On Fri, Sep 30, 2011 at 12:17 PM, Bjorn Helgaas <bhelgaas@xxxxxxxxxx> wrote: > On Fri, Sep 30, 2011 at 9:35 AM, Jon Mason <mason@xxxxxxxx> wrote: >> >> On Fri, Sep 30, 2011 at 12:01 AM, Bjorn Helgaas <bhelgaas@xxxxxxxxxx> wrote: >> > On Thu, Sep 29, 2011 at 6:16 PM, Jon Mason <mason@xxxxxxxx> wrote: >> >> Hey Avi, >> >> Can you try this patch? It should resolve the issue you are seeing. >> >> >> >> Thanks, >> >> Jon >> >> >> >> PCI: Workaround for Intel MPS errata >> >> >> >> Intel 5000 and 5100 series memory controllers have a known issue if read >> >> completion coalescing is enabled (the default setting) and the PCI-E >> >> Maximum Payload Size is set to 256B. To work around this issue, disable >> >> read completion coalescing if the MPS is 256B. >> > >> > I'd much rather see this done as an early quirk so it doesn't clutter probe.c. >> > >> > I don't know how you decide whether >> > - no coalescing with MPS=256, or >> > - coalescing with MPS=128 >> > is better. I suspect that having a quirk that doesn't change the >> > setting, but merely limits MPS to 128 if the BIOS enabled coalescing, >> > would be simplest and would stay in the best-tested chipset >> > configuration. >> >> This is what I was debating yesterday. Is it better to disable >> coalescing and get better throughput (which could be a net negative if >> the MPS isn't 256) or never allow it to be greater than 128? There is >> no way of knowing at quirk time if the disable is necessary or not, >> only when setting the MPS is it known (which is why I did it this >> way). I could, as you suggest, simply read the bit and see if it is >> enabled by the BIOS (which I'd bet it is every single time), and then >> limit the MPS to 128 as a quirk. This would be fairly simple to do. >> However, the errata from Intel says Windows 2008 always disables the >> coalescing and sets the MPS to 256B. With this known, Linux's I/O >> performance would be less than Windows on these systems. ... > > Presumably coalescing improves performance, too, and I don't have the > evidence that says "no coalescing with MPS=256" performs better than > "coalescing with MPS=128." > > But the fact that Windows 2008 disables coalescing is worth a lot (if > this is in a public erratum, a URL would be good). Given that, I'd The URLs were in the top of the patch, but perhaps they weren't obvious. http://www.intel.com/content/dam/doc/specification-update/5000-chipset-memory-controller-hub-specification-update.pdf http://www.intel.com/content/dam/doc/specification-update/5100-memory-controller-hub-chipset-specification-update.pdf Search for "MPS", as they are several pages into the errata chapter. > probably go with "no coalescing and MPS=256" just as you did. > > Maybe the quirk could be moved out of the generic code by making > pcie_set_mps() a weak function, so x86 could supply a version that > disables coalescing if MPS=256? Not sure what you mean here. Are you saying to make the function defined differently on each arch? > No news from Avi? Were you able to reproduce the problem and verify None so far. I believe he is in .il, so we might not hear back until Sunday. > that the quirk fixes it? I wish the kernel.org bugzilla were back. I just found a system to try it on! It has an Intel Corporation 5000X Chipset Memory Controller Hub, which should have the erratum. > Since it's not, maybe we should include the LKML URL > (https://lkml.org/lkml/2011/9/27/274) in the changelog. Will do. > > Bjorn > -- 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
