linux-kernel - Re: [PATCH v2 2/4] pciehp: Use link change notifications for hot-plug and removal

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Date:	Mon, 16 Dec 2013 18:36:01 -0800
From:	Guenter Roeck <linux@...ck-us.net>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
CC:	Rajat Jain <rajatjain@...iper.net>,
	Yinghai Lu <yinghai@...nel.org>,
	Rajat Jain <rajatjain.linux@...il.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kenji Kaneshige <kaneshige.kenji@...fujitsu.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Yijing Wang <wangyijing@...wei.com>,
	Paul Bolle <pebolle@...cali.nl>,
	Rajat Jain <rajatxjain@...il.com>,
	Guenter Roeck <groeck@...iper.net>
Subject: Re: [PATCH v2 2/4] pciehp: Use link change notifications for hot-plug
 and removal

On 12/16/2013 05:14 PM, Bjorn Helgaas wrote:
> On Mon, Dec 16, 2013 at 10:39 AM, Guenter Roeck <linux@...ck-us.net> wrote:
>> On Sun, Dec 15, 2013 at 05:18:26PM -0700, Bjorn Helgaas wrote:
>>> On Sun, Dec 15, 2013 at 4:24 PM, Rajat Jain <rajatjain@...iper.net> wrote:
>>>>>>>
>>>>>>>> Once again: the way I interpret this is: * Always enable Link events.
>>>>>>>> * Disable presence events if attention button is present.
>>>>>>>
>>>>>>> That sounds like a good plan to me.
>>>>>>
>>>>>> How about Diag_Reset from MPT2SAS and others?  link could up and down
>>>>>>
>>>>
>>>> I am assuming you are referring to
>>>>
>>>> static int _base_diag_reset(struct MPT2SAS_ADAPTER *ioc, int sleep_flag)
>>>>
>>>> Which as far as I could understand would cause link to go down and come up
>>>> again without the kernel knowing anything about it?  ...
>>>
>>>> In general, I guess the question is when a link goes down and back up,
>>>> whether or not we want to treat it as a hot unplug followed by a hotplug. I
>>>> think there may be cases such as AER (or the one Yinghai mentions) where we
>>>> don't want to treat it as a hotplug (see note below). And there may be cases
>>>> that we definitely want to treat it as hotplug (need link events!).
>>>> Situation gets more complex since there may be pciehp slots downstream of a
>>>> link getting reset.
>>>>
>>>> It seems to me that we are saying that a mechanism is needed so that a
>>>> voluntary Link flap is NOT treated like a hotplug temporarily?  ...
>>>
>>>> Notes: * it may not OK, if the kernel thinks the device is accessible when
>>>> it is really not.  What if during this downtime, someone tries to access the
>>>> device (say userspace)?  * How do we know after the link up, that the device
>>>> is really the same.  If during this reset, the device changed its
>>>> "character", say a different class?  I think a rescan should be mandated
>>>> after every such event.  * Do we need to unload and reload the driver after
>>>> the link reset, since it can be a different device?
>>>
>>> I am quite dubious about the idea of a voluntary link flap.  If the link goes
>>> down and comes back up, I don't see how we can make any assumptions about what
>>> device is there.
>>>
>>> I let Alex talk me into pciehp_reset_slot(), which disables presence detect
>>> interrupts while resetting a device, so we already have a bit of precedent for
>>> the idea.  But even in that case, the device could easily come out of reset as
>>> a different device, e.g., if the reset caused it to load updated firmware.
>>>
>>> I would feel much better if we treated link down as a remove and did a rescan
>>> on the link up.
>>>
>> Agreed. Question is if we might need some means for a driver to tell the PCIe
>> core about an upcoming "planned" link flap. pciehp_reset_slot() doesn't seem
>> to address the condition where a driver resets a connected chip by other means
>> than by calling pciehp_reset_slot(). Still not sure what happens when the
>> mpt2sas driver issues its diagnostic reset, to take Yinghai's example (or if
>> there would be a cleaner way to implement such a reset).
>
> In my opinion we should not add the concept of a planned link flap.
> We already have pci_reset_function(), and we can probably make that
> deal with link up/down events internally, so I think we should try to
> use that if we can.  I think it's too much of a mess to try to support
> link flaps for random driver-initiated resets that don't go through
> the PCI core.
>
Perfectly fine with me.

> That probably means going through and identifying all the drivers we
> can find that do their own internal resets, and converting them.
> We'll likely miss some, since the mechanisms are driver-specific.  And

Also might be difficult - that kind of work really asks for having
the hardware available for testing.

Guenter

> maybe there are some driver resets that think they add value over the
> core's pci_reset_function() (I'm not sure what that would be, but I'm
> open to discussion about it).
>
> Bjorn
>
>

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