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From:   Robin Gong <yibin.gong@nxp.com>
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Subject: RE: [PATCH v9 RESEND 01/13] spi: imx: add dma_sync_sg_for_device
 after fallback from dma
Thread-Topic: [PATCH v9 RESEND 01/13] spi: imx: add dma_sync_sg_for_device
 after fallback from dma
Thread-Index: AQHWPBYcMy4ZpJEEjkym0JBjSJknZajOy8sAgAABToCAAA6IgIAAsIcw
Date:   Tue, 9 Jun 2020 02:45:32 +0000
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On 2020/06/08 23:32 Mark Brown <broonie@kernel.org> wrote:=20
> On Mon, Jun 08, 2020 at 03:08:45PM +0000, Robin Gong wrote:
>=20
> > > > +	if (transfer->rx_sg.sgl) {
> > > > +		struct device *rx_dev =3D spi->controller->dma_rx->device->dev;
> > > > +
> > > > +		dma_sync_sg_for_device(rx_dev, transfer->rx_sg.sgl,
> > > > +				       transfer->rx_sg.nents, DMA_TO_DEVICE);
> > > > +	}
> > > > +
>=20
> > > This is confusing - why are we DMA mapping to the device after doing
> > > a PIO transfer?
>=20
> > '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.
>=20
> So we sync it back to the device so that we can then do another sync to C=
PU?
Yes, spi.c will sync to CPU again in spi_unmap_buf() after transfer done fi=
nally.
Otherwise, the fresh received data by CPU in this fallback case may be inva=
lidated
by spi.c, which led to the data corrupt on Matthias's side.

> TBH I'm a bit surprised that there's a requirement that we explicitly und=
o a
> sync and that a redundant double sync in the same direction might be an i=
ssue
Considering DMA transfer may be failed(for example, sdma firmware may not b=
e
updated as ERR009165 depends on), we'd better fallback to PIO to ensure no =
any
function break here. Thus should clean fresh rx data from cache into extern=
al memory
as real 'device' received by DMA. Understood a bit confusing here, but that=
 way could
be avoided by any code changing in spi.c. Or make some code changes in spi.=
c to cancel
spi_unmap_buf() in such fallback case?

> but I've not had a need to care so I'm perfectly prepared to believe ther=
e is.
>=20
> At the very least this needs a comment.
Okay, I'll add comment here in next.