arm_syscall cacheflush breakage on VIPT platforms

[Laurent Pinchart]

On Monday 28 September 2009 16:15:10 Jamie Lokier wrote:
> Laurent Pinchart wrote:
> > > I'd much rather just say "no, userspace DMA is *never* going to ever
> > > be supported" and call it a day, but I suspect no one's going to like
> > > that either.
> >
> > In that case developers will all create their own incompatible solutions
> > and the situation will likely get worse. Do you think it would be
> > possible to create a clean DMA-to-userspace API specific to the ARM
> > architecture ?
> 
> I hate to spell out the obvious, but a fine solution is to _not_ DMA
> directly to userspace, but to kmalloc() a large buffer in your own
> driver, DMA into the buffer (it's kernel memory so that's ok), and
> _then_ mmap() that buffer into userspace after the DMA.  Going the
> other way, mmap(), write from userspace, munmap(), then do the DMA to
> the device.
> 
> That's trivial to implement, and the developer's we're talking about
> should have no difficulty writing a simple driver like that.  They
> have a driver already, it's just a matter of adding the mmap method.
> 
> Russell, is there any reason why the above would not work?

Performance reasons mostly. Think about transferring uncompressed 1080p at 
30fps for instance.

-- 
Laurent Pinchart