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Those drivers want to know if the CXL link is > > live end-to-end (from endpoint, through switches, to the host bridge) > > and CXL memory operations are enabled. If not, a CXL accelerator may be > > able to fall back to PCI-only operation. Similar to the "Soft Reserve > > Memory" it needs to know that the CXL subsystem had a chance to probe > > the ancestor topology of the device and let that driver make a > > synchronous decision about CXL operation. > > > IMO, this is not the problem with accelerators, because this can not be > dynamically done, or not easily. Hmm, what do you mean can not be dynamically done? The observation is that a CXL card and its driver have no idea if the card is going to be plugged into a PCIe only slot. At runtime the driver only finds out the CXL is not there from the result of devm_cxl_add_memdev(). > The HW will support CXL or PCI, and if > CXL mem is not enabled by the firmware, likely due to a > negotiation/linking problem, the driver can keep going with CXL.io. Right, I think we are in violent agreement. > Of course, this is from my experience with sfc driver/hardware. Note > sfc driver added the check for CXL availability based on Terry's v13. Note that Terry's check for CXL availabilty is purely a hardware detection, there are still software reasons why cxl_acpi and cxl_mem can prevent devm_cxl_add_memdev() success. > But this is useful for solving the problem of module removal which can > leave the type2 driver without the base for doing any unwinding. Once a > type2 uses code from those other cxl modules explicitly, the problem is > avoided. You seem to have forgotten about this problem, what I think it > is worth to describe. What problem exactly? If it needs to be captured in these changelogs or code comments, let me know.