RE: [PATCH v9 RESEND 01/13] spi: imx: add dma_sync_sg_for_device after fallback from dma

[Robin Gong]

On 2020/06/09 Robin Murphy <robin.murphy@xxxxxxx> wrote: 
> On 2020-06-09 06:21, Robin Gong wrote:
> > On 2020/06/09 0:44 Robin Murphy <robin.murphy@xxxxxxx> wrote:
> >> On 2020-06-08 16:31, Mark Brown wrote:
> >>> On Mon, Jun 08, 2020 at 03:08:45PM +0000, Robin Gong wrote:
> >>>
> >>>>>> +	if (transfer->rx_sg.sgl) {
> >>>>>> +		struct device *rx_dev = spi->controller->dma_rx->device->dev;
> >>>>>> +
> >>>>>> +		dma_sync_sg_for_device(rx_dev, transfer->rx_sg.sgl,
> >>>>>> +				       transfer->rx_sg.nents, DMA_TO_DEVICE);
> >>>>>> +	}
> >>>>>> +
> >>>
> >>>>> This is confusing - why are we DMA mapping to the device after
> >>>>> doing a PIO transfer?
> >>>
> >>>> 'transfer->rx_sg.sgl' condition check that's the case fallback PIO
> >>>> after DMA transfer failed. But the spi core still think the buffer
> >>>> should be in 'device' while spi driver touch it by PIO(CPU), so
> >>>> sync it back to
> >> device to ensure all received data flush to DDR.
> >>>
> >>> So we sync it back to the device so that we can then do another sync
> >>> to CPU?  TBH I'm a bit surprised that there's a requirement that we
> >>> explicitly undo a sync and that a redundant double sync in the same
> >>> direction might be an issue but I've not had a need to care so I'm
> >>> perfectly prepared to believe there is.
> >>>
> >>> At the very least this needs a comment.
> >>
> >> Yeah, something's off here - at the very least, syncing with
> >> DMA_TO_DEVICE on the Rx buffer that was mapped with
> DMA_FROM_DEVICE is clearly wrong.
> >> CONFIG_DMA_API_DEBUG should scream about that.
> >>
> >> If the device has written to the buffer at all since dma_map_sg() was
> >> called then you do need a dma_sync_sg_for_cpu() call before touching
> >> it from a CPU fallback path, but if nobody's going to touch it from
> >> that point until it's unmapped then there's no point syncing it
> >> again. The
> >> my_card_interrupt_handler() example in DMA-API_HOWTO.txt
> demonstrates
> >> this.
> > Thanks for you post, but sorry, that's not spi-imx case now, because the rx
> data in device memory is not truly updated from 'device'/DMA, but from PIO,
> so that dma_sync_sg_for_cpu with DMA_FROM_DEVICE can't be used,
> otherwise the fresh data in cache will be invalidated.
> > But you're right, kernel warning comes out if CONFIG_DMA_API_DEBUG
> enabled...
> 
> Ah, I think I understand what's going on now. That's... really ugly :(
Yeah...The only reason is to avoid touch any spi core code...I'm trying to implement fallback at spi core so that can spi_unmap_buf directly if dma transfer error and no need
such dma_sync_* in spi client driver. Not sure if Mark could accept it. Thanks for your below great thoughts :) 
> 
> Looking at the SPI core code, I think a better way to handle this would be to
> have your fallback path call spi_unmap_buf() directly (or perform the same
> actions, if exporting that to drivers is unacceptable), then make sure
> ->can_dma() returns false after that such that spi_unmap_msg() won't try to
> unmap it again. That's a lot more reasonable than trying to fake up a
> DMA_TO_DEVICE transfer in the middle of a DMA_FROM_DEVICE operation on
> the same buffer.
> 
> Alternatively, is it feasible to initiate a dummy DMA request during probe, such
> that you can detect the failure condition and give up on the DMA channel early,
> and not have to deal with it during a real SPI transfer?
> 
> Robin.