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

[Martin Sperl]

On 19.09.2016 17:26, Noralf Trønnes wrote:
> 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.
I remember having seen similar interrupt issues with the SPI HW-block.
> In this situation wait_for_completion_timeout() apparently is unable to
> stop the madness.
That is because it is a level irq that immediately triggers another 
interrupt giving
no CPU no time to do other (threaded) OS activity - this might be slightly
different for multi-core machines...
> 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 at tronnes.org>
Reviewed-by: Martin Sperl <kernel at martin.sperl.org>