From: Ming Lei <tom.leiming@gmail.com>
To: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Cc: Jens Axboe <axboe@kernel.dk>,
linux-block <linux-block@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Artemi Ivanov <artemi.ivanov@cogentembedded.com>
Subject: Re: blk_queue_bounce_limit() broken for mask=0xffffffff on 64bit archs
Date: Fri, 13 Jan 2017 14:07:57 +0800 [thread overview]
Message-ID: <CACVXFVPgmaBh5OqjfTfqdQ9k78PWsmozuRZpSG5gMg84m4sDwQ@mail.gmail.com> (raw)
In-Reply-To: <632ad7f3-e837-a17c-5ba7-ed9800ba3ec0@cogentembedded.com>
Hi,
On Tue, Jan 10, 2017 at 4:48 AM, Nikita Yushchenko
<nikita.yoush@cogentembedded.com> wrote:
> 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
That should have been true in your case, since the b_pfn is smaller than
0xffffffff.
>
> 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?
Thanks,
Ming
>
>
> Not sure how to fix this properly. Any hints?
> --
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--
Ming Lei
next prev parent reply other threads:[~2017-01-13 6:09 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-09 20:48 blk_queue_bounce_limit() broken for mask=0xffffffff on 64bit archs Nikita Yushchenko
2017-01-09 20:59 ` Christoph Hellwig
2017-01-13 6:07 ` Ming Lei [this message]
2017-01-13 6:10 ` Nikita Yushchenko
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