[Q] block / zynq: DMA bouncing

[jgunthorpe@obsidianresearch.com (Jason Gunthorpe)]

On Tue, Jan 28, 2014 at 01:48:27PM +0000, Russell King - ARM Linux wrote:
> On Tue, Jan 28, 2014 at 02:28:28PM +0100, Guennadi Liakhovetski wrote:
> > +static void __init zynq_memory_init(void)
> > +{
> > +	/*
> > +	 * Reserve the 0-0x4000 addresses (before page tables and kernel)
> > +	 * which can't be used for DMA
> > +	 */
> > +	if (!__pa(PAGE_OFFSET))
> > +		memblock_reserve(0, 0x4000);
> 
> Or maybe this:
> 
> 	memblock_reserve(__pa(PAGE_OFFSET), __pa(swapper_pg_dir));
> 
> since that's actually what you mean here.

Can you have a non-zero PAGE_OFFSET and swapper_pg_dir != 0x4000?
Crash dump kernel maybe?

Perhaps:
	if (__pa(PAGE_OFFSET) < 0x8000)
 	   memblock_reserve(__pa(PAGE_OFFSET),
	                    min(__pa(swapper_pg_dir), 0x8000));
?

The Zynq TRM, UG585 pg 115, describes this issue. Physical addresses
below 0x8000 are not accessible to DMA from the AXI interconnect.

Though, how does a crash dump kernel know it needs to bounce buffer
the swapper if it dumps it via DMA?

Regards,
Jason