Re: question on spi_bitbang

[Kumar Gala]

On Mar 31, 2006, at 1:07 PM, Kumar Gala wrote:

> On Mar 31, 2006, at 12:11 PM, David Brownell wrote:
>
> > On Friday 31 March 2006 9:31 am, Kumar Gala wrote:
> > > I'm looking at using spi_bitbang for a SPI driver and was trying to
> > > understand were the right point is to handle MODE switches.
> > >
> > > There are 4 function pointers provided for each mode.
> >
> > That's if you are indeed "bit banging", or your controller is the
> > type that's basically a wrapper around a shift register:  each
> > txrx_word() function transfers (or bitbangs) a 1-32 bit word in
> > the relevant SPI mode (0-3).
> >
> > There's also a higher level txrx_bufs() routine for buffer-at-a-time
> > access, better suited to DMA, FIFOs, and half-duplex hardware.
>
> My controller is just a shift register that I can set the  
> characteristics of (bit length for example, reverse data).
>
> > > My controller
> > > HW has a mode register which allows setting clock polarity and clock
> > > phase.  I assume what I want is in my chipselect() function is to  
> > > set
> > > my mode register and have the four function pointers set to the same
> > > function.
> >
> > I don't know how your particular hardware works, but if you have a
> > real SPI controller it would probably be more natural to have your
> > setup() function handle that mode register earlier, out of the main
> > transfer loop ... unless that mode register is shared among all
> > chipselects, in which case you'd use the setup_transfer() call for
> > that, inside the transfer loop.  (That call hasn't yet been merged
> > into the mainline kernel yet; it's in the MM tree.)
>
> The mode register is shared between chipselects so I'll go pull the  
> patches from -mm for setup_transfer().  That sounds like the right  
> place for setting my mode register.

Now that I look at setup_transfer() more I dont think its what I  
want.  It's only going to get called if the transfer is different  
from the spi_device() settings.  I would seem I would want to  
configure all of my settings in chipselect().

> > The chipselect() call should only affect the chipselect signal and,
> > when you're activating a chip, its initial clock polarity.  Though
> > if you're not using the latest from the MM tree, that's also your
> > hook for ensuring that the SPI mode is set up right.
>
> Why deal with just clock polarity and not clock phase as well in  
> chipselect()?
>
> It sounds like with the new patch, I'll end up setting txrx_word[]  
> to the same function for all modes.
>
> - kumar
> -
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