Re: [PATCH V2] drivers/rtc/rtc-ds1307.c: Enable the mcp794xx alarm after programming time

[Grygorii.Strashko@xxxxxxxxxx]

On 04/23/2015 04:11 PM, Nishanth Menon wrote:
> On 04/23/2015 05:17 AM, Grygorii.Strashko@xxxxxxxxxx wrote:
> > On 04/23/2015 03:00 AM, Nishanth Menon wrote:
> > > On 04/22/2015 08:26 AM, Grygorii.Strashko@xxxxxxxxxx wrote:
> > > > Hi,
> > > >
> > > > On 04/21/2015 03:51 AM, Nishanth Menon wrote:
> > > > > Alarm interrupt enable register is at offset 0x7, while the time
> > > > > registers for the alarm follow that. When we program Alarm interrupt
> > > > > enable prior to programming the time, it is possible that previous
> > > > > time value could be close or match at the time of alarm enable
> > > > > resulting in interrupt trigger which is unexpected (and does not match
> > > > > the time we expect it to trigger).
> > > > >
> > > > > To prevent this scenario from occuring, program the ALM0_EN bit only
> > > > > after the alarm time is appropriately programmed.
> > > > >
> > > > > Ofcourse, I2C programming is non-atomic, so there are loopholes where
> > > > > the interrupt wont trigger if the time requested is in the past at
> > > > > the time of programming the ALM0_EN bit. However, we will not have
> > > > > unexpected interrupts while the time is programmed after the interrupt
> > > > > are enabled.
> > > >
> > > > I think it will be nice if you will mention that you going to follow
> > > > vendor recommendations - AN1491 Configuring the MCP794XX RTCC Family
> > > > http://ww1.microchip.com/downloads/en/AppNotes/01491A.pdf
> > > > ;)
> > > > "Also, it is recommended that the alarm registers be loaded
> > > > before the alarm is enabled."
> > >
> > > Hmm... i did not know that existed, thanks for digging it up.. that
> > > teaches me to look for docs before putting a scope/LA on the board
> > > (not that I regret doing that)... That said, reading the app note, I
> > > kind of realized:
> > > a) that playing with ST bit for programming time is not done, but
> > > then, that implies that oscillator will have to be restarted (upto a
> > > few seconds for certain crystals).. but that said, it does not seem
> > > mandatory or seem to (yet seen) functional issues...
> > >
> > > b) We dont have flexibility yet to describe if we do indeed have a
> > > backup battery or not - VBATEN should be set only if we have a backup
> > > battery on the platform :( - on some it might even be optional thanks
> > > to certain compliance requirements of shipping boards internationally
> > > and general "unlike" of lithium ion in cargo hold..
> > >
> > > c) we dont have capability to control the alarm polarity in the driver
> > > which, by the way, we probably should also control OUT polarity (when
> > > ALARM is not asserted)..
> > >
> > > d) we dont have support for external 32k oscillator(X1 only) instead
> > > of assuming we always have a 32k crystal(X1 and X2)...
> > >
> > > Ugghhh... more cleaning up to do for the future..
> > >
> > > that said, the sequence it does recommend (in page 4):
> > > The following steps show how the Alarm 0 is config-
> > > ured. Alarm 1 can be configured in the same manner.
> > > 1.    Write 0x23 to the Alarm0 Seconds register
> > > [0x0A].
> > > 2.    Write 0x47 to the Alarm0 Minutes register
> > > [0x0B].
> > > 3.    Write 0x71 to the Alarm0 Hours register [0x0C]
> > > – 11 hours in 12-hour format.
> > > 4.    Write 0x72 to the Alarm0 Day register [0x0D] –
> > > Tuesday + Alarm Polarity Low + Match on all.
> > > The Alarm0 Interrupt Flag is also cleared.
> > > 5.    Write 0x14 to the Alarm0 Date register [0x0E].
> > > 6.    Write 0x08 to the Alarm0 Month register [0x0F].
> > > With all the Alarm0 registers set we can now activate
> > > the Alarm0 on the Control register.
> > > 7.    Write 0x10 to the Control register [0x07] –
> > > Alarm0 enabled no CLKOUT, Alarm1 disabled
> > >
> > > before this patch we do ( http://pastebin.ubuntu.com/10863880/)
> > > 	CONTROL r[7] = 0x90 (OUT=1, ALM0EN=1)
> > > 	OSCTRIM r[8] = 0x00
> > > 	EEUNLOCK r[9] = 0x00
> > > 	ALM0SEC r[A] = 0x01
> > > 	ALM0MIN r[B] = 0x45
> > > 	ALM0HOUR r[C] = 0x23
> > > 	ALM0WKDAY r[D] = 0x75 <-ALMOIF is cleared
> > > 	ALM0DATE r[E] = 0x09
> > > 	ALM0MTH r[F] = 0x04
> > > 	RSRVED r[10] = 0x01
> > >
> > > with this patch, we do:
> > > burst(	CONTROL r[7] = 0x80 (OUT=1)
> > > 	OSCTRIM r[8] = 0x00
> > > 	EEUNLOCK r[9] = 0x00
> > > 	ALM0SEC r[A] = 0x01
> > > 	ALM0MIN r[B] = 0x45
> > > 	ALM0HOUR r[C] = 0x23
> > > 	ALM0WKDAY r[D] = 0x75 <-ALMOIF is cleared
> > > 	ALM0DATE r[E] = 0x09
> > > 	ALM0MTH r[F] = 0x04
> > > 	RSRVED r[10] = 0x01
> > > )
> > > 	CONTROL r[7] = 0x90 (OUT=1, ALM0EN=1)
> > >
> > > Which is slightly unoptimal way of what the app note recommends. - as
> > > I mentioned earlier in this thread, I will try and do optimizations in
> > > a later patch.
> > >
> > > Given that Andrew had picked up this patch, I dont see a reason to
> > > respin this yet. but will include the app note for future patches -
> > > thanks for pointing it out to me.
> >
> > ^^ Up to you. Np, Always yours!
>
> Considering the narrow focus of the current patch (which does fix an
> issue that it attempts to), can I get an Ack?

Reviewed-by: Grygorii Strashko <grygorii.strashko@xxxxxxxxxx>

--
regards,
-grygorii
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