Re: lpss_iosf_enter_d3_state breaks PWM on Baytrail

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 06/27/2017 03:29 PM, Andy Shevchenko wrote:
> On Mon, 2017-06-26 at 11:31 +0200, Hans de Goede wrote:
>> Hi Andy, et al.,
>>
>> I've spend quite some time this weekend debugging an issue
>> an a Bay Trail laptop with a Crystal Cove PMIC which uses
>> the LPSS PWM for backlight control rather then the PMIC's
>> PWM.
>>
>> The problem is lpss_iosf_enter_d3_state. Specifically when
>> called when the PWM runtime-suspends during i915 driver load,
>> during which the i915 driver disables then re-enables the PWM.
>>
>> This is the only time (including normal suspend(to-idle)
>> and resume) when this check:
>>
>>           pmc_status = (~(d3_sts_0 | func_dis)) & pmc_mask;
>>           if (pmc_status)
>>                   goto exit;
>>
>> Does not hit the goto exit path
> 
> ...meaning all LPSS devices (except maybe DMA) are in D3hot.

Correct AFAICT.
>>   and lpss_iosf_enter_d3_state
>> actually executes these 3 statements:
>>
>>           iosf_mbi_modify(LPSS_IOSF_UNIT_LPIO1, MBI_CFG_WRITE,
>>                           LPSS_IOSF_PMCSR, value2, mask2);
>>
>>           iosf_mbi_modify(LPSS_IOSF_UNIT_LPIO2, MBI_CFG_WRITE,
>>                           LPSS_IOSF_PMCSR, value2, mask2);
>>
>>           iosf_mbi_modify(LPSS_IOSF_UNIT_LPIOEP, MBI_CR_WRITE,
>>                           LPSS_IOSF_GPIODEF0, value1, mask1);
>>
>> If I comment out these 3 statements then everything works fine,
>> with these 3 statements in place though, then the PWM controller
>> gets stuck when the i915 driver tries to re-enable it (and it
>> runtime-resumes). After these 3 statements have been run once
>> then any subsequent reads from the PWM's control register
>> always return 0x00010000 and writes seem to be ignored.
> 
> Sounds like powered off LPSS power island.

Right, but this is after runtime resume and after lpss_iosf_exit_d3_state
has been called, I've added debug printk calls to verify that
lpss_iosf_exit_d3_state is called for the register reads which
only return 0x00010000

> 
>> Given the past troubles with the LPSS DMA controller suspend,
>> see all the links in this commit message (which introduces
>> the current fix) :
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/com
>> mit/?id=eebb3e8d8aaf2
>>
>> I'm tempted to just rip out lpss_iosf_enter_d3_state,
>> especially since it does not seem to do anything other
>> then during early boot when not all drivers are loaded yet.
> 
> Interesting...
> 
> You are telling that there is guaranteed that at least one LPSS device
> (except DMA) will be on at any time?

In my testing, yes that seems to be the case, a pr_err in
lpss_iosf_enter_d3_state after the if (...) goto ... only
triggers once during i915 driver load and never again. Not
even if I switch to a text console, then blank the
screen and wait a while.

Note that the enter / exit d3 state calls are only done on
runtime suspend / resume, so this also does not trigger on
normal suspend / resume.

>> Note that on Bay Trail devices with an AXP288 PMIC (most new
>> Bay Trail devices) and on any Cherry Trail device the LPSS I2C
>> controller connected to the PMIC will never suspend because
>> some DSTDs make ACPI OpRegion calls during late suspend /
>> early resume and as such we need to keep the I2C bus alive
>> the whole time. This means that the check in
>> lpss_iosf_enter_d3_state will always hit the goto exit path
>> on these devices,
> 
> Looks like an answer to the previous question.

