Re: PCIe hotplug

On Jul 20, 2012, at 8:04 PM, Julian Elischer wrote:

> Is anyone looking at PCIe hotplug support?
>=20
> 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.
>=20

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

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