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

Johan Hovold
2014-10-22T10:50:11Z

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@kernel.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