Right, except for Bay Trail devices with Crystal Cove PMIC,
there the I2C controller for the PMIC is not kept alive
(which we may need to change if we ever find late_suspend /
early_resume PMIC OpRegion use on these devices, but sofar
they don't seem to do this)

>>   leaving just (older) Bay Trail + non AXP288
>> PMIC devices as potentially hitting the code path where
>> lpss_iosf_enter_d3_state actually does something and these are
>> exactly the devices with which people are having a lot of
>> problems with freezes / hangs.
>>
>> So I believe that the best approach would be to simply remove
>> lpss_iosf_enter_d3_state. Which leaves me wondering what it
>> actually does. It would be nice for the future if undocumented
>> (not publicly documented) registers are used that some comments
>> are added describing what each register write actually does.
> 
> Do the commit messages describe this enough along with several comments
> inside acpi_lpss.c?

Nope, they give a hint what is going on, but don't make it
really clear what the individual register writes do, hence
my questions in this email.

>> These seem to simply put the DMA controller into D3:
>>
>>           u32 value2 = LPSS_PMCSR_D3hot;
>>           u32 mask2 = LPSS_PMCSR_Dx_MASK;
>>
>>           iosf_mbi_modify(LPSS_IOSF_UNIT_LPIO1, MBI_CFG_WRITE,
>>                           LPSS_IOSF_PMCSR, value2, mask2);
>>
>>           iosf_mbi_modify(LPSS_IOSF_UNIT_LPIO2, MBI_CFG_WRITE,
>>                           LPSS_IOSF_PMCSR, value2, mask2);
> 
> It overrides the lines which propagate clock and reset signals to the
> entire LPSS islands.

Overrides sounds weird if we set it to D0 / D3, I would expect there
to be 1 bit to say "override auto pm stuff" and another bit or bits
to select the value we are forcing when the override is active.

> DMA just happened to be a function 0 (in terms of PCI), so, if it is off
> the rest of the functions can not be on.

Ok.

>> But what does this one do?   :
>>
>>           u32 value1 = 0;
>>           u32 mask1 = LPSS_GPIODEF0_DMA_D3_MASK |
> 
>> LPSS_GPIODEF0_DMA_LLP;
> 
> (this one is a chicken bit to enable hardware linked list transfers on
> DMA)

Yes I noticed that was added in a separate commit. If we are
just setting that to enable a feature then why are not doing that
once on probe. That would seem more logical then toggling it
on / off with the power-state.

> 
>>
>>           iosf_mbi_modify(LPSS_IOSF_UNIT_LPIOEP, MBI_CR_WRITE,
>>                           LPSS_IOSF_GPIODEF0, value1, mask1);
>>
>> I guess this turns on(?) the automatically down powering of the
>> DMA controllers ? If so then why not just do only that ?
> 
> No, automatic power gating is a *hardware* (PMC firmware? I do not
> remember details now) feature which is overridden by the code above.

Then wat do the LPSS_GPIODEF0_DMA_D3_MASK bits do ?

> 
> It's something like logical gates where one of the input is what we
> supply through these chicken bits.
> 
>> And if this turns it on
>>   then lpss_iosf_exit_d3_state turns the
>> automagic off, if that is true,
> 
> The proper wording here I suppose *it allows or forbids the automatic
> power gating*. It's one layer up I would say. It means it might be a
> latency between writing these bits and actual hardware response on them.
> 
>>   then there is an ordering issue.
>> lpss_iosf_exit_d3_state gets only run on (runtime)resume, not
>> on the initial probe of LPSS devices, so that would mean that between
>> the initial probe and the first resume we have the undesirable
>> auto-off feature active ?
> 
> Looks like ACPI PM for LPSS should take care of this at probe. When I
> wrote the code I was assuming that ACPI enumerated devices are runtime
> powered on during ->probe(). For such I did explicit calls in DMA
> driver: bb32baf76e56 ("dmaengine: dw: enable runtime PM").

Hmm, doesn't the runtime-pm code assume that a device is on when
runtime pm gets initialized, rather then do an explicit runtime resume ?

Note this is a question, I'm not sure what the runtime pm code does.

Regards,

Hans