linux-kernel - Re: [PATCH] 3/5: Updates to SPI and mmc_spi: tx

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Message-Id: <200701250502.56467.david-b@pacbell.net>
Date:	Thu, 25 Jan 2007 05:02:56 -0800
From:	David Brownell <david-b@...bell.net>
To:	"Hans-Peter Nilsson" <hans-peter.nilsson@...s.com>
Cc:	linux-kernel@...r.kernel.org, mikael.starvik@...s.com,
	spi-devel-general@...ts.sourceforge.net
Subject: Re: [PATCH] 3/5: Updates to SPI and mmc_spi: tx_default, kernel 2.6.19

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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