linux-kernel - Re: [BUG] Devices breaking due to CONFIG_ZONE

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Message-ID: <CAPcyv4ivKNb2=505ytkVMbmOd=49da1EsHoG=Di60XJyoQTs8g@mail.gmail.com>
Date:	Fri, 22 Jan 2016 22:15:17 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Sudip Mukherjee <sudipm.mukherjee@...il.com>
Cc:	Christoph Hellwig <hch@....de>, "H. Peter Anvin" <hpa@...or.com>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Jerome Glisse <j.glisse@...il.com>, markk@...ra.co.uk,
	Joerg Roedel <jroedel@...e.de>, Borislav Petkov <bp@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] Devices breaking due to CONFIG_ZONE_DEVICE

On Fri, Jan 22, 2016 at 9:47 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Fri, Jan 22, 2016 at 8:46 PM, Sudip Mukherjee
> <sudipm.mukherjee@...il.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.