[Intel-gfx] [PATCH v9 06/17] pwm: lpss: Make pwm_lpss_apply() not rely on existing hardware state

[Hans de Goede]

Hi,

On 9/3/20 12:59 PM, Thierry Reding wrote:
> On Thu, Sep 03, 2020 at 12:51:03PM +0200, Hans de Goede wrote:
>> Before this commit pwm_lpss_apply() was making 2 assuming
>> 2 pre-conditions were met by the existing hardware state:
> 
> I think that "making 2" is too much.

You're right at first the sentence had something about making
2 assumptions, then I added pre-conditions in there for it
to better describe the problem...

>> 1. That the base-unit and on-time-div read back from the
>> control register are those actually in use, so that it
>> can skip setting the update bit if the read-back value
>> matches the desired values.
>>
>> 2. That the controller is enabled when the cached
>> pwm_state.enabled says that the controller is enabled.
>>
>> As the long history of fixes for subtle (often suspend/resume)
>> lpss-pwm issues shows, this assumptions are not necessary
>> always true.
>>
>> 1. Specifically is not true on some (*) Cherry Trail devices
>> with a nasty GFX0._PS3 method which: a. saves the ctrl reg value.
>> b. sets the base-unit to 0 and writes the update bit to apply/commit
>> c. restores the original ctrl value without setting the update bit,
>> so that the 0 base-unit value is still in use.
>>
>> 2. Assumption 2. currently is true, but only because of the code which
>> saves/restores the state on suspend/resume. By convention restoring the
>> PWM state should be done by the PWM consumer and the presence of this
>> code in the pmw-lpss driver is a bug. Therefor the save/restore code will
>> be dropped in the next patch in this series, after which this assumption
>> also is no longer true.
>>
>> This commit changes the pwm_lpss_apply() to make any assumptions about the
> 
> Did you mean to say "... to _not_ make any assumptions ..."?

Yes, oops. That is a small but important difference.

I'll do a v10 with your 2 Acked-by's added and both commit msg issues fixed.

Hopefully that will be the last version.

>> state the hardware is in. Instead it makes pwm_lpss_apply() always fully
>> program the PWM controller, making it much less fragile.
>>
>> *) Seen on the Acer One 10 S1003, Lenovo Ideapad Miix 310 and 320 models
>> and various Medion models.
>>
>> Signed-off-by: Hans de Goede <hdegoede at redhat.com>
>> ---
>>   drivers/pwm/pwm-lpss.c | 21 +++++++++------------
>>   1 file changed, 9 insertions(+), 12 deletions(-)
> 
> Other than the two small nits, this looks much more idiomatic and true
> to the atomic API, so:
> 
> Acked-by: Thierry Reding <thierry.reding at gmail.com>

Thank you.

Regards,

Hans