linux-kernel - Re: [RFC Patch V1] ioatdma: Ignore IOAT devices under hotplug-capable PCI host bridge

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Date:	Mon, 08 Jun 2015 19:44:43 +0800
From:	Jiang Liu <jiang.liu@...ux.intel.com>
To:	Vinod Koul <vinod.koul@...el.com>
CC:	Dave Jiang <dave.jiang@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	David Woodhouse <dwmw2@...radead.org>,
	Tony Luck <tony.luck@...el.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, dmaengine@...r.kernel.org
Subject: Re: [RFC Patch V1] ioatdma: Ignore IOAT devices under hotplug-capable
 PCI host bridge

On 2015/6/8 18:42, Vinod Koul wrote:
> On Tue, Jun 02, 2015 at 02:37:31PM +0800, Jiang Liu wrote:
>> Ccing Rafael, it's ACPI hotplug related.
>>
>> On 2015/6/2 14:36, Jiang Liu wrote:
>>> The dmaengine core assumes that async DMA devices will only be removed
>>> when they not used anymore, or it assumes dma_async_device_unregister()
>>> will only be called by dma driver exit routines. But this assumption is
>>> not true for the IOAT driver, which calls dma_async_device_unregister()
>>> from ioat_remove(). So current IOAT driver doesn't support device
>>> hot-removal because it may cause system crash to hot-remove an inuse
>>> IOAT device.
>>>
>>> To support CPU socket hot-removal, all PCI devices, including IOAT
>>> devices embedded in the socket, will be hot-removed. The idea solution
>>> is to enhance the dmaengine core and IOAT driver to support hot-removal,
>>> but that's too hard.
>>>
>>> This patch implements a hack to disable IOAT devices under hotplug-capable
>>> CPU socket so it won't break socket hot-removal.
>>>
> So below looks okay though I wonder how hard would it be to fix hot unplug ?
Hi Vinod,
	Thanks for review. About three years ago I worked out a
patch set to enhance the dmaengine core and ioat device driver to
support hot-removal. But it has been rejected due to concerns about
performance penalty caused by usage tracking.
	To support hot-removal, we need to track dma channel usage
and a way to reclaim dma channels when hot-removing. This may cause
sensible performance penalty. Recently I have tried again but still
haven't find a way to support hot-removal. So eventually I suggest
to disable IOAT device on hot-plug capable systems.
Thanks!
Gerry
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