On Thu, 2014-05-22 at 16:59 +0100, Catalin Marinas wrote:
On Mon, May 19, 2014 at 09:21:17AM +0100, Jon Medhurst (Tixy) wrote:
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.
Can you set a specific reserved memory region in the DT to be used by CMA (via linux,cma-default), or it's just for the default size?
The bindings and infrastructure got half merged in 3.15 but the patches to actually make this usable in drivers are stalled... http://lkml.org/lkml/2014/5/14/201
On arm64 we enabled CONFIG_DMA_CMA by default but compared to swiotlb it doesn't honour GFP_DMA. The arm32 port sets the CMA limit to arm_dma_limit which is 32-bit or a SoC-define one. So I'm tempted to default to 32-bit as well if it can be overridden via DT.
I believe the CMA pool could have been over-ridden if the stalled patches been accepted. I believe the specific patch for that is http://lkml.org/lkml/2014/2/28/278
The bindings for all this CMA stuf is in Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt