Re: [patch 2.6.28-rc9] spi: spi_write_then_read() regression fix

[Ingo Oeser <ioe-lkml@rameria.de>]

Hi David,

On Monday 22 December 2008, you wrote:
> On Sunday 21 December 2008, Linus Torvalds wrote:
> > 
> > On Sat, 20 Dec 2008, David Brownell wrote:
> > > 
> > > All SPI transfers are full duplex, and are packaged as half duplex
> > > by either discarding the data that's read ("write only"), or else
> > > by writing zeroes ("read only").  That patch wasn't ensuring that
> > > zeroes were getting written out during the "half duplex read" part
> > > of the transaction; instead, old RX bits were getting sent.
> > 
> > Hmm. In addition, isn't this broken (in that same function):
> 
> No -- this is full duplex.  The write_then_read() helper is
> simplifying a common half-duplex idiom for short operations,
> but the harware still does full duplex.  Buffer layout is:
> 
>   Before:	WWWWW0000000
>   After:	xxxxxRRRRRRR
> 
> That is, for every bit shifted out (W, 0) another one gets
> shifted in (x, R).  The I/O primitive essentially swaps
> contents of a one-word shift register between master and
> slave; or, sequences of such words.  Words don't need to
> be byte-size, though that's a common option.

> See above.  We only want the "R" bits which were shifted in
> right *after* the n_tx "W" bits.  If we offset rx_buf before
> the I/O, we'd start with the "x" don't-care bits and need to
> do something else to discard them.  (Plus, allocate more
> space at the end of the buffer.)

Wow, what interesting hardware logic and a nice explanation.

Could you put that into a comment somewhere close to those helpers?

You can safely assume, that any code which Linus doesn't understand
is non-trivial and needs a comment :-)


Best Regards

Ingo Oeser