linux-kernel - Re: [PATCH v2] intel8x0m: wait a bit before warm reset check

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Date:	Mon, 07 Mar 2011 10:55:26 +0100
From:	Takashi Iwai <tiwai@...e.de>
To:	Jesper Juhl <jj@...osbits.net>
Cc:	Paul Bolle <pebolle@...cali.nl>, Jaroslav Kysela <perex@...ex.cz>,
	alsa-devel@...a-project.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] intel8x0m: wait a bit before warm reset check

At Sun, 6 Mar 2011 01:12:26 +0100 (CET),
Jesper Juhl wrote:
> 
> On Sun, 6 Mar 2011, Paul Bolle wrote:
> 
> > At every resume a laptop I use prints this message (at KERN_ERR level):
> >     ALSA sound/pci/intel8x0m.c:904: AC'97 warm reset still in progress? [0x2]
> > 
> > The thing to note here is that 0x2 corresponds to ICH_AC97COLD. Ie, what
> > seems to be happening is that the register involved indicated a warm
> > reset for some time (as the ICH_AC97WARM bit was set) but by the time
> > the warning is printed, and that same register is checked again, that
> > bit is already cleared and only the ICH_AC97COLD bit is still set.
> > 
> > It turns out a warm reset needs some time to settle, but it is currently
> > checked right away. The test therefore fails the first time it is done
> > and schedule_timeout_uninterruptible() will be called. Once we return
> > from that jiffies is already (far) past end_time on this laptop, so we
> > exit the loop, print a warning, and exit the function while the warm
> > reset actually succeeded.
> > 
> > One way to fix this is to call udelay() after writing to the register
> > involved. A handful of tests suggests 500 usecs is a safe value. (This
> > might punish the "finish cold reset" case, but on this laptop such a
> > cold reset apparently never happens, so I can't say for sure.)
> > 
> > While we're at it drop the extra single tick from end_time, as it looks
> > rather silly.
> > 
> > Signed-off-by: Paul Bolle <pebolle@...cali.nl>
> > ---
> > 0) v1 was called " intel8x0m: schedule timeout before warm reset check".
> > 
> > 1) A udelay of 250 usecs always failed while 300 usecs always worked. So
> > 500 usecs might be a bit cautious.
> > 
> 
> Wouldn't it be nice to put these notes in a comment in the code so that 
> future readers don't have to scratch their heads and wonder where the 
> udelay() with some magical constant comes from?

That'd be better, indeed.

> Also, if udelay(300) always worked in your tests, wouldn't udelay(400) be 
> more than enough to provide a nice fat safety margin (33% more than 
> "always works")?

Well...  I think you know how meaningless to quote a percentage
representation in such a case...

> Are there no official documents that specify how long 
> this is expected to take that we could base this delay on instead of just 
> testing on one system?

Heh, can you really trust such a document? ;)

It's basically a workaround for a specific codec chip that doesn't set
the bit reflecting to the right status after resume/power-up.  That
is, it's just for the codec chip on Paul's machine.  Others work well
without this delay.

Also, as mentioned earlier, it's better to use usleep_range() than
udelay().  There is no hard upper-limit in this case, and any longer
sleep works.


thanks,

Takashi
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