[PATCH] ARM: optimize memset_io()/memcpy_fromio()/memcpy_toio()

[Will Deacon]

On Fri, Sep 28, 2012 at 11:31:10AM +0100, Russell King - ARM Linux wrote:
> On Fri, Sep 28, 2012 at 10:58:08AM +0100, Will Deacon wrote:
> > 	1. If an access other the first one generated by the instruction
> > 	   causes an abort, the CPU will ultimately re-execute the earlier
> > 	   accesses, which could be problematic to a device.
> 
> I don't think that's a problem for these.  They're used on RAM like
> regions.

[...]

> > So, unless we can guarantee that our accesses are all aligned, will never
> > fault, do not cross a page boundary and we are not running as a guest then
> > I'd be inclined to stick with byte-by-byte implementations for these
> > functions.
> 
> Well, that rather sucks if you're memset_io'ing various sizes (in
> megabytes - up to 8MB) of video memory.  We desperately need these
> functions optimized.

How are these regions mapped? If it's something like ioremap_wc, then this
gives us normal non-cacheable memory on ARMv7, which wouldn't be subject to
the restrictions that you have with device memory. 

> Either that or we allow DRM to be a security hole by omitting any kind
> of buffer clearing, because using the existing memset_io() is just far
> too expensive to clear 8MB a byte at a time.

If DRM is using normal memory, it sounds like the *_io string functions need
to be made to work with different memory types.

Will