Re: [Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1

[Laurent Pinchart]

On Wednesday 27 April 2011 12:43:16 Arnd Bergmann wrote:
> On Wednesday 27 April 2011, Catalin Marinas wrote:
> > On 21 April 2011 20:29, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > > I think the recent discussions on linaro-mm-sig and the BoF last week
> > > at ELC have been quite productive, and at least my understanding
> > > of the missing pieces has improved quite a bit. This is a list of
> > > things that I think need to be done in the kernel. Please complain
> > > if any of these still seem controversial:
> > > 
> > > 1. Fix the arm version of dma_alloc_coherent. It's in use today and
> > > 
> > >   is broken on modern CPUs because it results in both cached and
> > >   uncached mappings. Rebecca suggested different approaches how to
> > >   get there.
> > 
> > It's not broken since we moved to using Normal non-cacheable memory
> > for the coherent DMA buffers (as long as you flush the cacheable alias
> > before using the buffer, as we already do). The ARM ARM currently says
> > unpredictable for such situations but this is being clarified in
> > future updates and the Normal non-cacheable vs cacheable aliases can
> > be used (given correct cache maintenance before using the buffer).
> 
> Thanks for that information, I believe a number of people in the
> previous discussions were relying on the information from the
> documentation. Are you sure that this is not only correct for the
> cores made by ARM ltd but also for the other implementations that
> may have relied on documentation?
> 
> As I mentioned before, there are other architectures, where having
> conflicting cache settings in TLB entries for the same pysical page
> immediately checkstops the CPU, and I guess that this was also allowed
> by the current version of the ARM ARM.
> 
> > > 2. Implement dma_alloc_noncoherent on ARM. Marek pointed out
> > > 
> > >   that this is needed, and it currently is not implemented, with
> > >   an outdated comment explaining why it used to not be possible
> > >   to do it.
> > 
> > As Russell pointed out, there are 4 main combinations with iommu and
> > some coherency support (i.e. being able to snoop the CPU caches). But
> > in an SoC you can have different devices with different iommu and
> > coherency configurations. Some of them may even be able to see the L2
> > cache but not the L1 (in which case it would help if we can get an
> > inner non-cacheable outer cacheable mapping).
> > 
> > Anyway, we end up with different DMA ops per device via dev_archdata.
> 
> Having different DMA ops per device was the solution that I was suggesting
> with dma_mapping_common.h, but Russell pointed out that it may not be
> the best option.
> 
> The alternative would be to have just one set of dma_mapping functions
> as we do today, but to extend the functions to also cover the iommu
> case, for instance (example, don't take literally):
> 
> static inline dma_addr_t dma_map_single(struct device *dev, void *cpu_addr,
>                 size_t size, enum dma_data_direction dir)
> {
> 	dma_addr_t ret;
> 
> #ifdef CONFIG_DMABOUNCE
> 	if (dev->archdata.dmabounce)
> 		return dmabounce_map_single(dev, cpu_addr, size, dir);
> #endif
> 
> #ifdef CONFIG_IOMMU
> 	if (dev->archdata.iommu)
> 		ret = iommu_map_single(dev, cpu_addr, size, dir);
> 	else
> #endif

I wish it was that simple.

The OMAP4 ISS (Imaging Subsystem) has no IOMMU, but it can use the OMAP4 DMM 
(Dynamic Memory Manager) which acts as a memory remapper. Basically (if my 
understanding is correct), the ISS is configured to read/write from/to 
physical addresses. If those physical addresses are in the DMM address range, 
the DMM translates the accesses to physical accesses, acting as an IOMMU.

The ISS can thus write to physically contiguous memory directly, or to 
scattered physical pages through the DMM. Whether an IOMMU (or, to be correct 
in this case, the IOMMU-like DMM) needs to handle the DMA is a per-buffer 
decision, not a per-device decision.

> 		dma_addr = virt_to_dma(dev, ptr);
> 
> 	dma_sync_single_for_device(dev, dma_addr, size, dir);
> }
> 
> This would not even conflict with having a common implementation
> for iommu based dma_map_ops -- we would just call the iommu functions
> directly when needed rather than having an indirect function call.

-- 
Regards,

Laurent Pinchart
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