linux-kernel - Re: [PATCH v2] spi: core: Validate lenght of the transfers in message

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Message-ID: <20140218193747.GE4524@book.gsilab.sittig.org>
Date:	Tue, 18 Feb 2014 20:37:47 +0100
From:	Gerhard Sittig <gsi@...x.de>
To:	Mark Brown <broonie@...nel.org>
Cc:	"Ivan T. Ivanov" <iivanov@...sol.com>, linux-spi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] spi: core: Validate lenght of the transfers in message

On Tue, Feb 18, 2014 at 09:09 +0900, Mark Brown wrote:
> 
> On Sun, Feb 16, 2014 at 03:25:12PM +0100, Gerhard Sittig wrote:
> > On Sat, Feb 15, 2014 at 08:09 +0200, Ivan T. Ivanov wrote:
> 
> > - the total length of the SPI transfer cannot be empty (which I'd
> >   consider an optimization, not a violation, and may need a
> >   separate discussion)
> 
> We probably want to allow that for people doing fun stuff with cs_change
> though I'm not convinced anything doing that is actually a good idea.
> 
> > - the total length of the SPI transfer must be such that each
> >   "word" must be provided within a full 1/2/4 byte entity, with
> >   padding bits if the bits-per-word is "odd"
> 
> > Is this a misunderstanding on my side?  A terminology thing?  To
> > me, the "SPI transfer" is the total payload and may have any
> > arbitrary length.  What you check for is a constraint on the
> > transfer's length derived from or based on the "word length"
> > ('word' in SPI context).
> 
> > So the code may be appropriate, yet the description may need an
> > update, to not have the next person ask the same questions again.
> 
> It seems fairly clear to me - if we're transferring 16 bit words we need
> the transfer to me a multiple of 16 bits and so on?  The requirement for
> padding is unclear I have to say.

I meant "padding" in the sense that e.g. 12bit bits-per-word
require data to be provided or consumed in 16bit quantities (2
full bytes), 20bit bits-per-word require 4 bytes per SPI word.
Why not 3 bytes?  I'd guess this is due to FIFO port width.

At least this is how I read the check which this patch
implements.


virtually yours
Gerhard Sittig
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