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

[Robin Gong]

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...