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

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Date:   Mon, 9 Jan 2017 23:48:11 +0300
From:   Nikita Yushchenko <nikita.yoush@...entembedded.com>
To:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Artemi Ivanov <artemi.ivanov@...entembedded.com>
Subject: blk_queue_bounce_limit() broken for mask=0xffffffff on 64bit archs

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?