linux-kernel - Re: [PATCH] gpio: gpio driver for max7301 SPI GPIO expander

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Date:	Thu, 12 Jun 2008 10:00:43 +0200 (CEST)
From:	Guennadi Liakhovetski <g.liakhovetski@...gutronix.de>
To:	David Brownell <david-b@...bell.net>
cc:	linux-kernel@...r.kernel.org,
	spi-devel-general@...ts.sourceforge.net
Subject: Re: [PATCH] gpio: gpio driver for max7301 SPI GPIO expander

On Thu, 12 Jun 2008, David Brownell wrote:

> Some comments about one routine; presumably more to follow:

Thanks, David, I'll pass your comments on to the author and try to get 
them acted upon.

Thanks
Guennadi

> 
> 
> On Friday 06 June 2008, Guennadi Liakhovetski wrote:
> > +/**
> > + * max7301_read - Read back register content
> > + * @spi: The SPI device
> > + * @reg: Register offset
> > + *
> > + * A read from the MAX7301 means one message with two transfers
> 
> That's not what you implement though ... what it needs is:
> 
> 	- transfer #1:
> 		* select
> 		* write 16 bit command, discard rx data
> 		* deselect
> 	- transfer #2
> 		* select
> 		* write 16 bit command, saving rx data
> 		* deselect
> 
> That's from the data sheet.  Now, that *could* be implemented
> with a single message ... or, as you do, with two messages.
> 
> You should at least correct the comment, if you don't want to
> switch to a slightly faster single-message implementation.  :)
> 
> 
> > + *
> > + * Returns positive 8 bit value from device if successfull or a
> > + * negative value on error
> 
> Actually it returns a 16 bit value... also, "successful" has one "l".
> You seem to have forgotten to mask off the high byte ...
> 
> 
> > + */
> > +static int max7301_read(struct spi_device *spi, unsigned int reg)
> > +{
> > +       int ret;
> > +       u16 word;
> > +
> > +       word = 0x8000 | (reg << 8);
> > +       if ((ret = spi_write(spi, (const u8*)&word, siz
> 
> > eof(word))) != 0) 
> > +               return ret;
> > +       /*
> > +        * FIXME: This read should write 0x0000 (=NOOP at MAX7301 side)
> 
> Take out this FIXME, and just replace it with a note that this relies
> on the fact that TX always shifts out zeroes if there's no data specified.
> 
> > +        */
> > +       if ((ret = spi_read(spi, (u8*)&word, sizeof(word))))
> > +               return ret;
> > +       return word;
> > +}
> > +
> 
> 
> Oh, and in the probe():
> 
> > +       spi->master->setup(spi);
> 
> should be
> 
> 	ret = spi_setup(spi);
> 	if (ret < 0)
> 		return ret;
> 
> It's quite possible that something get misconfigured, so you end up trying
> to talk through a spi_master controller that doesn't support 16-bit words.
> 
> - Dave
> 

---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
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