>> There is a use cases when architecture is 64-bit but hardware supports >> only DMA to lower 4G of address space. E.g. NVMe device on RCar PCIe host. >> >> For such cases, it looks proper to call blk_queue_bounce_limit() with >> mask set to 0xffffffff - thus making block layer to use bounce buffers >> for any addresses beyond 4G. To support that, architecture provides >> GFP_DMA zone that covers exactly low 4G on arm64. >> >> However setting this limit does not work: >> >> if (b_pfn < (min_t(u64, 0xffffffffUL, BLK_BOUNCE_HIGH) >> PAGE_SHIFT)) >> dma = 1; >> >> When mask is 0xffffffff that condition is false > > That should have been true in your case, since the b_pfn is smaller than > 0xffffffff. b_pfn is exactly 0xffffffffUL >> SHIFT, thus contition is false >> q->limits.bounce_pfn = max(max_low_pfn, b_pfn); >> >> this line is executed and replaces any limit with end of memory (on >> 64bit arch all memory is low). > > I don't understand why max() is used? And why not min()? > > Looks the above line just disables bounce for 64bit arch, doesn't it? Effectively yes. And I don't understand logic behind this code. Nikita -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
