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

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Date:	Thu, 12 Jun 2008 00:15:50 -0700
From:	David Brownell <david-b@...bell.net>
To:	Guennadi Liakhovetski <g.liakhovetski@...gutronix.de>
Cc:	linux-kernel@...r.kernel.org,
	spi-devel-general@...ts.sourceforge.net
Subject: Re: [PATCH] gpio: gpio driver for max7301 SPI GPIO expander

Some comments about one routine; presumably more to follow:

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

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