On Mon, Jul 13, 2020 at 9:08 AM Doug Anderson <dianders@xxxxxxxxxxxx> wrote: > > Hi, > > On Mon, Jul 13, 2020 at 7:11 AM Rob Herring <robh+dt@xxxxxxxxxx> wrote: > > > > On Fri, Jul 10, 2020 at 5:02 PM Douglas Anderson <dianders@xxxxxxxxxxxx> 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.I358ea82de218ea5f4406572ade23f5e121297555@changeid/ > > 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
