On 19 October 2015 at 16:30, Russell King - ARM Linux <linux at arm.linux.org.uk> wrote: > On Mon, Oct 19, 2015 at 04:10:56PM +0200, Tomeu Vizoso wrote: >> On 19 October 2015 at 15:18, Russell King - ARM Linux >> <linux at arm.linux.org.uk> wrote: >> > On Mon, Oct 19, 2015 at 02:34:22PM +0200, Tomeu Vizoso wrote: >> >> ... If a device is available and has >> >> a compatible driver, but it cannot be probed because a dependency >> >> isn't going to be available, that's an error and is going to cause >> >> real-world problems unless the device is redundant. Currently we say >> >> nothing because with deferred probe the probe callbacks are also part >> >> of the mechanism that determines the dependency order. >> > >> > So what if device X depends on device Y, and we have a driver for >> > device Y built-in to the kernel, but the driver for device X is a >> > module? >> > >> > I don't see this being solvable in the way you describe above - it's >> > going to identify X as being unable to be satisfied, and report it as >> > an error - but it's not an error at all. >> >> It's going to probe Y at late_initcall, then probe X when its driver >> is registered. No deferred probes nor messages about it. >> >> But if you meant to write the opposite case (X built-in and Y in a >> module), then I have to ask you in what situation that would make >> sense. > > I did mean the opposite way around. It may not make sense if you're > targetting a single platform, but it may make sense in a single zImage > kernel. > > Consider something like a single zImage kernel that is built with > everything built-in to be able to boot and mount rootfs without > initramfs support on both platform A and platform B. Both platforms > share some hardware (eg, an I2C GPIO expander) which is built as a > module. It is a resource provider. Platform B contains a driver > which is required to boot on platform A, but not platform B (so the > kernel has to have that driver built-in.) On platform B, there is > a dependency to the I2C GPIO expander device.
I see, in this situation the person trying to find out why some device hadn't probed would enable debug logging of failed probes and would see one spurious message if there was a deferred probe because of the module. >> >> Having a specific switch for enabling deferred probe logging sounds >> >> good, but there's going to be hundreds of spurious messages about >> >> deferred probes that were just deferrals and only one of them is going >> >> to be the actual error in which a device failed to find a dependency. >> > >> > Why would there be? Sounds like something's very wrong there. >> >> Sorry about that, I have checked that only now and I "only" get 39 >> deferred probe messages on exynos5250-snow. > > I typically see one or two, maybe five maximum on the platforms I have > here, but normally zero. Hmm, I have given a look at our lava farm and have seen 2 dozens as common (with multi_v7). >> > So, really, after boot and all appropriate modules have been loaded, >> > you should end up with no deferred probes. Are you saying that you >> > still have "hundreds" at that point? If you do, that sounds like >> > there's something very wrong. >> >> I was talking about messages if we log each -EPROBE_DEFER, not devices >> that remain to be probed. The point being that right now we don't have >> a way to know if we are deferring because the dependency will be >> around later, or if we have a problem and the dependency isn't going >> to be there at all. > > What's the difference between a dependency which isn't around because > the driver is not built into the kernel but is available as a module, > and a dependency that isn't around because the module hasn't been > loaded yet? > > How do you distinguish between those two scenarios? In the former > scenario, the device will eventually come up when udev loads the > module. In the latter case, it's a persistent failing case. Agreed, but it's something that doesn't happen often and that's why such messages would be at the debug level instead of being warns or errors. >> Agreed, with the note from above on why it would be better to only >> print such a message only when the -EPROBE_DEFER is likely to be a >> problem. > > ... and my argument is that there's _no way_ to know for certain which > deferred probes will be a problem, and which won't. The only way to > definitely know that is if you disable kernel modules, and require > all drivers to be built into the kernel. > > What you can do is print those devices which have failed to probe at > late_initcall() time - possibly augmenting that with reports from > subsystems showing what resources are not available, but that's only > a guide, because of the "it might or might not be in a kernel module" > problem. Well, adding those reports would give you a changelog similar to the one in this series... Thanks, Tomeu > -- > FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up > according to speedtest.net. > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo at vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/
