Re: [PATCH] ads7846: allocate separate cache lines for tx and rx data

[David Brownell <david-b@pacbell.net>]

On Wednesday 15 July 2009, Alessandro Rubini wrote:
> From: Alessandro Rubini <rubini@gnudd.com>
> 
> Since the SPI master might use DMA, tx and rx buffers must live on
> different cache lines.

Not true.  Full duplex tranfsers using a single buffer are
explicitly allowed.


> Here one cache line is used for all tx and 
> another for all rx.  This will work whether the SPI master maps/unmaps
> one buffer at a time or all together.  The patch only fixes the analog
> inputs, as touch screen operation is not affected, although I didn't
> check why.  I tested on at91sam9263, where buffers are all mapped
> initially and unmapped individually at irq time.

If that spi_master driver mis-handles this, it's a bug in that
driver.  (And I hope you were using a version of the '9263 chip
which fixed its SPI bugs.  Initial silicon had a SPI controller
which was essentially unusuable in a multitasking environment.)

 
> The issue was discussed with Russell King on linux-arm-kernel.

Gee, but not with the author of that driver or the maintainer of
the SPI framework.   Who could have pointed out instantly where
the true bug resides.

 
> 
> Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
> Cc: Russell King <linux@arm.linux.org.uk>
> Cc: David Brownell <dbrownell@users.sourceforge.net>
> ---
>  drivers/input/touchscreen/ads7846.c |   53 +++++++++++++++++++++++++----------
>  1 files changed, 38 insertions(+), 15 deletions(-)
>