[PATCH 06/10] soc/qbman: Add ARM equivalent for flush_dcache_range()

[Arnd Bergmann]

On Monday, January 30, 2017 3:04:28 PM CET Russell King - ARM Linux wrote:
> On Wed, Jan 25, 2017 at 10:20:09PM +0100, Arnd Bergmann wrote:
> > On Monday, January 23, 2017 7:24:59 PM CET Roy Pledge wrote:
> > > > Is there a non-"under the covers" way to say "flush this region" without
> > > > the arch second-guessing whether it really needs to be flushed?
> > > Any advice on how to resolve this? I looked into trying to do a
> > > non-cacheable mapping of the
> > > memory so that the flush wouldn't be required but the ioremap code
> > > prevents mapping normal
> > > memory in this way.  The QMan device requiresthis memory to be zeroed at
> > > startup. Because the device
> > > does non coherent reads and writes to the memory we must ensure that any
> > > cache in the CPU cluster(s)
> > > is flushed in order to prevent a future castout from overwriting data.
> > > This happened before on PPC
> > > platforms and made for some very unfun debug session trying to
> > > understand what was causing the failure.
> > 
> > If this is normal RAM, you should be able to just write zeroes, and then
> > do a dma_map_single() for initialization. Are there any other requirements
> > what to do with the memory later, is it used for communication at all,
> > or just required to be zero?
> 
> Please don't encourage incorrect DMA API usage.
> 
> dma_map_single() must always be paired with an unmap at some point,
> otherwise it's a memory leak if DMA API debugging is enabled.  So,
> if you wish to suggest using dma_map_single(), please also suggest
> where to use dma_unmap_single() too.

The mapping obviously has to exist the whole time the device is in use,
and we should not call dma_unmap_single() until the device is released
by the driver.

Unfortunately it seems we can't use this interface though, as the device
is already configured to a specific reserved memory range, and we can't
just use get_free_pages() here.

	Arnd