RPmsg, DMA and ARM64

[Edgar E. Iglesias]

On Thu, Mar 26, 2015 at 03:26:07PM +0000, Russell King - ARM Linux wrote:
> On Tue, Mar 24, 2015 at 02:37:49PM +1000, Edgar E. Iglesias wrote:
> > I'm trying to run rpmsg and remoteproc on the ZynqMP but hitting an mm error.
> > I'm not sure who is breaking the rules, rpmsg or the dma allocators?
> > 
> > When rpmsg sets up the virtqueues, it allocates memory with
> > dma_alloc_coherent() and initializes a scatterlist with sg_init_one().
> > drivers/rpmsg/virtio_rpmsg_bus.c:rpmsg_probe().
> > sg_init_one() requires that the memory it gets is virt_addr_valid().
> > 
> > The problem I'm seeing is that on arm64, the dma alloc functions can
> > return vmalloced (via dma_common_contiguous_remap) memory. This
> > then causes havoc when the scatterlist code tries to go virt_to_page
> > and back to get hold of a physical adress (sg_phys()).
> 
> dma_alloc_coherent() is permitted to remap the memory which it returns;
> it's allowed not to be part of the linear mapping.  The underlying
> memory could even be sourced from highmem and mapped in on demand.
> 
> This means that using virt_to_page() et.al. on the return value from
> dma_alloc_coherent() is not permitted.

Thanks Russel, that's info is of great help.

> 
> The only way to pass such memory using scatterlists is by doing:
> 
> 	sg_init_table(sg, 1);
> 	sg_dma_address(sg) = addr;
> 	sg_dma_length(sg) = length;
> 
> Such a scatterlist must _never_ have the dma_(map|unmap|sync)_sg*()
> functions called on it - the only operations which would be permissible
> is to walk the scatterlist, and access it using the standard DMA
> accessors sg_dma_address() and sg_dma_length().

I'll have a closer look and see if I can change rpmsg to deal with
this.

Cheers,
Edgar