linux-kernel - Re: [PATCH 08/12] rtc: omap: restore irq state after reading TC registers

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Date:	Wed, 22 Oct 2014 12:50:11 +0200
From:	Johan Hovold <johan@...nel.org>
To:	Felipe Balbi <balbi@...com>
Cc:	Johan Hovold <johan@...nel.org>,
	Alessandro Zummo <a.zummo@...ertech.it>,
	Tony Lindgren <tony@...mide.com>,
	Benoît Cousson <bcousson@...libre.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lokesh Vutla <lokeshvutla@...com>,
	Guenter Roeck <linux@...ck-us.net>, nsekhar@...com,
	t-kristo@...com, j-keerthy@...com, linux-omap@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
	rtc-linux@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/12] rtc: omap: restore irq state after reading TC
 registers

On Sat, Oct 11, 2014 at 07:47:58PM -0500, Felipe Balbi wrote:
> On Sat, Oct 11, 2014 at 12:12:01PM +0200, Johan Hovold wrote:
> > On Fri, Oct 10, 2014 at 01:02:31PM -0500, Felipe Balbi wrote:
> > > Hi,
> > > 
> > > On Thu, Oct 09, 2014 at 09:06:30PM +0200, Johan Hovold wrote:
> > > > Make sure to restore local irq state when reading the timer/calendar
> > > > (TC) registers, so that omap_rtc_read_time() can be called with
> > > > interrupts disabled.
> > > > 
> > > > Signed-off-by: Johan Hovold <johan@...nel.org>
> > > > ---
> > > >  drivers/rtc/rtc-omap.c | 6 ++++--
> > > >  1 file changed, 4 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/drivers/rtc/rtc-omap.c b/drivers/rtc/rtc-omap.c
> > > > index 0ef016553a97..62e2e9a9887a 100644
> > > > --- a/drivers/rtc/rtc-omap.c
> > > > +++ b/drivers/rtc/rtc-omap.c
> > > > @@ -239,8 +239,10 @@ static void bcd2tm(struct rtc_time *tm)
> > > >  
> > > >  static int omap_rtc_read_time(struct device *dev, struct rtc_time *tm)
> > > >  {
> > > > +	unsigned long flags;
> > > > +
> > > >  	/* we don't report wday/yday/isdst ... */
> > > > -	local_irq_disable();
> > > > +	local_irq_save(flags);
> > > 
> > > you should really convert these to a real spin_lock_irq*(), that's
> > > because local_irq* do not get re-written with RT patchset, so this
> > > pretty much "breaks" RT.
> > 
> > The driver uses local_irq* throughout to guarantee registers are not
> > read or written during an update event.
> > 
> > In fact, at least on AM33xx, this is not even necessary when reading (as
> > opposed to writing) the TC registers, but I did not dare change that
> > without knowing how the legacy platforms work in this respect.
> > 
> > Do you suggest doing this conversion as part of, or on top of, this
> > series?
> 
> probably on top of is. Safer that way.

I didn't include this conversion in v2 as you may have noticed. In fact,
I'm not even sure it should be done.

Using local_irq* "breaks" RT, but if we change it to spin_lock_irq* and
RT preempts us, we may miss the 15us access window which would break the
driver:

	"If the ARM accesses the TC registers outside of the access
	 period, then the access is not guaranteed."

On am33xx this only affects updating the TC (timer/calendar) or alarm
registers, but the driver currently disables interrupts also on reading
the time (possibly because the early IP revisions did not have the
shadow read registers).

Johan
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