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

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Date:	Fri, 22 Jan 2016 21:47:54 -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 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.