Re: [PATCH v8 07/17] pwm: lpss: Always update state and set update bit

Andy Shevchenko
2020-08-31T08:56:59Z

On Sun, Aug 30, 2020 at 02:57:43PM +0200, Hans de Goede wrote:
> This commit removes a check where we would skip writing the ctrl register
> and then setting the update bit in case the ctrl register already contains
> the correct values.
>
> In a perfect world skipping the update should be fine in these cases, but
> on Cherry Trail devices the AML code in the GFX0 devices' PS0 and PS3
> methods messes with the PWM controller.
>
> The "ACPI / LPSS: Resume Cherry Trail PWM controller in no-irq phase" patch
> earlier in this series stops the GFX0._PS0 method from messing with the PWM
> controller and on the DSDT-s inspected sofar the _PS3 method only reads
> from the PWM controller (and turns it off before we get a change to do so):
>
> {
> PWMB = PWMC /* \_SB_.PCI0.GFX0.PWMC */
> PSAT |= 0x03
> Local0 = PSAT /* \_SB_.PCI0.GFX0.PSAT */
> }
>
> The PWM controller getting turning off before we do this ourselves is
> a bit annoying but not really an issue.
>
> The problem this patch fixes comes from a new variant of the GFX0._PS3 code
> messing with the PWM controller found on the Acer One 10 S1003 (1):
>
> {
> PWMB = PWMC /* \_SB_.PCI0.GFX0.PWMC */
> PWMT = PWMC /* \_SB_.PCI0.GFX0.PWMC */
> PWMT &= 0xFF0000FF
> PWMT |= 0xC0000000
> PWMC = PWMT /* \_SB_.PCI0.GFX0.PWMT */
> PWMT = PWMC /* \_SB_.PCI0.GFX0.PWMC */
> Sleep (0x64)
> PWMB &= 0x3FFFFFFF
> PWMC = PWMB /* \_SB_.PCI0.GFX0.PWMB */
> PSAT |= 0x03
> Local0 = PSAT /* \_SB_.PCI0.GFX0.PSAT */
> }
>
> This "beautiful" piece of code clears the base-unit part of the ctrl-reg,
> which effectively disables the controller, and it sets the update flag
> to apply this change. Then after this it restores the original ctrl-reg
> value, so we do not see it has mucked with the controller.
>
> *But* it does not set the update flag when restoring the original value.
> So the check to see if we can skip writing the ctrl register succeeds
> but since the update flag was not set, the old base-unit value of 0 is
> still in use and the PWM controller is effectively disabled.
>
> IOW this PWM controller poking means that we cannot trust the base-unit /
> on-time-div value we read back from the PWM controller since it may not
> have been applied/committed. Thus we must always update the ctrl-register
> and set the update bit.
>
> 1) And once I knew what to look for also in a bunch of other devices
> including the popular Lenovo Ideapad Miix 310 and 320 models and
> various Medion models.

Despite the above mentioned issue I'm always in favour of not micro-optimizing I/O.
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
> ---
> Changes in v8:
> - New patch in v8 of this patch-set
> ---
> drivers/pwm/pwm-lpss.c | 10 ++++------
> 1 file changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/pwm/pwm-lpss.c b/drivers/pwm/pwm-lpss.c
> index 9a7400c6fb6e..20f6b6d6f874 100644
> --- a/drivers/pwm/pwm-lpss.c
> +++ b/drivers/pwm/pwm-lpss.c
> @@ -85,7 +85,7 @@ static void pwm_lpss_prepare(struct pwm_lpss_chip *lpwm, struct pwm_device *pwm,
> unsigned long long on_time_div;
> unsigned long c = lpwm->info->clk_rate, base_unit_range;
> unsigned long long base_unit, freq = NSEC_PER_SEC;
> - u32 orig_ctrl, ctrl;
> + u32 ctrl;
>
> do_div(freq, period_ns);
>
> @@ -104,16 +104,14 @@ static void pwm_lpss_prepare(struct pwm_lpss_chip *lpwm, struct pwm_device *pwm,
> do_div(on_time_div, period_ns);
> on_time_div = 255ULL - on_time_div;
>
> - orig_ctrl = ctrl = pwm_lpss_read(pwm);
> + ctrl = pwm_lpss_read(pwm);
> ctrl &= ~PWM_ON_TIME_DIV_MASK;
> ctrl &= ~((base_unit_range - 1) << PWM_BASE_UNIT_SHIFT);
> ctrl |= (u32) base_unit << PWM_BASE_UNIT_SHIFT;
> ctrl |= on_time_div;
>
> - if (orig_ctrl != ctrl) {
> - pwm_lpss_write(pwm, ctrl);
> - pwm_lpss_write(pwm, ctrl | PWM_SW_UPDATE);
> - }
> + pwm_lpss_write(pwm, ctrl);
> + pwm_lpss_write(pwm, ctrl | PWM_SW_UPDATE);
> }
>
> static inline void pwm_lpss_cond_enable(struct pwm_device *pwm, bool cond)
> --
> 2.28.0
>

--
With Best Regards,
Andy Shevchenko