On Tue, Jun 02, 2020 at 08:23:54PM -0700, Raj, Ashok wrote: > Hi Bjorn > > I was trying to fix pcie_write_mrrs() since it seems to not follow PCIe > SIG recommendations. > > It appears to that 010 (512b) is the default and 4096b is the max spec > allowed limit. > > Current code seems to match MRRS and MPS, which seem to be completely > different purposes. Yes, I agree that MRRS and MPS have different purposes, and from a hardware point of view, there's no reason MRRS should be related to MPS. The reason Linux tries to set MRRS == MPS is so we can set MPS on shared paths to be higher than the smallest MPS of the leaves. For the leaf with a small MPS, we set MRRS == MPS and assume that is enough to limit the size of TLPs it receives. That assumption is true for *reads* initiated by the leaf, but not it's necessarily true for *writes* to the leaf, e.g., peer-to-peer writes. There's a little more explanation here: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b03e7495a862 > But trying to fix i got confused with all these pcie_bus_config_types. > > enum pcie_bus_config_types { > PCIE_BUS_TUNE_OFF, /* Don't touch MPS at all */ > PCIE_BUS_DEFAULT, /* Ensure MPS matches upstream bridge */ > PCIE_BUS_SAFE, /* Use largest MPS boot-time devices support */ > PCIE_BUS_PERFORMANCE, /* Use MPS and MRRS for best performance */ > PCIE_BUS_PEER2PEER, /* Set MPS = 128 for all devices */ > }; > > Not sure what the difference between BUS_TUNE_OFF and BUS_DEFAULT is. I'm not sure either, but there is a case in pci_configure_mps() where we test for PCIE_BUS_TUNE_OFF but not BUS_DEFAULT, so it's not completely obvious that they're identical. > MPS matching upstream bridge is required in all cases right? The MRRS trick above is intended to relax this requirement. > BUS_SAFE/BUS_PERFORMANCE? Not sure why that is special for BUS_DEFAULT. > > Wouldn't it be simple to say: > > BUS_DEFAULT : Just use BIOS settings, no change to anything. > > BUS_SAFE: Says Choose largest MPS boot-time settings? What does that > actually mean? Why isn't that BUS_PERFORMANCE? > > P2P: Choose smallest setting makes sense. > > I think only 3 settings make sense. > > BUS_DEFAULT == TUNE_OFF : Choose BIOS values, don't touch anything > BUS_SAFE == BUS_PERFORMANCE : Should actually be the default. > BUS_PEER2PEER - Same as now > > Do we really have value for the other settings? Maybe for BUS_DEFAULT we > should call it BUS_BIOS (to indicate real meaning) > > BUS_PERFORMANCE should be the default setting for the system. > > BUS_P2P if someone needs to configure for p2p. I would love it if somebody reworked this a bit. I think it's a little confusing right now. Bjorn
