Re: [PATCH v2 13/13] drm/i915: Acquire P-Unit access when modifying P-Unit settings

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

Hi,

On 30-01-17 16:38, Ville Syrjälä wrote:
> On Mon, Jan 30, 2017 at 04:27:58PM +0100, Hans de Goede wrote:
>> Hi,
>>
>> On 30-01-17 16:11, Ville Syrjälä wrote:
>>> On Mon, Jan 30, 2017 at 04:02:19PM +0100, Hans de Goede wrote:
>>>> Hi,
>>>>
>>>> On 30-01-17 14:10, Ville Syrjälä wrote:
>>>>> On Sat, Jan 28, 2017 at 06:18:45PM +0100, Hans de Goede wrote:
>>>>>> Hi,
>>>>>>
>>>>>> On 01/28/2017 05:25 PM, Hans de Goede wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> On 01/27/2017 02:51 PM, Ville Syrjälä wrote:
>>>>>>>> On Mon, Jan 23, 2017 at 10:09:58PM +0100, Hans de Goede wrote:
>>>>>>>>> Make sure the P-Unit or the PMIC i2c bus is not in use when we send a
>>>>>>>>> request to the P-Unit by calling iosf_mbi_punit_acquire() / _release()
>>>>>>>>> around P-Unit write accesses.
>>>>>>>>
>>>>>>>> Can't we just stuff the calls into the actual punit write function
>>>>>>>> rather than sprinkling them all over the place?
>>>>>>>
>>>>>>> punit access is acquired across sections like this:
>>>>>>>
>>>>>>>         iosf_mbi_punit_acquire();
>>>>>>>
>>>>>>>         val = vlv_punit_read(dev_priv, PUNIT_REG_DSPFREQ);
>>>>>>>         val &= ~DSPFREQGUAR_MASK;
>>>>>>>         val |= (cmd << DSPFREQGUAR_SHIFT);
>>>>>>>         vlv_punit_write(dev_priv, PUNIT_REG_DSPFREQ, val);
>>>>>>>         if (wait_for((vlv_punit_read(dev_priv, PUNIT_REG_DSPFREQ) &
>>>>>>>                       DSPFREQSTAT_MASK) == (cmd << DSPFREQSTAT_SHIFT),
>>>>>>>                      50)) {
>>>>>>>                 DRM_ERROR("timed out waiting for CDclk change\n");
>>>>>>>         }
>>>>>>>         iosf_mbi_punit_release();
>>>>>>>
>>>>>>> Where we want to wait for the requested change to have taken
>>>>>>> effect before releasing the punit.
>>>>>
>>>>> Hmm. That's somewhat unfortunate. It also highlights a problem with the
>>>>> patch wrt. RPS. We don't wait for the GPU to actually change frequencies
>>>>> in set_rps() because that would slow things down too much. So I have to
>>>>> wonder how much luck is needed to make this workaround really effective.
>>>>
>>>> So the history of this patch-set is that I wrote this patch before
>>>> writing the patch to get FORCEWAKE_ALL before the pmic bus becomes
>>>> active (patch 12/13). Since a lot of testing was done with this
>>>> patch included in the patch-set and since it seemed a good idea
>>>> regardless (given my experience with accessing the punit vs
>>>> pmic bus accesses) I decided to leave it in.
>>>>
>>>> Possibly just the patch to get FORCEWAKE_ALL is enough, that one
>>>> actually fixed things for me. That is also why I made this the
>>>> last patch in the set. I asked tagorereddy to test his system
>>>> without this patch, but he did not get around to that.
>>>>
>>>> After all we do tell the punit to not touch the bus by acquiring
>>>> the pmic bus semaphore from i2c-desigware-baytrail.c, so maybe
>>>> for RPS freq changes it honors that and properly waits. Maybe it
>>>> honors that for all punit requests i915 does and the only real
>>>> problem is the forcewake stuff ?
>>>>
>>>> I can try to drop this patch from my queue and run without it
>>>> for a while and see if things don't regress. And also ask
>>>> tagorereddy again to test his system that way.
>>>>
>>>> Does that (dropping this patch for now) sound like a good idea?
>>>
>>> More test results couldn't hurt at least. It also makes me wonder if
>>> just bumping the timeouts to some ridiculously high value would fix
>>> the problem as well.
>>
>> I've already tried bumping the forcewake timeout from 50 to 250ms,
>> before writing the patch to just get forcewake_all before the pmic
>> bus access begins, that does not fix things,
>
> And you bumped the i2c mutex timeout as well? Or does that fail somehow
> gracefully if it can't get the mutex?

It will fail the i2c transfer with -ETIMEOUT, which will make the driver
report an error instead of e.g. the battery level, but it should not
affect the forcewake calls and those still failed with the large
timeout. So yes basically the i2c mutex fails gracefully.

>
>> and since we busy wait
>> for this timeout from non-sleeping context 250ms already is way too
>> high.
>
> Sure, but I'm just trying to understand if the problem is simply caused
> by proceeding with some hardware access without getting the i2c mutex.

Understood.

>>>>>>>> + a comment would be nice why it's there.
>>>>>>>
>>>>>>> I will add comments to the acquire calls.
>>>>>>>
>>>>>>>> Do we need a kconfig select/depends on the iosf_mbi thing? Or some
>>>>>>>> ifdefs?
>>>>>>>
>>>>>>> No, the iosf_mbi header defines empty inline versions of
>>>>>>> iosf_mbi_punit_acquire / _release if IOSF_MBI is disabled,
>>>>>>> this does mean that iosf_mbi must be builtin if the i915
>>>>>>> driver is. I'll add:
>>>>>>>
>>>>>>>     depends on DRM_I915=IOSF_MBI || IOSF_MBI=y
>>>>>>>
>>>>>>> To the i915 Kconfig to enforce this.
>>>>>>
>>>>>> Hmm, ok so that does not work (long cyclic dependency through the
>>>>>> selection of ACPI_VIDEO).
>>>>>>
>>>>>> So I've now added this instead:
>>>>>>
>>>>>> 	# iosf_mbi needs to be builtin if we are builtin
>>>>>> 	select IOSF_MBI if DRM_I915=y
>>>>>
>>>>> That's probably not going to help anyone since i915 is usually a module.
>>>>
>>>> Right, that is fine, then either the IOSF_MBI symbols are available,
>>>> or IOSF_MBI is disabled and we get the inline nops from the header.
>>>>
>>>> The problem scenario is DRM_I915=y and IOSF_MBI=m, which is not very
>>>> realistic IMHO, but will get triggered by the random-config testing
>>>> several contributors do and lead to an unresolved symbol error there.
>>>
>>> Well, from the user POV anything with IOSF_MBI==n can be a problem.
>>> So I'm not sure if we should allow that.
>>
>> So you're suggesting we just add an unconditional "select IOSF_MBI"
>> to the i915 Kconfig entry?
>
> Yeah, that should at least cut down the number of people accidentally
> misconfiguring their kernels and hitting this problem in the future.

Ok.

Regards,

Hans

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