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

Hans de Goede
2020-08-31T11:50:53Z

Hi,

On 8/31/20 10:56 AM, Andy Shevchenko wrote:
> 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>

Thank you.

Regards,

Hans

>
>> 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
>>
>