linux-kernel - Re: [PATCH RFC 2/2] staging: fbtft: fix 9-bit SPI support detection

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Date:	Tue, 25 Aug 2015 19:34:55 +0200 (CEST)
From:	Stefan Wahren <stefan.wahren@...e.com>
To:	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Noralf Trønnes <noralf@...nnes.org>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org,
	Henri Chain <henri.chain@...ves.ec-nantes.fr>,
	devel@...verdev.osuosl.org, Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH RFC 2/2] staging: fbtft: fix 9-bit SPI support detection


> Noralf Trønnes <noralf@...nnes.org> hat am 25. August 2015 um 00:00
> geschrieben:
>
>
>
> Den 24.08.2015 20:33, skrev Stefan Wahren:
> > Since bits_per_word isn't usually checked during SPI setup the 9-bit
> > support must be checked manually.
> >
> > Signed-off-by: Stefan Wahren <stefan.wahren@...e.com>
> > ---
> > drivers/staging/fbtft/fbtft-core.c | 7 +++++++
> > drivers/staging/fbtft/flexfb.c | 7 +++++++
> > 2 files changed, 14 insertions(+)
> >
> > diff --git a/drivers/staging/fbtft/fbtft-core.c
> > b/drivers/staging/fbtft/fbtft-core.c
> > index 3638554..bd71487 100644
> > --- a/drivers/staging/fbtft/fbtft-core.c
> > +++ b/drivers/staging/fbtft/fbtft-core.c
> > @@ -1438,6 +1438,13 @@ int fbtft_probe_common(struct fbtft_display *display,
> > if (par->spi && display->buswidth == 9) {
> > par->spi->bits_per_word = 9;
> > ret = spi_setup(par->spi);
> > + if (!ret) {
> > + struct spi_master *ma = par->spi->master;
> > +
> > + if (!(ma->bits_per_word_mask & SPI_BPW_MASK(9)))
> > + ret = -EINVAL;
> > + }
> > +
> > if (ret) {
>
> There's no point in calling spi_setup() when it doesn't check bits_per_word.

If checking of bits_per_word is the only intention of the setup call, then i
agree.
But i'm not sure it is safe to remove the setup call complete.

Couldn't this cause regressions since there is no common spi setup call for all
drivers?

> Apparently this changed with the commit:
> spi: convert drivers to use bits_per_word_mask.
> This has not been detected earlier, because FBTFT was previously mostly
> used on the Raspberry Pi which had a downstream SPI driver that did this
> check.
>
> How about this:
>
> - par->spi->bits_per_word = 9;
> - ret = par->spi->master->setup(par->spi);
> + if (par->spi->master->bits_per_word_mask &
> SPI_BPW_MASK(9)) {
> + par->spi->bits_per_word = 9;
> - if (ret) {
> + } else {
> dev_warn(&par->spi->dev,
> "9-bit SPI not available, emulating
> using 8-bit.\n");
> - par->spi->bits_per_word = 8;

I think this assignment should stay.

> - ret = par->spi->master->setup(par->spi);
> - if (ret)
> - goto out_release;
> /* allocate buffer with room for dc bits */
> par->extra = devm_kzalloc(par->info->device,
>
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