On Tue, Dec 10, 2019 at 08:41:15AM -0600, Bjorn Helgaas wrote: > On Mon, Dec 09, 2019 at 04:06:38PM +0000, Andre Przywara wrote: > > From: Deepak Pandey <Deepak.Pandey@xxxxxxx> > > > > The Arm N1SDP SoC suffers from some PCIe integration issues, most > > prominently config space accesses to not existing BDFs being answered > > with a bus abort, resulting in an SError. > > Can we tease this apart a little more? Linux doesn't program all the > bits that control error signaling, so even on hardware that works > perfectly, much of this behavior is determined by what firmware did. > I wonder if Linux could be more careful about this. > > "Bus abort" is not a term used in PCIe. IIUC, a config read to a > device that doesn't exist should terminate with an Unsupported Request > completion, e.g., see the implementation note in PCIe r5.0 sec 2.3.1. > > The UR should be an uncorrectable non-fatal error (Table 6-5), and > Figures 6-2 and 6-3 show how it should be handled and when it should > be signaled as a system error. In case you don't have a copy of the > spec, I extracted those two figures and put them at [1]. > > Can you collect "lspci -vvxxx" output to see if we can correlate it > with those figures and the behavior you see? > > [1] https://drive.google.com/file/d/1ihhdQvr0a7ZEJG-3gPddw1Tq7cTFAsah/view?usp=sharing > > > To mitigate this, the firmware scans the bus before boot (catching the > > SErrors) and creates a table with valid BDFs, which acts as a filter for > > Linux' config space accesses. > > > > Add code consulting the table as an ACPI PCIe quirk, also register the > > corresponding device tree based description of the host controller. > > Also fix the other two minor issues on the way, namely not being fully > > ECAM compliant and config space accesses being restricted to 32-bit > > accesses only. > > As I'm sure you've noticed, controllers that support only 32-bit > config writes are not spec compliant and devices may not work > correctly. The comment in pci_generic_config_write32() explains why. > > You may not trip over this problem frequently, but I wouldn't call it > a "minor" issue because when you *do* trip over it, you have no > indication that a register was corrupted. > > Even ECAM compliance is not really minor -- if this controller were > fully compliant with the spec, you would need ZERO Linux changes to > support it. Every quirk like this means additional maintenance > burden, and it's not just a one-time thing. It means old kernels that > *should* "just work" on your system will not work unless somebody > backports the quirk. With regards to URs resulting in unwanted aborts or similar - this seems to be a very common theme amongst ARM PCI controller drivers. For example both ARM32 imx6 and ARM32 keystone have fault handlers to handle an abort and fabricate a 0xffffffff read value. The ARM32 rcar driver, whilst it doesn't appear to produce an abort, does read the PCI_STATUS register after making a config read to determine if any aborts have happened - in which case it reports PCIBIOS_DEVICE_NOT_FOUND. And as recently reported [1], the rockchip driver also appears to produce aborts. I suspect that this ARM64 controller driver won't be the last either. Thus any solution here may form the basis of copy-cat solutions for subsequent controllers. >From my understanding of the issues, the ARM64 serrors are imprecise and as a result there isn't a sensible way of using them to determine that a read is a UR. So where there are no other solutions to suppress the generation of an abort by the controller, the only solutions that seem to exist are 1) pre-scan the devices in firmware and only talk to those devices in Linux - a safe option but limiting - perhaps with side effects for CRS and 2) the approach rcar takes in using the PCI_STATUS register - though you'd end up having to mask the serror (PSTATE.A) for a limited period of time - a risky option (you'll miss real serrors) - but with no side effects. (I don't know if option 2 is feasible in this case by the way). [1] https://lore.kernel.org/linux-pci/2a381384-9d47-a7e2-679c-780950cd862d@xxxxxxxxxxxxxx/2-0001-WFT-PCI-rockchip-play-game-with-unsupported-request-.patch Thanks, Andrew Murray > > > This allows the Arm Neoverse N1SDP board to boot Linux without crashing > > and to access *any* devices (there are no platform devices except UART).
