Re: [PATCH 1/3] i2c: bcm2835: Fix hang for writing messages larger than 16 bytes

From: Noralf TrÃnnes
Date: Mon Sep 19 2016 - 13:36:32 EST

Den 19.09.2016 18:51, skrev Eric Anholt:
Noralf TrÃnnes <noralf@xxxxxxxxxxx> writes:

Writing messages larger than the FIFO size results in a hang, rendering
the machine unusable. This is because the RXD status flag is set on the
first interrupt which results in bcm2835_drain_rxfifo() stealing bytes
from the buffer. The controller continues to trigger interrupts waiting
for the missing bytes, but bcm2835_fill_txfifo() has none to give.
In this situation wait_for_completion_timeout() apparently is unable to
stop the madness.

The BCM2835 ARM Peripherals datasheet has this to say about the flags:
TXD: is set when the FIFO has space for at least one byte of data.
RXD: is set when the FIFO contains at least one byte of data.
TXW: is set during a write transfer and the FIFO is less than full.
RXR: is set during a read transfer and the FIFO is or more full.

Implementing the logic from the downstream i2c-bcm2708 driver solved
the hang problem.

Signed-off-by: Noralf TrÃnnes <noralf@xxxxxxxxxxx>
Patches 1, 3 are:

Reviewed-by: Eric Anholt <eric@xxxxxxxxxx>

For patch 2 I followed some of it, but couldn't quite say it's a review.
I trust your testing, and that the path has had a lot more testing in
the downstream tree, so it's:

I don't know how much testing downstream really has had since this feature
is disabled by default. So there is a possibility that this can break some
devices (while at the same time enable others).
I wondered about adding a module parameter so it could be disabled at
runtime, but decided to see what the review would be first.
It is of course possible to add a this module parameter later should it
turn out to break some device.


Acked-by: Eric Anholt <eric@xxxxxxxxxx>