On Fri, Jun 25, 2021 at 05:22:04PM -0500, Bjorn Helgaas wrote: > On Fri, Jun 25, 2021 at 05:30:29PM +0800, Huacai Chen wrote: > > In new revision of LS7A, some PCIe ports support larger value than 256, > > but their maximum supported MRRS values are not detectable. Moreover, > > the current loongson_mrrs_quirk() cannot avoid devices increasing its > > MRRS after pci_enable_device(), and some devices (e.g. Realtek 8169) > > will actually set a big value in its driver. So the only possible way is > > configure MRRS of all devices in BIOS, and add a PCI device flag (i.e., > > PCI_DEV_FLAGS_NO_INCREASE_MRRS) to stop the increasing MRRS operations. > > > > However, according to PCIe Spec, it is legal for an OS to program any > > value for MRRS, and it is also legal for an endpoint to generate a Read > > Request with any size up to its MRRS. As the hardware engineers says, > > the root cause here is LS7A doesn't break up large read requests (Yes, > > that is a problem in the LS7A design). > > "LS7A doesn't break up large read requests" claims to be a root cause, > but you haven't yet said what the actual *problem* is. > > Is the problem that an endpoint reports a malformed TLP because it > received a completion bigger than it can handle? Is it that the LS7A > root port reports some kind of error if it receives a Memory Read > request with a size that's "too big"? Maybe the LS7A doesn't know > what to do when it receives a Memory Read request with MRRS > MPS? > What exactly happens when the problem occurs? > > MRRS applies only to the read request. It is not directly related to > the size of the completions that carry the data back to the device > (except that obviously you shouldn't get a completion larger than the > read you requested). > > The setting that directly controls the size of completions is MPS > (Max_Payload_Size). One reason to break up read requests is because > the endpoint's buffers can't accommodate big TLPs. One way to deal > with that is to set MPS in the hierarchy to a smaller value. Then the > root port must ensure that no TLP exceeds the MPS size, regardless of > what the MRRS in the read request was. > > For example, if the endpoint's MRRS=4096 and the hierarchy's MPS=128, > it's up to the root port to break up completions into 128-byte chunks. > > It's also possible to set the endpoint's MRRS=128, which means reads > to main memory will never receive completions larger than 128 bytes. > But it does NOT guarantee that a peer-to-peer DMA from another device > will be limited to 128 bytes. The other device is allowed to generate > Memory Write TLPs with payloads up to its MPS size, and MRRS is not > involved at all. > > It's not clear yet whether the LS7A problem is with MRRS, with MPS, or > with some combination. It's important to understand exactly what is > broken here so the quirk doesn't get in the way of future changes to > the generic MRRS and MPS configuration. > > Here's a good overview: > > https://www.xilinx.com/support/documentation/white_papers/wp350.pdf > > > Signed-off-by: Huacai Chen <chenhuacai@xxxxxxxxxxx> > > --- > > drivers/pci/pci.c | 5 +++++ > > drivers/pci/quirks.c | 8 +++++++- > > include/linux/pci.h | 2 ++ > > 3 files changed, 14 insertions(+), 1 deletion(-) > > > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c > > index b717680377a9..6f0d2f5b6f30 100644 > > --- a/drivers/pci/pci.c > > +++ b/drivers/pci/pci.c > > @@ -5802,6 +5802,11 @@ int pcie_set_readrq(struct pci_dev *dev, int rq) > > > > v = (ffs(rq) - 8) << 12; > > > > + if (dev->dev_flags & PCI_DEV_FLAGS_NO_INCREASE_MRRS) { > > + if (rq > pcie_get_readrq(dev)) > > + return -EINVAL; > > + } > > + > > ret = pcie_capability_clear_and_set_word(dev, PCI_EXP_DEVCTL, > > PCI_EXP_DEVCTL_READRQ, v); > > > > diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c > > index dee4798a49fc..8284480dc7e4 100644 > > --- a/drivers/pci/quirks.c > > +++ b/drivers/pci/quirks.c > > @@ -263,7 +263,13 @@ static void loongson_mrrs_quirk(struct pci_dev *dev) > > * anything larger than this. So force this limit on > > * any devices attached under these ports. > > */ > > - if (pci_match_id(bridge_devids, bridge)) { > > + if (bridge && pci_match_id(bridge_devids, bridge)) { > > + dev->dev_flags |= PCI_DEV_FLAGS_NO_INCREASE_MRRS; > > + > > + if (pcie_bus_config == PCIE_BUS_DEFAULT || > > + pcie_bus_config == PCIE_BUS_TUNE_OFF) > > + break; Another approach might be to make a quirk that prevents Linux from touching MPS and MRRS at all under any circumstances. You'd have to do this without reference to pcie_bus_config so future MPS/MRRS algorithm changes wouldn't be affected. And the quirk bit would have to be in struct pci_host_bridge, similar to no_ext_tags. > > if (pcie_get_readrq(dev) > 256) { > > pci_info(dev, "limiting MRRS to 256\n"); > > pcie_set_readrq(dev, 256); > > diff --git a/include/linux/pci.h b/include/linux/pci.h > > index 24306504226a..5e0ec3e4318b 100644 > > --- a/include/linux/pci.h > > +++ b/include/linux/pci.h > > @@ -227,6 +227,8 @@ enum pci_dev_flags { > > PCI_DEV_FLAGS_NO_FLR_RESET = (__force pci_dev_flags_t) (1 << 10), > > /* Don't use Relaxed Ordering for TLPs directed at this device */ > > PCI_DEV_FLAGS_NO_RELAXED_ORDERING = (__force pci_dev_flags_t) (1 << 11), > > + /* Don't increase BIOS's MRRS configuration */ > > + PCI_DEV_FLAGS_NO_INCREASE_MRRS = (__force pci_dev_flags_t) (1 << 12), > > }; > > > > enum pci_irq_reroute_variant { > > -- > > 2.27.0 > >
