battery switch-over detection on pcf2127

battery switch-over detection on pcf2127

[Rasmus Villemoes]

Hi Bruno

I just noticed your "rtc: pcf2127: add tamper detection support"
(03623b4b04) from 5.4. Unfortunately, clearing the BTSE bit breaks a use
case of ours:

We rely on the battery switch-over detection to distinguish a powerfail
during boot from a PORESET by the external watchdog (in the latter case,
the RTC is still powered throughout, meaning there is no battery
switch-over event). OTOH, we do not use the tamper detection - in fact,
the TS signal is unconnected on our board.

We're currently still on 4.19, but we will eventually upgrade to a
kernel containing the above commit. So I was wondering if we could
figure out a way that would work for both of us - either some CONFIG
knob, or perhaps something in the device-tree. Any ideas?

Rasmus

Re: battery switch-over detection on pcf2127

[Alexandre Belloni]

On 05/05/2020 21:54:47+0200, Rasmus Villemoes wrote:
> Hi Bruno
> 
> I just noticed your "rtc: pcf2127: add tamper detection support"
> (03623b4b04) from 5.4. Unfortunately, clearing the BTSE bit breaks a use
> case of ours:
> 
> We rely on the battery switch-over detection to distinguish a powerfail
> during boot from a PORESET by the external watchdog (in the latter case,
> the RTC is still powered throughout, meaning there is no battery
> switch-over event). OTOH, we do not use the tamper detection - in fact,
> the TS signal is unconnected on our board.
> 
> We're currently still on 4.19, but we will eventually upgrade to a
> kernel containing the above commit. So I was wondering if we could
> figure out a way that would work for both of us - either some CONFIG
> knob, or perhaps something in the device-tree. Any ideas?
> 

Yes, I was working on a patch series last week allowing to read BF. I'm
not sure clearing BTSE is your issue but clearing BF is.

I'm going to send it tonight, I'll copy you, let me now if that works
for you. You can then read BF using the RTC_VL_READ ioctl. The
RTC_VL_BACKUP_SWITCH flag will be set if a switchover happened.
The RTC_VL_CLR ioctl can be used to clear the flag.

I think clearing BTSE is still the right thing to do.

Regards,

-- 
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

Re: battery switch-over detection on pcf2127

[Bruno Thomsen]

Hi Rasmus

Den tir. 5. maj 2020 kl. 22.07 skrev Alexandre Belloni
<alexandre.belloni@bootlin.com>:
>
> On 05/05/2020 21:54:47+0200, Rasmus Villemoes wrote:
> > Hi Bruno
> >
> > I just noticed your "rtc: pcf2127: add tamper detection support"
> > (03623b4b04) from 5.4. Unfortunately, clearing the BTSE bit breaks a use
> > case of ours:
> >
> > We rely on the battery switch-over detection to distinguish a powerfail
> > during boot from a PORESET by the external watchdog (in the latter case,
> > the RTC is still powered throughout, meaning there is no battery
> > switch-over event). OTOH, we do not use the tamper detection - in fact,
> > the TS signal is unconnected on our board.
> >
> > We're currently still on 4.19, but we will eventually upgrade to a
> > kernel containing the above commit. So I was wondering if we could
> > figure out a way that would work for both of us - either some CONFIG
> > knob, or perhaps something in the device-tree. Any ideas?
> >
>
> Yes, I was working on a patch series last week allowing to read BF. I'm
> not sure clearing BTSE is your issue but clearing BF is.
>
> I'm going to send it tonight, I'll copy you, let me now if that works
> for you. You can then read BF using the RTC_VL_READ ioctl. The
> RTC_VL_BACKUP_SWITCH flag will be set if a switchover happened.
> The RTC_VL_CLR ioctl can be used to clear the flag.
>
> I think clearing BTSE is still the right thing to do.

I think your use case is valid and it sounds like Alexandre solution will
solve it as you just need to know if a battery switch-over has happened
not when exactly it happened.

