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

["Grygorii.Strashko@linaro.org" <grygorii.strashko@linaro.org>]

On 04/23/2015 04:11 PM, Nishanth Menon wrote:
> On 04/23/2015 05:17 AM, Grygorii.Strashko@linaro.org wrote:
>> On 04/23/2015 03:00 AM, Nishanth Menon wrote:
>>> On 04/22/2015 08:26 AM, Grygorii.Strashko@linaro.org 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@linaro.org>

-- 
regards,
-grygorii