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Groups > linux.kernel > #1554704

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

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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?

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Thread

blk_queue_bounce_limit() broken for mask=0xffffffff on 64bit archs Nikita Yushchenko <nikita.yoush@cogentembedded.com> - 2017-01-09 21:50 +0100
  Re: blk_queue_bounce_limit() broken for mask=0xffffffff on 64bit  archs Christoph Hellwig <hch@infradead.org> - 2017-01-09 22:10 +0100
  Re: blk_queue_bounce_limit() broken for mask=0xffffffff on 64bit archs Ming Lei <tom.leiming@gmail.com> - 2017-01-13 07:10 +0100
    Re: blk_queue_bounce_limit() broken for mask=0xffffffff on 64bit  archs Nikita Yushchenko <nikita.yoush@cogentembedded.com> - 2017-01-13 07:20 +0100

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