Messages in this thread | | | Date | Thu, 5 Mar 2015 20:17:53 +0000 | From | Russell King - ARM Linux <> | Subject | Re: ARM: OMPA4+: is it expected dma_coerce_mask_and_coherent(dev, DMA_BIT_MASK(64)); to fail? |
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On Thu, Mar 05, 2015 at 08:55:07PM +0200, Grygorii.Strashko@linaro.org wrote: > Now I can see very interesting behavior related to dma_coerce_mask_and_coherent() > and friends which I'd like to explain and clarify. > > Below is set of questions I have (why - I explained below): > - Is expected dma_coerce_mask_and_coherent(DMA_BIT_MASK(64)) and friends to fail on 32 bits HW?
Not really.
> - What is expected value for max_pfn: max_phys_pfn or max_phys_pfn + 1?
mm/page_owner.c: /* Find an allocated page */ for (; pfn < max_pfn; pfn++) {
drivers/base/platform.c: u32 low_totalram = ((max_pfn - 1) << PAGE_SHIFT); drivers/base/platform.c: u32 high_totalram = ((max_pfn - 1) >> (32 - PAGE_SHIFT));
So, there's ample evidence that max_pfn is one more than the greatest pfn which may be used in the system.
> - What is expected value for struct memblock_region->size: mem_range_size or mem_range_size - 1?
A size is a size - it's a number of bytes contained within the region. If it is value 1, then there is exactly one byte in the region. If there are 0x7fffffff, then there are 2G-1 bytes in the region, not 2G.
> - What is expected value to be returned by memblock_end_of_DRAM(): > @base + @size(max_phys_addr + 1) or @base + @size - 1(max_phys_addr)?
The last address plus one in the system. However, there's a problem here. On a 32-bit system, phys_addr_t may be 32-bit. If it is 32-bit, then "last address plus one" could be zero, which makes no sense. Hence, it is artificially reduced to 0xfffff000, thereby omitting the final page.
> Example 3 CONFIG_ARM_LPAE=y (but system really works with 32 bit address space): > memory { > device_type = "memory"; > reg = <0x80000000 0x80000000>; > }; > > memblock will be configured as: > memory.cnt = 0x1 > memory[0x0] [0x00000080000000-0x000000ffffffff], 0x80000000 bytes flags: 0x0 > ^^^^^^^^^^ > max_pfn = 0x00100000 > > The dma_coerce_mask_and_coherent() will fail in case 'Example 3' and succeed in cases 1,2. > dma-mapping.c --> __dma_supported() > if (sizeof(mask) != sizeof(dma_addr_t) && <== true for all OMAP4+ > mask > (dma_addr_t)~0 && <== true for DMA_BIT_MASK(64) > dma_to_pfn(dev, ~0) < max_pfn) { <== true only for Example 3
Hmm, I think this may make more sense to be "< max_pfn - 1" here, as that would be better suited to our intention.
The result of dma_to_pfn(dev, ~0) is the maximum PFN which we could address via DMA, but we're comparing it with the maximum PFN in the system plus 1 - so we need to subtract one from it.
Please think about this and test this out; I'm not back to normal yet (post-op) so I could very well not be thinking straight yet.
Thanks.
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