Hi, just copy pasting Matt's original cover letter: We're periodically facing problems in CI where all registers read back as 0xFFFFFFFF. In general this is what happens when the CPU is unable to communicate with a PCI device, so the transaction autocompletes with all F's as a placeholder. Sometimes the device will recover on its own, sometimes it will never come back. We already have some attempts to detect when this happens (e.g., when checking FPGA_DBG), but let's add a couple more checks with descriptive error messages to identify the problem in other cases: - When the device is first probed, we'll do an initial check of the GT forcewake register. As a masked register, the upper bits should always come back as 0's if device access is behaving properly, so if we see all F's, we can conclude that the device is already in a bad state. We'll wait two seconds to see if it recovers on its own, then give up on the device. - When we encounter a 'forcewake timed out while waiting for clear' error, we'll do one more read of the register to see if it's because we're just reading back all F's. If so, we'll print a more meaningful message clarifying that it isn't the forcewake itself that's the problem, but rather communication with the device. Note that this only captures the failure case where accessing the device is problematic (resulting in registers giving all F's). There's a separate class of problems where the device is okay, but the GT inside the device is busted and all GT registers read back as 0's (other registers like sgunit registers are usually still readable). This series does not address that class of errors. This is just a quick change to get some better CI error messages. Some ideas for future enhancements: - Try something to reset the device if we detect a problem at driver load (e.g., PCI FLR, toggling the PCI power state, etc.)? - Use something more standard like pci_read_config_dword() instead of a device register read to determine when we're not communicating properly? Generally the PCI config space is also giving all F's at this point. - Also handle the "device OK, GT dead" case by finding some GT register(s) that should never be 0 on a functioning system. Maybe one of the fuse registers would work for this? Matt Roper (2): drm/i915: Sanitycheck MMIO access early in driver load drm/i915: Check for unreliable MMIO during forcewake drivers/gpu/drm/i915/intel_uncore.c | 46 +++++++++++++++++++++++++++-- 1 file changed, 43 insertions(+), 3 deletions(-) -- 2.39.2
