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

[Robin Murphy <robin.murphy@arm.com>]

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.