blk_queue_bounce_limit() broken for mask=0xffffffff on 64bit archs

[Nikita Yushchenko]

Hi

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

  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).


Not sure how to fix this properly. Any hints?