On Mon, Jul 13, 2020 at 9:08 AM Doug Anderson dianders@chromium.org wrote:
Hi,
On Mon, Jul 13, 2020 at 7:11 AM Rob Herring robh+dt@kernel.org wrote:
On Fri, Jul 10, 2020 at 5:02 PM Douglas Anderson dianders@chromium.org wrote:
I found that if I ever had a little mistake in my kernel config, or device tree, or graphics driver that my system would sit in a loop at bootup trying again and again and again. An example log was:
Why do we care about optimizing the error case?
It actually results in a _fully_ infinite loop. That is: if anything small causes a component of DRM to fail to probe then the whole system doesn't boot because it just loops trying to probe over and over again. The messages I put in the commit message are printed over and over and over again.
Sounds like a bug as that's not what should happen.
If you defer during boot (initcalls), then you'll be on the deferred list until late_initcall and everything is retried. After late_initcall, only devices getting added should trigger probing. But maybe the adding and then removing a device is causing a re-trigger.
msm ae00000.mdss: bound ae01000.mdp (ops 0xffffffe596e951f8) msm_dsi ae94000.dsi: ae94000.dsi supply gdsc not found, using dummy regulator msm_dsi_manager_register: failed to register mipi dsi host for DSI 0 [drm:ti_sn_bridge_probe] *ERROR* could not find any panel node ...
I finally tracked it down where this was happening:
- msm_pdev_probe() is called.
- msm_pdev_probe() registers drivers. Registering drivers kicks off processing of probe deferrals.
- component_master_add_with_match() could return -EPROBE_DEFER. making msm_pdev_probe() return -EPROBE_DEFER.
- When msm_pdev_probe() returned the processing of probe deferrals happens.
- Loop back to the start.
It looks like we can fix this by marking "mdss" as a "simple-bus". I have no idea if people consider this the right thing to do or a hack. Hopefully it's the right thing to do. :-)
It's a simple test. Do the child devices have any dependency on the parent to probe and/or function? If so, not a simple-bus.
Great! You can see in the earlier patch in the series that the very first thing that happens when the parent device probes is that it calls devm_of_platform_populate(). That means no dependencies, right?
It should. But then I reviewed the MDSS binding today and it looks like the MDSS is the interrupt parent for at least some child devices?
So that means it's fine/correct to add "simple-bus" here?
Once I do this I notice that my boot gets marginally faster (you don't need to probe the sub devices over and over) and also if I
Can you quantify that?
I'd say < 100 us. I can try to quantify more if needed, but it wasn't the point of this patch.
Have you run with devlinks enabled. You need a command line option to enable. That too should reduce deferred probes.
Ah, good idea! I will try it. However, even with devlinks, if there is any chance of deferred probes then we need a fix like this. The point of the patch isn't about speeding things up but about avoiding an infinite loop at bootup due to a small bug.
I think a deferred probe would only happen if there's a dependency we don't track (but we're tracking about everything that's common). But if there's some error, I'm not sure what would happen. Seems like a good test case. :)
have a problem it doesn't loop forever (on my system it still gets upset about some stuck clocks in that case, but at least I can boot up).
Deferred probe only runs when a device is added, so it's not like it is continually running.
If you don't mind looking at the code patch, see:
https://lore.kernel.org/r/20200710160131.4.I358ea82de218ea5f4406572ade23f5e1...
Specifically you can see that each time we try to probe we were calling of_platform_populate(). That appeared to be enough to trigger things.
Like I said, sounds like a bug. Even if 'simple-bus' is the appropriate thing to do here, it should be fixed or at least understood.
Rob