Re: [PATCH 2/4] spi: gpio: Fix reading for 3WIRE

[Lorenzo Bianconi <lorenzo.bianconi@xxxxxxxxxx>]

On Sep 06, Linus Walleij wrote:
> On Thu, Sep 6, 2018 at 11:05 AM Lorenzo Bianconi
> <lorenzo.bianconi@xxxxxxxxxx> wrote:
> > > On Tue, Sep 04, 2018 at 11:49:49PM +0200, Linus Walleij wrote:
> > >
> > > > So remains to add two new permutations:
> > > > bitbang_txrx_be_3wire_cpha[0|1]() and double the
> > > > code in spi-bitbang-txrx.h. Not good for maintenance.
> > >
> > > > So damned if I do, damned if I don't.
> > >
> > > > But I guess I go for the latter approach?
> > >
> > > Yes, I think so - there's obviously a lot of overhead from bitbanging
> > > this stuff so it's worth it to try to minimize it.
> >
> > Hi Linus and Mark,
> >
> > maybe here we can move flags definition in spi_bitbang_bufs routine, e.g:
> >
> > -       return cs->txrx_bufs(spi, cs->txrx_word, nsecs, t, 0);
> > +
> > +       if (spi->master->flags & (SPI_MASTER_NO_TX | SPI_MASTER_NO_RX))
> > +               return cs->txrx_bufs(spi, cs->txrx_word, nsecs, t,
> > +                                    spi->master->flags);
> > +       else
> > +               return cs->txrx_bufs(spi, cs->txrx_word, nsecs, t, 0);
> >
> > in this way we can unify spi_gpio_txrx_word_mode{0-3}/spi_gpio_spec_txrx_word_mode{0-3} definitions
> > in spi-gpio.c and other drivers in the future can use this optimization. What
> > do you think? Are there any side affects with that approach?
> 
> I think it's not working because real half-duplex adapters, those
> who *REALLY* cannot do RX or TX will attempt to do RX and TX
> anyways if we do that. Not that I know if there are any.

please correct me if I am wrong but for half-duplex adapters we
will have SPI_MASTER_NO_TX or SPI_MASTER_NO_RX set in master flags,
so we will pass these flags to the master driver. Moreover at the moment
txrx_word[] is set just in (except spi-gpio):
- spi-ath79.c
- spi-butterfly.c
- spi-sh-sci.c
- spi-lm70llp.c

where flags was always set to 0 before commit 304d34360b09 and where
SPI_MASTER_NO_{TX|RX} flags are not actually used.
Anyway maybe there is a better solution :)

Regards,
Lorenzo


> 
> AT lunch I've finally understood, maybe, what needs to be done
> so I' working on some better solution.
> 
> Yours,
> Linus Walleij