On Fri, 2014-05-16 at 18:40 +0100, Catalin Marinas wrote:
On Fri, May 16, 2014 at 06:08:45PM +0100, Jon Medhurst (Tixy) wrote:
On Fri, 2014-05-16 at 13:55 +0100, Catalin Marinas wrote: [...]
It could if arm64 would restrict the DMA addresses to 32-bit, but it doesn't and I end up on my platform with USB DMA buffers allocated >4GB address.
dma_alloc_coherent() on arm64 should return 32-bit addresses if the coherent_dma_mask is set to 32-bit.
Not if you have CONFIG_DMA_CMA. Unless I have misread the code, enabling CMA means memory comes from a common pool carved out at boot with no way for drivers to specify it's restrictions [1]. It's what I've spent most of the week trying to work around in a clean way, and have finally given up.
The easiest "hack" would be to pass a limit dma_contiguous_reserve() in arm64_memblock_init(), something like this:
That is by far the easiest but I dismissed it because it affects all platforms built from that source tree; and if the hack were extended to include a kernel config option, that still may not work on a single kernel binary expected to work on multiple platforms. Basically, I've tried various was to do it 'properly' and after failing am resorting to truly hideous hacks to the (out-of-tree driver) code - so at least other platforms won't be impacted.