Re: [PATCH] spi: add a bits_per_word to struct spi_board_info

[Grant Likely <grant.likely-s3s/WqlpOiPyB63q8FvJNQ@public.gmane.org>]

On Thu, Sep 02, 2010 at 02:13:48PM +0800, Feng Tang wrote:
> Hi David,
> 
> 
> > > Grant also asked me the same question.
> > 
> > 
> > I saw him ask a different question (which
> > I quoted) ...
> 
> My mistake, I tried to answer Grant's and your questions in one email to
> keep the discussion in one thread.
> 
> > 
> > 
> >  Yes, driver itself
> > > know the bits_per_word info best in most case, but there
> > > is
> > > some device (Option GTM501L spi modem) which supports
> > > multiple
> > > bits_per_mode option, and here platform code
> > is the good place to set it.
> > 
> > 
> > Not unless it knows which mode the driver uses...
> > a kind of layering violation.
> 
> spi_board_info has a member of "mode", so for my platform, I prefer to
> set all these infos in platform code, while protocol driver can
> check if the preset infos like speed_hz/mode/bits_per_word are supported.

The real problem is that spi_drivers are normally allowed to change
bits_per_word as needed.  If the spi_driver gets unbound and
rebound, then the original platform-set value for bits_per_word is
lost and the behaviour changes.  If you really want to give the driver
hints about the best setting for bits_per_word or similar, then you
should probably use a driver-specific pdata structure.

g.


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