linux-kernel - Re: [PATCHv2 2/4] ACPI / LPSS: custom power domain for LPSS

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Message-ID: <20140523130252.GH18308@xps8300>
Date:	Fri, 23 May 2014 16:02:52 +0300
From:	Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Mike Turquette <mturquette@...aro.org>,
	Jin Yao <yao.jin@...ux.intel.com>,
	Li Aubrey <aubrey.li@...ux.intel.com>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 2/4] ACPI / LPSS: custom power domain for LPSS

On Fri, May 23, 2014 at 03:10:08PM +0200, Rafael J. Wysocki wrote:
> On Friday, May 23, 2014 03:30:53 PM Heikki Krogerus wrote:
> > On Thu, May 22, 2014 at 01:28:16AM +0200, Rafael J. Wysocki wrote:
> > > On Wednesday, May 21, 2014 01:52:58 PM Heikki Krogerus wrote:
> > > > On Wed, May 21, 2014 at 01:01:31PM +0200, Rafael J. Wysocki wrote:
> > > > > On Wednesday, May 21, 2014 01:05:11 PM Heikki Krogerus wrote:
> > > > > > On Tue, May 20, 2014 at 11:33:09PM +0200, Rafael J. Wysocki wrote:
> > > > > > > First, is the 10 ms sleep really necessary?  I'd expect the AML to take care of
> > > > > > > such delays (this is not a PCI device formally).
> > > > > > 
> > > > > > Unfortunately that is not the case. There is nothing in the AML for
> > > > > > this. Mika, correct me if I'm wrong.
> > > > > > 
> > > > > > > And because this is not a PCI device formally, why is the comment talking about
> > > > > > > the PCI spec?  Why is PCI relevant in any way here?
> > > > > > 
> > > > > > Under the hood the devices are still PCI devices, even if they
> > > > > > formally aren't. Maybe I should point that out in the comment..
> > > > > > 
> > > > > > We put the sleep there because without it there was no guarantee if
> > > > > > the device was properly resumed by the time the drivers resume hooks
> > > > > > were called. The symptom in case of a failure was simply that the
> > > > > > registers could not be written, which leads into timeouts at least in
> > > > > > case of the I2C and UART and making them unusable until the next
> > > > > > suspend followed by resume.
> > > > > 
> > > > > OK, so the msleep() is functionally necessary.  Instead of talking about the
> > > > > PCI in the comment, which will make a casual reader think "What the heck?",
> > > > > please say something like "the delay is necessary for the subsequent register
> > > > > writes to succeed on <example system>".
> > > > 
> > > > OK.
> > > 
> > > So I have one more concern.  Namely, async suspend is not enabled for the LPSS
> > > devices, so the delays will accumulate for them and that may become a big deal
> > > at one point.
> > > 
> > > This may be addressed either (1) by enabling async suspend for them or, which would
> > > be more complicated, by doing the msleep() once for the whole LPSS in .resume_early()
> > > and restoring the register values in .resume() without delaying.
> > > 
> > > For (1) I have the following untested patch (on top of my bleeding-edge branch, but
> > > it should apply to the mainline too if I haven't overlooked anything).  Can you
> > > please try it on boxes with LPSS and see if it doesn't break suspend/resume on them?
> > 
> > Done, and there were no problems. I tested it with HSW, BYT and also BDW.
> 
> Great, thanks!
> 
> So I'll add a changelog and I'm going to push it along with your series.
> 
> Are you going to update the $subject patch, or send an update on top of
> linux-next?

I'll send an update right away. On top of explaining the problem if
left without the msleep(10), I still left the comment about the PCI
spec plus a mention that all LPSS devices are actually PCI devices. I
hope that's OK with you. It just felt like we need to point out that
the 10ms is not pulled out of the hat.


-- 
heikki
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