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

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Date:	Mon, 17 May 2010 09:14:39 +0100
From:	"Hennerich, Michael" <Michael.Hennerich@...log.com>
To:	Oskar Schirmer <os@...ix.com>,
	Mike Frysinger <vapier.adi@...il.com>
CC:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Daniel Glöckner <dg@...ix.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Oliver Schneidewind <osw@...ix.com>
Subject: RE: [PATCH] ad7877: fix spi word size to 16 bit

Oskar Schirmer wrote on 2010-05-17:
> 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

100% agreed.

Two ways to address the issue:
1) Forcing spi->bits_per_word = 16 like this patch does.
2) Or going with the default 8-bit transfers and using be16_to_cpu().

Personally I prefer 1) unless someone tells me that not all SPI bus drivers
support 16-bit transfers.

Greetings,
Michael

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