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

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Date:   Mon, 19 Sep 2016 19:36:23 +0200
From:   Noralf Trønnes <noralf@...nnes.org>
To:     Eric Anholt <eric@...olt.net>, wsa@...-dreams.de,
        swarren@...dotorg.org
Cc:     linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-rpi-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/3] i2c: bcm2835: Fix hang for writing messages larger
 than 16 bytes


Den 19.09.2016 18:51, skrev Eric Anholt:
> Noralf Trønnes <noralf@...nnes.org> 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 Trønnes <noralf@...nnes.org>
> Patches 1, 3 are:
>
> Reviewed-by: Eric Anholt <eric@...olt.net>
>
> 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.

Noralf.

> Acked-by: Eric Anholt <eric@...olt.net>
>
> Thanks!