Linus Torvalds wrote:
On Wed, 15 Nov 2006, Arjan van de Ven wrote:
well we could cheat some. And have the generic code for this just
register the irq handler for both somehow.
Well, not generic code. It would have to be the driver itself that does
it, since generic code doesn't even know (at irq request time - and when
they are generated - it just gets the irq number).
And the thing is, once you do that, all the advantages of MSI totally go
away - both the "nice" ones and the "really good ones" (the latter being
the hopeful eventual removal of irq routing confusions). So if you do
that, the better solution is for the driver to say "I won't use MSI at
all".
Really.
It all boils down to the same thing: either we have to know that MSI works
(where "know" is obviously relative - it's not like you can avoid _all_
bugs, but dammit, even a single report of "not working" means that there
are probably a ton of machines like that, and we did something wrong), or
we shouldn't use it. There is no middle ground. You can't really safely
"test" for it, and while you _can_ say "just do both", it doesn't really
help anything (and potentially exposes you to just more bugs: if enablign
MSI actually _does_ disable INTx, but then doesn't work, at a minimum you
end up with a device that doesn't work, even if the rest of the kernel
might be ok).
And btw, I say this as a person whose new main machine used to have HDA
routed over MSI, and the decision to default to it off meant that it went
back to the regular INTx thing.
(Btw, MSI interrupts also seem to not participate in CPU balancing:
22: 41556 43005 IO-APIC-fasteoi HDA Intel
506: 110417 0 PCI-MSI-edge eth0
mine do:$ cat /proc/interrupts
CPU0 CPU1
0: 7986702 7971263 IO-APIC-edge timer
...
20: 90626 95073 IO-APIC-fasteoi uhci_hcd:usb2
220: 1722 1415 PCI-MSI-edge HDA Intel
NMI: 0 0
LOC: 15957069 15957071
ERR: 0
MIS: 0
Also, I find it disturbing that we are forcing users to have know about
all these
magic options that have to be put on the kernel boot line. My hard drive
on my
new laptop would only run at 1.2mbs until I found out I had to use
combined_mode=libata
and build a new ramdisk that included ata_piix.
My $.02
Steve
which is another semantic change introduced by using MSI)
Linus
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