in any scenario is via the mutex
> I've already implemented and a second flag for the removing path (device_lock
> will be dropped, of course).
>
>>
>> So Sashiko is kinda reporting a false positive here.
>>
>> > +
>> > + chip = dev_get_drvdata(dev);
>> > + if (!chip) {
>> > + ret = -ENODEV;
>> > + goto err_dev_unlock;
>> > + }
>> > +
>> > + rp1 = pwmchip_get_drvdata(chip);
>> > + if (!rp1) {
>> > + ret = -ENODEV;
>> > + goto err_dev_unlock;
>> > + }
>> > +
>> > + mutex_lock(&rp1->lock);
>> > + if (!rp1->clk_enabled) {
>> > + ret = -EBUSY;
>> > + goto err_clk_unlock;
>> > + }
>> > +
>> > + ret = regmap_read(rp1->regmap, RP1_PWM_PHASE(2), &tach_val);
>> > + if (ret)
>> > + goto err_clk_unlock;
>> > +
>> > + ret = (int)tach_val;
>> > +
>> > +err_clk_unlock:
>> > + mutex_unlock(&rp1->lock);
>> > +err_dev_unlock:
>> > + device_unlock(dev);
>> > +
>> > + return ret;
>> > +}
>> > +EXPORT_SYMBOL_NS_GPL(rp1_pwm_read_tachometer, "RP1_PWM_FAN");
>> > +
>> > +static int rp1_pwm_probe(struct platform_device *pdev)
>> > +{
>> > + struct device *dev = &pdev->dev;
>> > + unsigned long clk_rate;
>> > + struct pwm_chip *chip;
>> > + void __iomem *base;
>> > + struct rp1_pwm *rp1;
>> > + int ret;
>> > +
>> > + chip = devm_pwmchip_alloc(dev, RP1_PWM_NUM_PWMS, sizeof(*rp1));
>> > + if (IS_ERR(chip))
>> > + return PTR_ERR(chip);
>> > +
>> > + rp1 = pwmchip_get_drvdata(chip);
>> > + ret = devm_mutex_init(dev, &rp1->lock);
>> > + if (ret)
>> > + return ret;
>> > +
>> > + base = devm_platform_ioremap_resource(pdev, 0);
>> > + if (IS_ERR(base))
>> > + return PTR_ERR(base);
>> > +
>> > + rp1->regmap = devm_regmap_init_mmio(dev, base, &rp1_pwm_regmap_config);
>> > + if (IS_ERR(rp1->regmap))
>> > + return dev_err_probe(dev, PTR_ERR(rp1->regmap), "Cannot initialize regmap\n");
>>
>> You mentioned "rework regmap error paths" in cover letter.
>>
>> But the issue is that regmap doesn't need be used here at all. RP1 is on PCIe
>> so there is no need for bus abstraction, direct use of MMIO is sufficient.
>> MMIO accessors have no error paths, so you're saying yourself from having to
>> handle that, and also reduce the overhead by not having to go through an
>> abstraction w/ indirect funicton calls.
>>
>> I suppose regmap was used when syscon was there; but it's not needed anymore.
>
> True, and I don't have any issue in converting back to MMIO call and drop the conditional for
> error checking, but please consider the following, since the driver may be extended in the
> future to support more features:
>
> - regmap gives you free debugfs view on the registers, which may be useful to test
> the new features.
Do you have any register that we want to access that is not part of the PWM
facility, other than tachometer?
> - regmap_write/read may still return an error in case the passed register is not in range.
> This will be trapped at runtime only, but could still be useful during development
I think this is rather a anti-feature. Having additional error paths for some
thing that never happens is not a good idea, especially that you basically get 0
coverage for these paths.
You already know the shape of the register region, so the bounds checking
provided by regmap would be better served by an ahead-of-time check:
#define RP1_PWM_REG_MAX (RP1_PWM_DUTY(RP1_PWM_NUM_PWMS) + 4)
struct resource *res;
base = devm_platform_get_and_ioremap_resource(pdev, 0, &res);
if (IS_ERR(base))
return PTR_ERR(base);
if (resource_size(res) < RP1_PWM_REG_MAX) ...
Shameless plug: Rust abstractions for I/O access is actually pretty good in this
regard in the sense that we try to prove statically that access canot fail. It
also has PWM and platform abstractions. If you're interested in learning Rust
this driver might be a good candidate :) If you're going to LPC we can chat
about this there.
Best,
Gary
> - I expect the PWM driver to be a access with very low frequency, so I guess the overhead
> imposed by regmap is negligible.
>
> Since we already have it, I'd prefer to leave regmap if possible for the aforementioned reasons,
> but I'm obviously open to drop it in favor of direct MMIO calls in case you or anyone else are
> not seeing those as real benefits.
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