On Tue, Feb 21, 2023 at 10:46:11PM +0100, Pali Rohár wrote: > On Sunday 19 February 2023 20:46:19 Lukas Wunner wrote: > > > On Sun, 5 Feb 2023, Maciej W. Rozycki wrote: > > > > This is v6 of the change to work around a PCIe link training phenomenon > > > > where a pair of devices both capable of operating at a link speed above > > > > 2.5GT/s seems unable to negotiate the link speed and continues training > > > > indefinitely with the Link Training bit switching on and off repeatedly > > > > and the data link layer never reaching the active state. > > > > Philipp is witnessing similar issues with a Pericom PI7C9X2G404EL > > switch below a Broadcom STB host controller: On some rare occasions, > > when booting the system the link trains correctly at 5 GT/s and the > > switch is accessible without any issues. But most of the time, > > the switch is inaccessible on boot. The Broadcom STB host controller > > claims not to support Link Active Reporting, but in reality has a > > link status indicator in a custom register. It indicates that the > > link is up, the link trains to 2.5 GT/s but the switch is inaccessible. > > This is interesting. Do you know which layer it indicates that is up? > I can image that PCIe physical layer or data link layer is up but > PCIe transaction layer not yet up and so sending config requests fail. > Existence of custom register may explain that it indicates different > "link up" meaning. drivers/pci/controller/pcie-brcmstb.c defines the following bits: #define PCIE_MISC_PCIE_STATUS_PCIE_PORT_MASK 0x80 #define PCIE_MISC_PCIE_STATUS_PCIE_DL_ACTIVE_MASK 0x20 #define PCIE_MISC_PCIE_STATUS_PCIE_PHYLINKUP_MASK 0x10 #define PCIE_MISC_PCIE_STATUS_PCIE_LINK_IN_L23_MASK 0x40 And brcm_pcie_link_up() checks that both DL_ACTIVE and PHYLINKUP are set. A public spec for the Broadcom STB PCIe controller does not seem to exist, so I do not know what the register bits mean exactly. > > Due to a quirk of the Broadcom STB host controller, ECAM access to > > the inaccessible switch raises an unhandled CPU exception and thus > > causes a kernel panic, making the issue difficult to debug. > > Is this ARM Cortex A53 core and unhandled exception is asynchronous one > with syndrome 0xbf000002? It's a Cortex A72 and yes the exception looks like this: SError Interrupt on CPU1, code 0x00000000bf000002 -- SError I was wondering why we're not checking in the exception handler whether the accessed address is in ECAM space, and just return from the handler since such exceptions could be handled by returning "all ones" in software from the PCI core. Then again, perhaps there's a method to stop the controller from raising an exception on ECAM access to an inaccessible device. If such a method exists (e.g. some register bit), that would obviously be preferred. > > The switch works fine 100% when plugged into a TI Sitara AM64 board > > (contains a DesignWare-derived PCIe host controller). > > It is really DesignWare? I had an impression that TI uses PCIe IPs from > Cadence, not from DesignWare. And Cadence controllers behave in some > cases different from Designware controllers. You're right, I was mistaken, it's indeed a Cadence. > > Next step is to hook up > > a Teledyne T28 analyzer to see what's going on. > > Can you use Teledyne T28 for debugging this issue? Because this is > something which can finally show what is happing there. Yes it should be possible to debug this, the analyzer is capable of logging the link training sequence and present it in a Wireshark-esque interface. Thanks, Lukas
