Hi Russell,
On 03/05/2015 10:17 PM, Russell King - ARM Linux wrote:
> On Thu, Mar 05, 2015 at 08:55:07PM +0200, Grygorii.Strashko@xxxxxxxxxx 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.
Thanks - it seems clear now.
>> - 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.
^ this part seems not fully true now, because for ARM32 + DT the
fdt.c->early_init_dt_add_memory_arch() is called instead of arm_add_memory()
and it works in a different way a bit.
For example, I don't see below message when reg = <0x80000000 0x80000000>:
"Truncating memory at 0x80000000 to fit in 32-bit physical address space"
instead memblock silently configured as
memory.cnt = 0x1
memory[0x0].base = 0x80000000
memory[0x0].size = 0x7fffffff
>
>> 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.
Ok. I'll try 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 for your comments. I hope you feel better.
--
regards,
-grygorii
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