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Date: Fri, 22 Jan 2016 22:15:17 -0800
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Subject: Re: [BUG] Devices breaking due to CONFIG_ZONE_DEVICE
From: Dan Williams <dan.j.williams@intel.com>
To: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>, "H. Peter Anvin" <hpa@zytor.com>,
        Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Rik van Riel <riel@redhat.com>, Mel Gorman <mgorman@suse.de>,
        Jerome Glisse <j.glisse@gmail.com>, markk@clara.co.uk,
        Joerg Roedel <jroedel@suse.de>, Borislav Petkov <bp@suse.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Fri, Jan 22, 2016 at 9:47 PM, Dan Williams <dan.j.williams@intel.com> wrote:
> On Fri, Jan 22, 2016 at 8:46 PM, Sudip Mukherjee
> <sudipm.mukherjee@gmail.com> wrote:
>> Hi All,
>> Commit 033fbae988fc ("mm: ZONE_DEVICE for "device memory"") has
>> introduced CONFIG_ZONE_DEVICE while sacrificing CONFIG_ZONE_DMA.
>> Distributions like Ubuntu has started enabling CONFIG_ZONE_DEVICE and
>> thus breaking parallel port. Please have a look at
>> https://bugzilla.kernel.org/show_bug.cgi?id=110931 for the bug report.
>>
>> Apart from parallel port I can see some sound drivers will also break.
>>
>> Now what is the possible solution for this?
>
> The tradeoff here is enabling direct-I/O for persistent memory vs
> support for legacy devices.
>
> One possible solution is to alias ZONE_DMA and ZONE_DEVICE.  At early
> boot if pmem is detected disable these legacy devices, or the reverse
> disable DMA to persistent memory if a legacy device is detected.  The
> latter is a bit harder to do as I think we would want to make the
> decision early during memory init before we would know if any parallel
> ports or ISA sound cards are present.

...another option that might be cleaner is to teach GFP_DMA to get
memory from a different mechanism.  I.e. don't use the mm-zone
infrastructure to organize that small 16MB pool of memory.