On Thu, 6 Feb 2014, Mark Brown wrote:
> On Thu, Feb 06, 2014 at 01:56:41PM +0100, Geert Uytterhoeven wrote:
> > I'll see whether I can make use of this in the RSPI driver.
> > However, I think there's an incompatibility with Dual and Quad SPI
> > transfers: those are unidirectional, so you must not supply a buffer in
> > the unused direction.
>
> Right, but as a result of that any hardware which can do dual or quad
> mode must of necessity not require full duplex operation. I suppose
> it's possible that some hardware might be sufficiently idiotic to
> require full duplex operation only for normal SPI mode but that's a
> separate quirk. I guess from the above that the rspi hardware has this
> problem?
Yes it does ;-)
As the MISO/MOSI pins are reused for Dual/Quad, it cannot be full-duplex,
except for loopback.
> It is, of course, also always possible that someone would implement quad
> and dual with separate pins for both directions - I can see people doing
> this for use with DSPs for example.
Sure, but it'll be incompatible (without adding glue logic) with e.g. the
current line of Quad SPI FLASHes from Spansion etc.
Anyway, I think it's safer to not set up rx/tx_buf for the unused direction
in the SPI core code for Dual/Quad transfers, until someone proves the above
hardware does exist. If both rx/tx_buf are filled in, the SPI master driver
cannot know in which direction the Dual/Quad transfer is operating.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
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