[BUG] i2c-designware silently fails on long transfers

[Russell King - ARM Linux]

On Mon, Nov 21, 2016 at 12:29:01PM +0200, Mika Westerberg wrote:
> On Fri, Nov 18, 2016 at 07:35:42PM +0000, Russell King - ARM Linux wrote:
> > Another mitigation would be to lower the I2C bus frequency on Juno from
> > 400kHz to 100kHz, so that there's 4x longer IRQ latency possible.
> > However, even that isn't going to be reliable - even going to 100kHz
> > isn't going to allow the above case to be solved - the interrupt is
> > delayed by around 2ms, and it takes about 1.4ms to send/receive 16 bytes
> > at 100kHz.  (9 * 16 / (100*10^3)).
> > 
> > So, I think all hope is lost for i2c-designware on Juno to cope with
> > reading the EDID from TDA998x reliably.
> 
> :-(
> 
> I wonder if we can get it work more reliably by using DMA (provided that
> there are DMA channels available for I2C in Juno)? That would allow the
> hardware to perform longer reads without relying on how fast the
> interrupt handler is able to empty the Rx FIFO.

It would need to DMA to the Tx FIFO to keep it filled - it triggers the
stop condition when the Tx FIFO empties.  From what I can see in the
driver, the Tx FIFO not only takes the data but also a "command" to tell
the hardware what to do.

The Rx FIFO would also need DMA to avoid it overflowing due to high
interrupt latency.

I don't know what state DMA is in on the Juno, or even whether it has
DMA - it has a PL330 DMA controller, but I see nothing in the DT files
making use of it.

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