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From:   Robin Gong <yibin.gong@nxp.com>
To:     Mark Brown <broonie@kernel.org>
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Subject: RE: [PATCH v1 RFC 1/2] spi: introduce fallback to pio
Thread-Topic: [PATCH v1 RFC 1/2] spi: introduce fallback to pio
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Date:   Fri, 12 Jun 2020 02:18:32 +0000
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On 2020/06/11 21: 41 Mark Brown <broonie@kernel.org> wrote:
> On Thu, Jun 11, 2020 at 08:58:29PM +0800, Robin Gong wrote:
> > Add SPI_CONTROLLER_FALLBACK to fallback to pio mode in case dma
> > transfer failed.
> > If spi client driver want to enable this feature please set
> > master->flags with SPI_MASTER_FALLBACK and add master->fallback
> > checking in its can_dma() as spi-imx.c
>=20
> If we were going to do this I don't see why we'd have a flag for this rat=
her than
> just doing it unconditionally but...
What do you mean flag here, 'master->flags' or SPI_MASTER_FALLBACK? 'master=
->flags'
could let client fallback to PIO finally and spi core clear this flag once =
this transfer done,
so that DMA could be tried again in the next transfer. Client could enable =
this feature by choosing SPI_MASTER_FALLBACK freely without any impact on o=
thers.
>=20
> >  			ret =3D ctlr->transfer_one(ctlr, msg->spi, xfer);
> >  			if (ret < 0) {
> > +				if (ctlr->cur_msg_mapped &&
> > +				   (ctlr->flags & SPI_CONTROLLER_FALLBACK)) {
> > +					__spi_unmap_msg(ctlr, msg);
> > +					ctlr->fallback =3D true;
> > +					goto fallback_pio;
> > +				}
>=20
> ...I don't think this can work sensibly - this is going to try PIO if the=
re's *any*
> error.  We might have had some sort of issue during the transfer for exam=
ple
> so have some noise on the bus.  Like I said on a prior version of this I =
really
Any error happen in DMA could fallback to PIO , seems a nice to have, becau=
se it could
give chance to run in PIO which is more reliable. But if there is also erro=
r in PIO, thus may loop here, it's better adding limit try times here?   =20
> think that we need to be figuring out if the DMA controller can support t=
he
> transaction before we even map the buffer for it, having the controller j=
ust
> randomly fail underneath the consumer just does not sound robust.
But dmaengine_prep_slave_sg still may return failure even if anything about
DMA is ok before spi transfer start, such as dma description malloc failure=
. This
patch seems could make spi a bit robust...