On Thu, 25 Mar 2021 10:37:54 +0200 Leon Romanovsky <l...@kernel.org> wrote:
> On Wed, Mar 24, 2021 at 11:17:29AM -0600, Alex Williamson wrote: > > On Wed, 24 Mar 2021 17:13:56 +0200 > > Leon Romanovsky <l...@kernel.org> wrote: > > <...> > > > > Yes, and real testing/debugging almost always requires kernel rebuild. > > > Everything else is waste of time. > > > > Sorry, this is nonsense. Allowing users to debug issues without a full > > kernel rebuild is a good thing. > > It is far from debug, this interface doesn't give you any answers why > the reset didn't work, it just helps you to find the one that works. > > Unless you believe that this information will be enough to understand > the root cause, you will need to ask from the user to perform extra > tests, maybe try some quirk. All of that requires from the users to > rebuild their kernel. > > So no, it is not debug. It allows a user to experiment to determine (a) my device doesn't work in a given scenario with the default configuration, but (b) if I change the reset to this other thing it does work. That is a step in debugging. It's absurd to think that a sysfs attribute could provide root cause, but it might be enough for someone to further help that user. It would be a useful clue for a bug report. Yes, reaching root cause might involve building a kernel, but that doesn't invalidate that having a step towards debugging in the base kernel might be a useful tool. > > > > > > For policy preference, I already described how I've configured QEMU > > > > > > to > > > > > > prefer a bus reset rather than a PM reset due to lack of > > > > > > specification > > > > > > regarding the scope of a PM "soft reset". This interface would > > > > > > allow a > > > > > > system policy to do that same thing. > > > > > > > > > > > > I don't think anyone is suggesting this as a means to avoid quirks > > > > > > that > > > > > > would resolve reset issues and create the best default general > > > > > > behavior. > > > > > > This provides a mechanism to test various reset methods, and thereby > > > > > > identify broken methods, and set a policy. Sure, that policy might > > > > > > be > > > > > > to avoid a broken reset in the interim before it gets quirked and > > > > > > there's potential for abuse there, but I think the benefits outweigh > > > > > > the risks. > > > > > > > > > > This interface is proposed as first class citizen in the general sysfs > > > > > layout. Of course, it will be seen as a way to bypass the kernel. > > > > > > > > > > At least, put it under CONFIG_EXPERT option, so no distro will enable > > > > > it > > > > > by default. > > > > > > > > Of course we're proposing it to be accessible, it should also require > > > > admin privileges to modify, sysfs has lots of such things. If it's > > > > relegated to non-default accessibility, it won't be used for testing > > > > and it won't be available for system policy and it's pointless. > > > > > > We probably have difference in view of what testing is. I expect from > > > the users who experience issues with reset to do extra steps and one of > > > them is to require from them to compile their kernel. > > > > I would define the ability to generate a CI test that can pick a > > device, unbind it from its driver, and iterate reset methods as a > > worthwhile improvement in testing. > > Who is going to run this CI? At least all kernel CIs (external and > internal to HW vendors) that I'm familiar are building kernel themselves. > > Distro kernel is too bloat to be really usable for CI. At this point I'm suspicious you're trolling. A distro kernel CI certainly uses the kernel they intend to ship and support in their environment. You're concerned about a bloated kernel, but the proposal here adds 2-bytes per device to track reset methods and a trivial array in text memory, meanwhile you're proposing multiple per-device memory allocations to enhance the feature you think is too bloated for CI. > > > The root permissions doesn't protect from anything, SO lovers will use > > > root without even thinking twice. > > > > Yes, with great power comes great responsibility. Many admins ignore > > this. That's far beyond the scope of this series. > > <...> > > > > I'm trying to help you with your use case of providing reset policy > > > mechanism, which can be without CONFIG_EXPERT. However if you want > > > to continue path of having specific reset type only, please ensure > > > that this is not taken to the "bypass kernel" direction. > > > > You've lost me, are you saying you'd be in favor of an interface that > > allows an admin to specify an arbitrary list of reset methods because > > that's somehow more in line with a policy choice than a userspace > > workaround? This seems like unnecessary bloat because (a) it allows > > the same bypass mechanism, and (b) a given device is only going to use > > a single method anyway, so the functionality is unnecessary. Please > > help me understand how this favors the policy use case. Thanks, > > The policy decision is global logic that is easier to grasp. At some > point of our discussion, you presented the case where PM reset is not > defined well and you prefer to do bus reset (something like that). > > I expect that QEMU sets same reset policy for all devices at the same > time instead of trying per-device to guess which one works. > > And yes, you will be able to bypass kernel, but at least this interface > will be broader than initial one that serves only SO and workarounds. I still think allocating objects for a list and managing that list is too bloated and complicated, but I agree that being able to have more fine grained control could be useful. Is it necessary to be able to re-order reset methods or might it still be better aligned to a policy use case if we allow plus and minus operators? For example, a device might list: [pm] [bus] Indicating that PM and bus reset are both available and enabled. The user could do: echo -pm > reset_methods This would result in: pm [bus] Indicating that both PM and bus resets are available, but only bus reset is enabled (note this is the identical result to "echo bus >" in the current proposal). "echo +pm" or "echo default" could re-enable the PM reset. Would something like that be satisfactory? If we need to allow re-ording, we'd want to use a byte-array where each byte indicates a type of reset and perhaps a non-zero value in the array indicates the method is enabled and the value indicates priority. For example writing "dev_spec,flr,bus" would parse to write 1 to the byte associated with the device specific reset, 2 to flr, 3 to bus reset, then we'd process low to high (or maybe starting at a high value to count down to zero might be more simple). We could do that with only adding less than a fixed 8-bytes per device and no dynamic allocation. Thoughts? Thanks, Alex
