Messages in this thread | | | Date | Wed, 17 Aug 2022 14:43:46 +0100 | Subject | Re: [PATCH] mmc: sdhci-xenon: Fix 2G limitation on AC5 SoC | From | Robin Murphy <> |
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On 2022-08-16 21:51, Vadym Kochan wrote: [...] >> The one thing to watch out for is that SWIOTLB doesn't necessarily interact >> very well with DMA offsets. Given the intent of >> of_dma_get_max_cpu_address(), I think it ought to work out OK now for >> current kernels on DT systems if everything is described correctly, but >> otherwise it's likely that you end up with ZONE_DMA either being empty or >> containing all memory, so the SWIOTLB buffer ends up being allocated >> anywhere such that it might not actually work as expected. >> >> Robin. > > Hi Robin, > > Thank you for the reply. > > My understanding is that swiotlb is allocated (in case of arm64) > in the following cases: > > #1 when it is forced from the kernel cmdline > > #2 when max_pfn is greater than arm64_dma_phys_limit (and this is used > as the end from which to allocate the swiotlb pool in the > top-botom direction via memblock API). > > #3 using restricted dma-pool > > Of course option #3 works fine because swiotlb is kind of forced to use > particulary this range of memory. > > Both options #1 & #2 causes to use full memory mask even if to specify > dma-ranges in the DT: > > dma-ranges = <0x0 0x0 0x2 0x0 0x0 0x80000000>; > > or if to specify the opposite: > > dma-ranges = <0x2 0x0 0x0 0x0 0x0 0x80000000>; > > just to make it lower than U32 to pass > > zone_dma_bits = min3(32U, dt_zone_dma_bits, acpi_zone_dma_bits) > > condition, but then it will be re-set in max_zone_phys() by: > > > if (phys_start > U32_MAX) > zone_mask = PHYS_ADDR_MAX; > else if (phys_start > zone_mask) > zone_mask = U32_MAX;
Ah, indeed I missed that, sorry. It seems that that change to stop assuming an offset kind of crossed over with the introduction of *_dma_get_max_cpu_address(), but now that that firmware property parsing *is* implemented, in principle it should be equally possible to evaluate the actual offsets as well, and decide whether an offset ZONE_DMA is appropriate or not. Either way, this is definitely the area which needs work if we want to to able to support topologies like this properly.
> So, currently I dont see how to pin swiotlb (I see it as a main problem) to some specific range of physical > memory (particulary to the first 2G of RAM).
Indeed, if ZONE_DMA and/or ZONE_DMA32 can't be set appropriately, then there's no way to guarantee correct allocation of any DMA buffers, short of hacking it with explicitly placed reserved-memory carveouts.
Thanks, Robin.
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