On Wednesday 24 January 2007 8:52 pm, Hans-Peter Nilsson wrote:
> (Please CC me on replies, I'm not subscribed to LKML or the SPI list. Thanks.)
>
> The SD/MMC SPI-based protocol isn't really duplex. In the
> normal case there's either information transmitted or received,
> not both simultaneously. The unused data is always 0xff; ones.
> Unfortunately, the SPI framework leaves outgoing data for a
> left-out tx_buf just as "undefined"
In current kernels that's actually changed. The value to be shifted
out is now specified ... as all-zeroes, which is what various chips
need to receive, and various (half-duplex/Microwire) controllers seem
to be doing in any case.
If that's an issue -- and MMC-over-SPI needs to specify some other
value -- then please re-issue this patch against 2.6.20, including
the update to the bitbang driver ("reference implementation").
I think a better way to package this would be to define a new
flag for spi->mode, since controller drivers are already supposed
to be checking that to make sure they handle all the options which
have been specified. That flag could work in conjunction with
a byte provided in the spi_device, that would be used instead of
zero. (Some folk have noted that when debugging, it's easier if
the pattern there is neither all-ones nor all-zeroes ... something
that a digital scope will show is easier to work with.)
- Dave
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