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

[Thierry Reding <thierry.reding@gmail.com>]

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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.

Doesn't this now make patch 6/17 obsolete?

Thierry

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