linux-kernel - Re: [PATCH] ad7877: fix spi word size to 16 bit

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Date:	Mon, 17 May 2010 09:29:36 +0200
From:	"Oskar Schirmer" <os@...ix.com>
To:	Mike Frysinger <vapier.adi@...il.com>
Cc:	Oskar Schirmer <os@...ix.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Daniel Glöckner <dg@...ix.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	Oliver Schneidewind <osw@...ix.com>,
	Michael Hennerich <michael.hennerich@...log.com>
Subject: Re: [PATCH] ad7877: fix spi word size to 16 bit

On Sun, May 16, 2010 at 15:25:34 -0400, Mike Frysinger wrote:
> On Sat, May 15, 2010 at 14:15, Oskar Schirmer wrote:
> > On Thu, May 13, 2010 at 00:53:35 -0700, Dmitry Torokhov wrote:
> >> On Fri, May 07, 2010 at 02:23:07PM -0400, Mike Frysinger wrote:
> >> > On Fri, May 7, 2010 at 05:41, Daniel Glöckner wrote:
> >> > > On 05/06/2010 08:26 PM, Mike Frysinger wrote:
> >> > >> i think it'd be a better idea to do something like:
> >> > >>   if (spi->bits_per_word != 16) {
> >> > >>     if (spi->bits_per_word) {
> >> > >>       dev_err(&spi->dev, "Invalid SPI settings; bits_per_word must be 16\n");
> >> > >>       return -EINVAL;
> >> > >>     }
> >> > >>     spi->bits_per_word = 16;
> >> > >>     spi_setup(spi);
> >> > >>   }
> >> > >
> >> > > There is no way to set bits_per_word using struct spi_board_info. The
> >> > > description of that structure in spi.h explicitly lists the wordsize as
> >> > > one of the parameters drivers should set themself in probe().
> >> > >
> >> > > Only struct bfin5xx_spi_chip allows to set this value in the board code.
> >> >
> >> > an obvious shortcoming in the SPI framework that should be fixed, but
> >> > that doesnt make any difference to the above code now does it ?  it'll
> >> > operate correctly regardless of the SPI bus master.
> >>
> >> So is the updated patch coming?
> >
> > The basic question I see is, whether it is in the
> > responsibility of ad7877 to check a wrong setting
> > possibly caused in board specific code. If so,
> > then the proposal by Mike should be used, but if not
> > so, it would introduce unneeded code.
> >
> > Remember: both versions end up in correctly setting
> > bits_per_word, with the difference merely in feedback
> > level.
> 
> imo, unsupported board settings should always be detected & rejected.
> all SPI master drivers do this (detect & reject unsupported SPI slave
> settings).

please note, that bits_per_word is not a board setting,
it's a demand of the device. consequently, there is no one
to set unsupported values and thus none to be detected.

the only architecture setting bits_per_word thru spi_chip
is blackfin, but I cannot see a good reason, why the board
settings should engage with a fixed demand of the device?

  Oskar
-- 
oskar schirmer, emlix gmbh, http://www.emlix.com
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