On Sun, 2011-06-05 at 22:54 -0500, Jon Mason wrote: .../... So I had a chat with our (IBM) firmware people and got some interesting feedback here: > > Same with max read req. size except that for that one we don't even have > > a register telling us what the bridge max is in the first place. > > Yes, I was going to look into that as well, since it makes sense to > check and set it at the same time as the MPS. Interestingly, our FW folks also clamp the MRRS to the MRS (never set it to higher than the MPS). > > I have some in-house code to fix that up on powerpc at boot that I > > haven't released yet (for a platform that we haven't published yet) but > > it's only boot time, I haven't dealt with the hotplug case. > > > > The problem I see with hotplug is that potentially, one could have to > > change the entire hierarchy below the PHB when a new device is plugged > > with a smaller max payload size than what was used before. > > > > However, I didn't think it was "safe" to just change the size on a live > > system, devices have to be quiescent. > > > > That makes it impossible to do reliably in my book, ie, anything with a > > hotplug might have to be hard wired to the smallest size unless the > > hotplug happens at the PHB level in which case there is no risk of > > disrupting another device while changing the size. > > On the limited amount of systems I've access to, I see the PCI-E slots > wired directly to the RC. So, this would not be an issue for these. > Though, we might want to add a check for a bus between the RC and the > hotpluggable PCI-E slot, and force it to the minimum in that case. For that case, what our FW does, is that it basically ignores the downstream path and only cares about upstream payloads (DMAs). IE. It configures newly plugged devices to have a smaller or equal MRS to the bridge they plug into, and that's it. The assumption is that we never get large downstream packets, probably because we never really support device-to-device DMA and our host bridges don't generate requests larger than a small payload anyways. So I think that's the right policy for me to implement long run for hotplug on ppc, tho I don't know if that can be accepted generically. Cheers, Ben. > > > > Ben. > > > > > > -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
