Re: [PATCH] rtc: pcf8563: add SMBus support

[Alexandre Belloni <alexandre.belloni@bootlin.com>]

On 01/10/2026 15:24:07+0200, Michael Walle wrote:
> On Wed Sep 30, 2026 at 5:25 PM CEST, Alexandre Belloni wrote:
> >> Because of this fallback, when pcf8563_rtc_read_time() reads the time:
> >> 
> >> drivers/rtc/rtc-pcf8563.c:pcf8563_rtc_read_time() {
> >>     ...
> >>     err = regmap_bulk_read(pcf8563->regmap, PCF8563_REG_ST1, buf,
> >>                            sizeof(buf));
> >>     ...
> >> }
> >> 
> >> The bulk read executes as a loop of separate single-byte SMBus reads instead
> >> of an atomic transaction. Since each SMBus byte transaction issues its own
> >> STOP condition, the RTC's internal register latch is released between reads.
> >> 
> >> Does this allow the hardware time to roll over mid-read and result in
> >> corrupt time data?
> >
> > This is a valid concern, did you test?
> 
> Yes, did you read my first reply? I'm not sure how this is handled
> in the RTC subsystem. Most regmap converted RTC should fall into
> this case. How do you propose to handle that? Instead of removing
> the check, we could just test for either I2C or SMBus block read.

For some reason, I didn't get you reply in my inbox. For the RTCs that
are not latching, we do read the sec register twice. I guess testing for
I2C or SMBus block read would make more sense here.


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