Re: PCIe hotplug

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