ACPI / LPSS: Save Cherry Trail PWM ctx registers only once (at activation)

ACPI / LPSS: Save Cherry Trail PWM ctx registers only once (at activation)

The DSDTs on most Cherry Trail devices have an ugly clutch where the PWM
controller gets turned off from the _PS3 method of the graphics-card dev:

            Method (_PS3, 0, Serialized)  // _PS3: Power State 3
            {
                ...
                            PWMB = PWMC /* \_SB_.PCI0.GFX0.PWMC */
                            PSAT |= 0x03
                            Local0 = PSAT /* \_SB_.PCI0.GFX0.PSAT */
                ...
            }

Where PSAT is the power-status register of the PWM controller.

Since the i915 driver will do a pwm_get on the pwm device as it uses it to
control the LCD panel backlight, there is a device-link marking the i915
device as a consumer of the pwm device. So that the PWM controller will
always be suspended after the i915 driver suspends (which is the right
thing to do). This causes the above GFX0 PS3 AML code to run before
acpi_lpss.c calls acpi_lpss_save_ctx().

So on these devices the PWM controller will already be off when
acpi_lpss_save_ctx() runs. This causes it to read/save all 1-s (0xffffffff)
as ctx register values.

When these bogus values get restored on resume the PWM controller actually
keeps working, since most bits are reserved, but this does set bit 3 of
the LPSS General purpose register, which for the PWM controller has the
following function: "This bit is re-used to support 32kHz slow mode.
Default is 19.2MHz as PWM source clock".

This causes the clock of the PWM controller to switch from 19.2MHz to
32KHz, which is a slow-down of a factor 600. Surprisingly enough so far
there have been few bug reports about this. This is likely because the
i915 driver was hardcoding the PWM frequency to 46 KHz, which divided
by 600 would result in a PWM frequency of approx. 78 Hz, which mostly
still works fine. There are some bug reports about the LCD backlight
flickering after suspend/resume which are likely caused by this issue.

But with the upcoming patch-series to finally switch the i915 drivers
code for external PWM controllers to use the atomic API and to honor
the PWM frequency specified in the video BIOS (VBT), this becomes a much
bigger problem. On most cases the VBT specifies either 200 Hz or 20
KHz as PWM frequency, which with the mentioned issue ends up being either
1/3 Hz, where the backlight actually visible blinks on and off every 3s,
or in 33 Hz and horrible flickering of the backlight.

There are a number of possible solutions to this problem:

1. Make acpi_lpss_save_ctx() run before GFX0._PS3
 Pro: Clean solution from pov of not medling with save/restore ctx code
 Con: As mentioned the current ordering is the right thing to do
 Con: Requires assymmetry in at what suspend/resume phase we do the save vs
      restore, requiring more suspend/resume ordering hacks in already
      convoluted acpi_lpss.c suspend/resume code.
2. Do some sort of save once mode for the LPSS ctx
 Pro: Reasonably clean
 Con: Needs a new LPSS flag + code changes to handle the flag
3. Detect we have failed to save the ctx registers and do not restore them
 Pro: Not PWM specific, might help with issues on other LPSS devices too
 Con: If we can get away with not restoring the ctx why bother with it at
      all?
4. Do not save the ctx for CHT PWM controllers
 Pro: Clean, as simple as dropping a flag?
 Con: Not so simple as dropping a flag, needs a new flag to ensure that
      we still do lpss_deassert_reset() on device activation.
5. Make the pwm-lpss code fixup the LPSS-context registers
 Pro: Keeps acpi_lpss.c code clean
 Con: Moves knowledge of LPSS-context into the pwm-lpss.c code

1 and 5 both do not seem to be a desirable way forward.

3 and 4 seem ok, but they both assume that restoring the LPSS-context
registers is not necessary. I have done a couple of test and those do
show that restoring the LPSS-context indeed does not seem to be necessary
on devices using s2idle suspend (and successfully reaching S0i3). But I
have no hardware to test deep / S3 suspend. So I'm not sure that not
restoring the context is safe.

That leaves solution 2, which is about as simple / clean as 3 and 4,
so this commit fixes the described problem by implementing a new
LPSS_SAVE_CTX_ONCE flag and setting that for the CHT PWM controllers.

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200903112337.4113-3-hdegoede@redhat.com