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

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Date:   Fri, 13 Jan 2017 09:10:37 +0300
From:   Nikita Yushchenko <nikita.yoush@...entembedded.com>
To:     Ming Lei <tom.leiming@...il.com>
Cc:     Jens Axboe <axboe@...nel.dk>,
        linux-block <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Artemi Ivanov <artemi.ivanov@...entembedded.com>
Subject: Re: blk_queue_bounce_limit() broken for mask=0xffffffff on 64bit
 archs


>> 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