Re: [PATCH REGRESSION-FIX resend] pwm: lpss: Set enable-bit before waiting for update-bit

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

Hi,

On 31-03-17 22:07, Andy Shevchenko wrote:
> On Wed, 2017-03-29 at 20:25 +0200, Hans de Goede wrote:
>> Hi,
>>
>> On 29-03-17 19:41, Andy Shevchenko wrote:
>>> On Wed, 2017-03-29 at 14:42 +0200, Hans de Goede wrote:
>
> Thanks for your patience and valuable input.
>
> So, I found CharryTrail with enabled PWM (UP board v0.4) and confirm the
> bug.
>
> Moreover, I have re-tested again all 4 platforms with and without your
> fix, and I dunno how I did not notice this before, but looks like either
> mine (though commit message shows that I have tested on 3 platforms at
> least, so, I can re-test for sure) or Ilkka's patch broke it on all
> platforms except Broxton / Apollo Lake.
>
> So, summurize what we need is a quirk for Broxton / Apollo Lake. I need
> to check Gemini Lake also to be sure.
>
> And we definitely need this as a fix for stable. I would appreciate if
> you can figure out which patch from previous series (b14e8ceff034
> or 10d56a4cb1c6) broke it.

My patch commit msg says: "fixes 10d56a4cb1c6c894c60acbaec0f8aa44aba833b0"
and unless my memory deceives me I tested that that was the bad commit
by reverting it.

The problem for Cherry Trail is not perse the order in which the
update / enable bits are written (AFAICT), but the waiting for
the update bit to clear while the enable bit is not set, which
is why my latest version only moves the wait.

That wait was already present before 10d56a4cb1c6, but with a
reasonable short timeout, and just continuing on after the
timeout. 10d56a4cb1c6 makes pwm_lpss_apply() exit with an
error (without ever setting the enable bit) when the timeout
expires.

The troublesome commit caused 2 issues:
1) An IMHO unacceptable long timeout (0.5 seconds of near busy waiting on each enable)
2) Erroring out when the timeout expires without the update flag clearing

Regards,

Hans