linux-kernel - RE: [PATCH] [drivers] [SPI] SPI_GPIO: add support for controllers with missing MISO pin

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Message-id: <000501c9d792$f8feb950$eafc2bf0$%szyprowski@samsung.com>
Date:	Mon, 18 May 2009 10:30:55 +0200
From:	Marek Szyprowski <m.szyprowski@...sung.com>
To:	'Ben Dooks' <ben-linux@...ff.org>
Cc:	'LKML' <linux-kernel@...r.kernel.org>,
	linux-arm-kernel@...ts.arm.linux.org.uk, kyungmin.park@...sung.com,
	Marek Szyprowski <m.szyprowski@...sung.com>
Subject: RE: [PATCH] [drivers] [SPI] SPI_GPIO: add support for controllers with
 missing MISO pin

Hello,

On Thursday, May 07, 2009 2:32 PM Ben Dooks wrote:

> On Thu, May 07, 2009 at 02:24:11PM +0200, Marek Szyprowski wrote:
> > There are some boards that do not strictly follow SPI standard and
> use only 3 wires (SCLK, MOSI, SS) for connecting some simple auxiliary
> chips and controls them with GPIO based 'spi controller'. In this
> configuration the MISO line is missing (it is not required if the chip
> does not transfer any data back to host). The example of such board is
> a NCP ARM S3C64XX based machine. This patch adds support for such non-
> standard configuration in GPIO-based SPI controller.
> [...]
> > diff --git a/drivers/spi/spi_gpio.c b/drivers/spi/spi_gpio.c
> > index 26bd03e..5b75601 100644
> > --- a/drivers/spi/spi_gpio.c
> > +++ b/drivers/spi/spi_gpio.c
> > @@ -114,7 +114,10 @@ static inline void setmosi(const struct
> spi_device *spi, int is_on)
> >
> >  static inline int getmiso(const struct spi_device *spi)
> >  {
> > -	return !!gpio_get_value(SPI_MISO_GPIO);
> > +	if (SPI_MISO_GPIO)
> > +		return !!gpio_get_value(SPI_MISO_GPIO);
> > +	else
> > +		return 0;
> >  }
> 
> Is zero a good approximation for 'no gpio' ?

Now I found that zero might be a valid gpio pin number on some architectures
(it just means GPIO0 pin). This is imho a bit strange behavior of gpiolib as
there should be also a special values for INVALID or NOGPIO cases. Does 
anyone have any ideas how such cases should be handled properly? 

Best regards
--
Marek Szyprowski
Samsung Poland R&D Center



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