Mismatched aliases with DMA mappings?

[Kyungmin Park]

Hi Dave,

Marek is on vacation and will back 24 Sep. he will explain it in detail.

I just show how CMA addresses mismatched aliases codes.

In reserve function, it declares the require memory size with base
address. At that time it calles 'dma_contiguous_early_fixup()'. it
just registered address and size.

void __init dma_contiguous_early_fixup(phys_addr_t base, unsigned long size)
{
        dma_mmu_remap[dma_mmu_remap_num].base = base;
        dma_mmu_remap[dma_mmu_remap_num].size = size;
        dma_mmu_remap_num++;
}

These registerd base and size will be remap at dma_contiguous_remap
function at paging_init.

void __init dma_contiguous_remap(void)
{
        int i;
        for (i = 0; i < dma_mmu_remap_num; i++) {
                phys_addr_t start = dma_mmu_remap[i].base;
                phys_addr_t end = start + dma_mmu_remap[i].size;
                struct map_desc map;
                unsigned long addr;

                if (end > arm_lowmem_limit)
                        end = arm_lowmem_limit;
                if (start >= end)
                        continue;

                map.pfn = __phys_to_pfn(start);
                map.virtual = __phys_to_virt(start);
                map.length = end - start;
                map.type = MT_MEMORY_DMA_READY;

                /*
                 * Clear previous low-memory mapping
                 */
                for (addr = __phys_to_virt(start); addr < __phys_to_virt(end);
                     addr += PMD_SIZE)
                        pmd_clear(pmd_off_k(addr));

                iotable_init(&map, 1);
        }
}

Thank you,
Kyungmin Park

On 9/22/12, Dave Martin <dave.martin at linaro.org> wrote:
> Hi Marek,
>
> I've been trying to understand whether (and if so, how) and DMA buffer
> allocation code in dma-mapping.c avoids mismatched alises in the kernel
> linear map.
>
>
> I need a way of getting some uncached memory for communicating with
> temporarily noncoherent CPUs during CPU bringup/teardown.  Although
> the DMA API does not seem quite the right solution for this, nothing
> else currently feels like quite the right solution either.  Approaches
> based on memblock_steal() and on using cacheable memory with explicit
> flushing both have problems, and reserving specific physical memory
> via DT seems ugly, because we really don't care where the memory is.
>
> What is needed is something like an ioremap of anonymous memory with
> specific attributes, using largely the same infrastructure as the DMA
> API, but eliminating a mismatched alias of the allocated memory in the
> kernel linear mapping is likely to be important.
>
> Can you explain how the DMA mapping code eliminates mismatched aliases?
> I can see the attributes of new mappings being set, but currently I
> don't see how the linear map gets modified.
>
> Cheers
> ---Dave
>
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