PCIe hotplug

[Warner Losh]

On Jul 23, 2012, at 1:45 AM, Julian Elischer wrote:

> On 7/22/12 9:11 PM, Warner Losh wrote:
>> On Jul 22, 2012, at 9:12 PM, Alexander Kabaev wrote:
>> 
>>> On Sun, 22 Jul 2012 20:22:33 -0600
>>> Scott Long <scottl at samsco.org> wrote:
>>> 
>>>> On Jul 20, 2012, at 8:04 PM, Julian Elischer wrote:
>>>> 
>>>>> Is anyone looking at PCIe hotplug support?
>>>>> 
>>>>> I'm especially interested if anyone has a strategy for device
>>>>> re-insertion and reassociating the reinserted device with its old
>>>>> device_t so that it gets the same unit number.. (assumes access to
>>>>> a serial number or similar) Even if it is put back into a different
>>>>> slot.
>>>>> 
>>>> Would the PCI system be responsible for figuring out this serial
>>>> number?  I don't think that it can, but it's a question to answer, I
>>>> guess.  If it can't then it's up to the driver to generate a unique
>>>> cookie that would be stored by the PCI subsystem.  This cookie would
>>>> have to be based off of data that can be retrieved from the PCI
>>>> config space and/or VPD space, since anything more would require
>>>> resource allocation, which is only allowed in the DEV_ATTACH phase,
>>>> and once you've hit that phase you've already pretty much sealed the
>>>> deal on unit number assignment.
>>>> 
>>>> So what would probably happen is that the PCI layer provides a ring
>>>> buffer of cookie storage and a set of accessors for the drivers.  The
>>>> cookies would map to a key-value pair with the device unit name and
>>>> number.  During probe, a driver can look at PCI config space and
>>>> generate a cookie.  That cookie can then be communicated up to the
>>>> PCI layer for storage.  Maybe the driver calls a match routine that
>>>> returns a unit number on match and a store on failure, then the
>>>> driver calls a set_unit_number accessor.  Only the driver that wins
>>>> the bid would win the unit number reassignment or cookie storage.  Or
>>>> maybe the driver passes the cookie up as part of its return code, and
>>>> the match and unit assignment happens automatically.  Drivers that
>>>> don't want to participate in this simply wouldn't, and everything
>>>> would continue to operate the same way.  The two sticky parts are
>>>> rogue/buggy drivers that abuse the api and cause a flood of cookies
>>>> to be generated, and questions on when a unit number is eligible for
>>>> reuse.  For the first one, a ring buffer of cookies would solve the
>>>> immediate problem, but you might still have some risk of drivers
>>>> selectively wrapping the buffer for whatever accidental or evil
>>>> purpose.  For the second problem, maybe a unit number stays
>>>> persistent only if the PCIe hot remove mechanism requests it, and
>>>> then only until the ring-buffer wraps.
>>>> 
>>>> Scott
>>>> 
>>> I do not think the whole problem as depicted by Julian is even worth
>>> solving. Why keeping any data for the device that might _never_ come
>>> back? What if the device hierarchy just starts from the PCI-e and
>>> extends upwards and user still holds on to some vestiges of a previous
>>> device chain (say, by keeping a character control device sharing the
>>> same unit number open, common practice)? Reusing unit number is much
>>> trickier then, and might not be even possible. So, before one jumps
>>> into 'how', can we agree on 'why' first? When device goes away, it is
>>> not just this device's device_t that is disappearing, it is a whole
>>> tree rooted at that device. I see no point in trying to reconstruct
>>> that.
>> There's a reason that PC Card and CardBus never supported this at all.  The assumption was that reconnecting devices is so cheap that it isn't worth the bother.  This is true for all but some specialized devices today: network information is easy to reconstruct, storage drives are easy to reconfigure (since we already fail all in-flight transactions when the device goes away), etc.  I can see some advantage to having storage cope, but there already geom classes that can help people code when drives can go away.
>> 
>>> PCI-e hotplug proper is very much orthogonal to the question of unit
>>> numbering and IS worth supporting.
>> Yes.  totally agreed.
> 
> I'm not saying that it's vitally important but was wondering if people had a strategy for it..
> i.e. is it a question worth worrying about?
> 
> In a separate forum Warner and I (yeah I know I'm answering Warner, but I'm addressing the others) discussed the feasibility  of surviving an "oops pulled the wrong card" event with regards to a particular flash memory card. I was just carrying that forwards as a thought experiment (There is actually a strategy that sounds feasible).
> 
> The problem of getting a serial number out of the BAR space during probe is also possibly solvable in our case but the question of how long to remember a device is legitimate an My answer would be that
> 1/ a particular driver would be able to specify whether it could handle this, and
> 2/ it might be limited to some pragmatic number such as 16 or 32, or a time limit.

Actually,  for the case where there's expensive state to reconstruct, you'd likely want to keep the state around for an estimated time it would take to reconstruct it.  I think that this may necessarily have to be done outside the framework of newbus, or you'd need a new mechanism/state that the driver could request as part of its detach routine that says effectively 'keep this around'.  Any later probe on insert would have to be able to find this and tell newbus about it.  However, that does start to get ugly in a hurry.

It would be better, imho, that if a driver wanted to keep this info around, it would have to take responsibility for finding any such pending state and cope appropriately.  In the case of storage devices, you'd be able to have those storage volumes on a separate bus, and then you'd have all the control that you need.  But then you start wondering into issues with what to do with the pending transactions, what to do with new transactions, etc before giving up.

Then again, all these are nice to haves, especially since we don't have pcie hot plug at all right now.

Warner