On 6/2/24 14:45, Warner Losh wrote:
On Sun, Jun 2, 2024, 3:37 PM Colin Percival <[email protected]
<mailto:[email protected]>> wrote:
I don't think there's any good mechanism for doing that, unfortunately.
Part
of the problem is that our device probing scheme is designed around the idea
that by the time you return from device_attach, you know if the device has
successfully attached; if you discover that the device is broken at a later
time it's too late to assign the unit number to a different device.
This is false. We assign unit in probe, and once assigned you don't want to
reassign it.
Ah, I misremembered. It's a while since I was thinking about these details.
I remember talking to Warner about this a while back in the context of nvme,
but the problem of "the spec says we have to wait a long time and we don't
want to do that serially for every disk" was resolved by our driver learning
to be opportunistic and ask the disks if they were ready yet instead of
simply
waiting the time stipulated by the spec.
If there's no status register to read, you can't do much. And in an SoC
there's only going to be one. So between yhe two, i don't see much benefit to
be had. And even if we could do this wait asynchronously, there is nothing
else to do in parallel.
I suppose it's unlikely that an SoC would have two PCI buses...
Maybe something to consider when someone decides to write newnewbus. ;-)
Parallel discovery is something we've talked about, but there's a number of
logistical issues to sort out first.
Yep, definitely nontrivial.
--
Colin Percival
FreeBSD Release Engineering Lead & EC2 platform maintainer
Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid
