Re: [PATCH][RFC] asm-generic:remove calling flush_write_buffers() in dma_sync_*_for_cpu

[Arnd Bergmann <arnd@arndb.de>]

On Monday 29 June 2009, Joerg Roedel wrote:
> On Sun, Jun 28, 2009 at 03:34:35PM +0000, Arnd Bergmann wrote:
> > On Sunday 28 June 2009 14:39:19 tom.leiming@gmail.com wrote:
> > > From: Ming Lei <tom.leiming@gmail.com>
> > > 
> > > dma_sync_*_for_cpu() is introduced to make cpu access dma buffers safely when
> > > dma transfer is over, it seems there is nothing to do with cpu write buffer,
> > > so remove it.
> > > 
> > > Signed-off-by: Ming Lei <tom.leiming@gmail.com>
> > 
> > Right, this looks correct. On a related note, flush_write_buffers is
> > architecture specific right now: only x86 and frv implement it at all,
> > though and with slightly different semantics.
> 
> This doen't look correct to me. The sync functions may do bounce buffering
> which is all about copying data from one place in main memory to another. So we
> need these flush_write_buffer() calls in the _for_cpu path too.

Right, I didn't consider that.

Wouldn't it be better to put the flush_write_buffer in the specific
operation (swiotlb_sync_*_for_*) rather than the multiplexer?

Maybe in that case, smp_wmb() would be more appropriate because
it is defined on all architectures.

> > Maybe it would be more consistent to change the dma_map_* and
> > dma_sync_*_for_device stuff there to wmb() to make it  portable
> > to other architectures.
> 
> If we change it to wmb() it would be executed every time there even if the
> processor doesn't require it. Other architectures could simply add a
> flush_write_buffers() implemention if they want to adapt the common dma-mapping
> implementation, no?

As mentioned, the definition of flush_write_buffers() seems a little dodgy,
I would feel much more comfortable with putting it into the architecture
specific code or using one of the existing common barriers, since we already
have so many of them.

	Arnd <><