Messages in this thread | | | From | Ingo Oeser <> | Subject | Re: [patch 2.6.28-rc9] spi: spi_write_then_read() regression fix | Date | Tue, 23 Dec 2008 02:53:59 +0100 |
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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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