[rtc-linux] Re: [PATCH 4/6] rtc: rv8803: Always apply the I²C workaround

On 21/07/2016 at 14:34, Beno=C3=AEt Th=C3=A9baudeau wrote:

On 21/07/2016 at 13:10, Alexandre Belloni wrote:

quoted

On 21/07/2016 at 12:41:30 +0200, Beno=C3=AEt Th=C3=A9baudeau wrote :

quoted

The I=C2=B2C NACK issue of the RV-8803 may occur after any I=C2=B2C STA=

RT

quoted

quoted

condition, depending on the timings. Consequently, the workaround must
be applied for all the I=C2=B2C transfers.

This commit abstracts the I=C2=B2C transfer code into register access
functions. This avoids duplicating the I=C2=B2C workaround everywhere. =

This

quoted

quoted

also avoids the duplication of the code handling the return value of
i2c_smbus_read_i2c_block_data(). Error messages are issued in case of
definitive register access failures (if the workaround fails). This
change also makes the I=C2=B2C transfer return value checks consistent.

Well, my initial idea was that the workaround is actually needed only
for operations that are not restartable from userspace.

Did you it that bug? On which RTC?

No, I've not seen that bug. However, the errata sheet says that this issu=

e may

occur if two consecutive I=C2=B2C START conditions are slightly more than=

 950 ms

apart. It does not even say that this is restricted to I=C2=B2C transfers
addressing
this RTC. Moreover, if the time is read or set from userspace, this might
occur
at any time relatively to an RTC interrupt or to a previous time read/set
operation, so possibly 950 ms afterwards.

All in all, it's safer and easier to always apply this workaround, all th=

e

more
it is applied at almost no cost when there is no issue.

To fully answer your questions, it's the job of the driver to execute the
userspace requests properly without returning spurious errors. Userspace mi=
ght
consider that such errors are definitive and that there is no reason to try=
 and
restart the operation, which could cause all sorts of issues. In other word=
s,
the driver should not rely on userspace operation restarts as a kind of
workaround if it can easily handle this itself.

Also, the kernel itself (not only userspace) may read the RTC time (with
CONFIG_RTC_HCTOSYS), without userspace caring.

I'm rather working with the RX8900.

Best regards,
Beno=C3=AEt

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