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
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