I can help test the patches too. Now without google auto html..

Bruno

Re: battery switch-over detection on pcf2127

[Rasmus Villemoes]

On 05/05/2020 22.38, Bruno Thomsen wrote:
> Hi Rasmus
> 
> Den tir. 5. maj 2020 kl. 22.07 skrev Alexandre Belloni
> <alexandre.belloni@bootlin.com>:
>>
>> On 05/05/2020 21:54:47+0200, Rasmus Villemoes wrote:
>>> Hi Bruno
>>>
>>> I just noticed your "rtc: pcf2127: add tamper detection support"
>>> (03623b4b04) from 5.4. Unfortunately, clearing the BTSE bit breaks a use
>>> case of ours:
>>>
>>> We rely on the battery switch-over detection to distinguish a powerfail
>>> during boot from a PORESET by the external watchdog (in the latter case,
>>> the RTC is still powered throughout, meaning there is no battery
>>> switch-over event). OTOH, we do not use the tamper detection - in fact,
>>> the TS signal is unconnected on our board.
>>>
>>> We're currently still on 4.19, but we will eventually upgrade to a
>>> kernel containing the above commit. So I was wondering if we could
>>> figure out a way that would work for both of us - either some CONFIG
>>> knob, or perhaps something in the device-tree. Any ideas?
>>>
>>
>> Yes, I was working on a patch series last week allowing to read BF. I'm
>> not sure clearing BTSE is your issue but clearing BF is.
>>
>> I'm going to send it tonight, I'll copy you, let me now if that works
>> for you. You can then read BF using the RTC_VL_READ ioctl. The
>> RTC_VL_BACKUP_SWITCH flag will be set if a switchover happened.
>> The RTC_VL_CLR ioctl can be used to clear the flag.
>>
>> I think clearing BTSE is still the right thing to do.
> 
> I think your use case is valid and it sounds like Alexandre solution will
> solve it as you just need to know if a battery switch-over has happened
> not when exactly it happened.
> 
> I can help test the patches too.

Thanks for the quick replies, both. Unfortunately, being able to read BF
from linux is not relevant to us - all the handling happens early in the
bootloader (including clearing BF, so that we can detect that the
previous boot failed only because of power fail - hence whether the
linux driver clears BF or not is not relevant). We really just want
linux to not touch the bits in CTRL3 at all.

Hm, wait. Re-reading the above suggests that BF can get set even if BTSE
is not, and a quick experiment shows that is true - I must have misread
the data sheet. While I think that's fine for now (currently I only
print the time of last switch-over as a diagnostic), I did have some use
case in mind for comparing that timestamp to the current time and make
decisions based on that. But until I figure out exactly what I want to
use it for, and until we actually upgrade to 5.4+, there's no rush.

Thanks,
Rasmus

Re: battery switch-over detection on pcf2127

[Alexandre Belloni]

On 05/05/2020 23:01:19+0200, Rasmus Villemoes wrote:
> Thanks for the quick replies, both. Unfortunately, being able to read BF
> from linux is not relevant to us - all the handling happens early in the
> bootloader (including clearing BF, so that we can detect that the
> previous boot failed only because of power fail - hence whether the
> linux driver clears BF or not is not relevant). We really just want
> linux to not touch the bits in CTRL3 at all.
> 

Well, in that case, Linux doesn't touch the BF bit anymore unless
userspace uses the ioctls so you should be ok using it from your
bootloader.

> Hm, wait. Re-reading the above suggests that BF can get set even if BTSE
> is not, and a quick experiment shows that is true - I must have misread
> the data sheet. While I think that's fine for now (currently I only
> print the time of last switch-over as a diagnostic), I did have some use
> case in mind for comparing that timestamp to the current time and make
> decisions based on that. But until I figure out exactly what I want to
> use it for, and until we actually upgrade to 5.4+, there's no rush.
> 

Configuring the timestamps is something else I want to do but I still
didn't finish to design the proper interface.

It is definitively on the todo list.

-- 
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com