On Sat, Jul 7, 2018 at 6:24 AM, Pingfan Liu <kernelfans@xxxxxxxxx> wrote: > On Fri, Jul 6, 2018 at 9:55 PM Pingfan Liu <kernelfans@xxxxxxxxx> wrote: >> >> On Fri, Jul 6, 2018 at 4:47 PM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote: >> > >> > On Fri, Jul 6, 2018 at 10:36 AM, Lukas Wunner <lukas@xxxxxxxxx> wrote: >> > > [cc += Kishon Vijay Abraham] >> > > >> > > On Thu, Jul 05, 2018 at 11:18:28AM +0200, Rafael J. Wysocki wrote: >> > >> OK, so calling devices_kset_move_last() from really_probe() clearly is >> > >> a mistake. >> > >> >> > >> I'm not really sure what the intention of it was as the changelog of >> > >> commit 52cdbdd49853d doesn't really explain that (why would it be >> > >> insufficient without that change?) >> > > >> > > It seems 52cdbdd49853d fixed an issue with boards which have an MMC >> > > whose reset pin needs to be driven high on shutdown, lest the MMC >> > > won't be found on the next boot. >> > > >> > > The boards' devicetrees use a kludge wherein the reset pin is modelled >> > > as a regulator. The regulator is enabled when the MMC probes and >> > > disabled on driver unbind and shutdown. As a result, the pin is driven >> > > low on shutdown and the MMC is not found on the next boot. >> > > >> > > To fix this, another kludge was invented wherein the GPIO expander >> > > driving the reset pin unconditionally drives all its pins high on >> > > shutdown, see pcf857x_shutdown() in drivers/gpio/gpio-pcf857x.c >> > > (commit adc284755055, "gpio: pcf857x: restore the initial line state >> > > of all pcf lines"). >> > > >> > > For this kludge to work, the GPIO expander's ->shutdown hook needs to >> > > be executed after the MMC expander's ->shutdown hook. >> > > >> > > Commit 52cdbdd49853d achieved that by reordering devices_kset according >> > > to the probe order. Apparently the MMC probes after the GPIO expander, >> > > possibly because it returns -EPROBE_DEFER if the vmmc regulator isn't >> > > available yet, see mmc_regulator_get_supply(). >> > > >> > > Note, I'm just piecing the information together from git history, >> > > I'm not responsible for these kludges. (I'm innocent!) >> > >> > Sure enough. :-) >> > >> > In any case, calling devices_kset_move_last() in really_probe() is >> > plain broken and if its only purpose was to address a single, arguably >> > kludgy, use case, let's just get rid of it in the first place IMO. >> > >> Yes, if it is only used for a single use case. >> > Think it again, I saw other potential issue with the current code. > device_link_add->device_reorder_to_tail() can break the > "supplier<-consumer" order. During moving children after parent's > supplier, it ignores the order of child's consumer. What do you mean? > Beside this, essentially both devices_kset_move_after/_before() and > device_pm_move_after/_before() expose the shutdown order to the > indirect caller, and we can not expect that the caller can not handle > it correctly. It should be a job of drivers core. Arguably so, but that's how those functions were designed and the callers should be aware of the limitation. If they aren't, there is a bug in the caller. > It is hard to extract high dimension info and pack them into one dimension > linked-list. Well, yes and no. We know it for a fact that there is a linear ordering that will work. It is inefficient to figure it out every time during system suspend and resume, for one and that's why we have dpm_list. Now, if we have it for suspend and resume, it can also be used for shutdown. > And in theory, it is warranted that the shutdown seq is > correct by using device tree info. More important, it is cheap with > the data structure in hand. So I think it is time to resolve the issue > once for all. Not the way you want to do that, though. Thanks, Rafael
