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Subject: Re: [PATCH v9 RESEND 01/13] spi: imx: add dma_sync_sg_for_device
 after fallback from dma
To:     Mark Brown <broonie@kernel.org>, Robin Gong <yibin.gong@nxp.com>
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From:   Robin Murphy <robin.murphy@arm.com>
Message-ID: <59ce3620-00b9-bac1-30e1-011a29583642@arm.com>
Date:   Mon, 8 Jun 2020 17:44:21 +0100
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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.

Robin.