Messages in this thread | | | Subject | Re: [PATCH v9 RESEND 01/13] spi: imx: add dma_sync_sg_for_device after fallback from dma | From | Robin Murphy <> | Date | Tue, 9 Jun 2020 11:00:33 +0100 |
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On 2020-06-09 06:21, Robin Gong wrote: > On 2020/06/09 0:44 Robin Murphy <robin.murphy@arm.com> 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 :(
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.
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