[PATCH 2/5] spi: pl022: use generic DMA slave configuration if possible

Linus Walleij
2013-02-08T22:10:02Z

On Fri, Feb 8, 2013 at 5:28 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> On Friday 08 February 2013 16:22:48 Russell King - ARM Linux wrote:
>> If it's DMA _to_ a device, then we will only ever clean the lines prior to
>> a transfer, never invalidate them. So that's not really a concern. (There
>> better not be any dirty cache lines associated with the empty zero page
>> either.)
>
> Right, makes sense. I thought I had read about a CPU that
> could not flush a cache line without also invalidating
> it, but that must have been something other than ARM,
> or maybe I'm misremembering it.

I don't think it matters one bit. The page can contain a bitmap
of Donald Duck or zero FWIW. It's just that the DMA
controller just neeeds to read *something* that does not cause
a bus stall.

It's due to the syncronous nature of the SPI protocol, to get
something out you need to put something in. So when reading,
this is a way to feed in some junk.

So this goes on my TODO...

Yours,
Linus Walleij