RE: [BUG] ls1046a: eDMA does not transfer data from I2C

[Leo Li]

> >
> > Despite the DMA completing successfully, no data was copied into the
> > buffer, leaving the original (now junk) contents. I probed the I2C bus
> > with an oscilloscope, and I verified that the transfer did indeed occur.
> > The timing between submission and completion seems reasonable for the
> > bus speed (50 kHz for whatever reason).
> >
> > I had a look over the I2C driver, and nothing looked obviously
> > incorrect. If anyone has ideas on what to try, I'm more than willing.
> 
> Is the DMA controller cache-coherent? I see the mainline LS1046A DT doesn't
> have a "dma-coherent" property for it, but the behaviour is entirely
> consistent with that being wrong - dma_map_single() cleans the cache,
> coherent DMA write hits the still-present cache lines,

So the coherent DMA write only gets data into the cache not also the DRAM?  Otherwise a read back would get the updated data too.

- Leo

> dma_unmap_single() invalidates the cache, and boom, the data is gone and
> you read back the previous content of the buffer that was cleaned out to
> DRAM beforehand.
> 
> Robin.
> 
> > --Sean