On Sun, Jul 8, 2018 at 4:25 PM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote: > > 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? > The drivers use device_link_add() to build "supplier<-consumer" order without knowing each other. Hence there is the following potential odds: (consumerX, child_a, ...) (consumer_a,..) (supplierX), where consumer_a consumes child_a. When device_link_add()->device_reorder_to_tail() moves all descendant of consumerX to the tail, it breaks the "supplier<-consumer" order by "consumer_a <- child_a". And we need recrusion to resolve the item in (consumer_a,..), each time when moving a consumer behind its supplier, we may break "parent<-child". > > 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. > If we consider device_move()-> device_pm_move_after/_before() more carefully like the above description, then we can hide the detail from caller. And keep the info of the pm order inside the core. > > It is hard to extract high dimension info and pack them into one dimension > > linked-list. > > Well, yes and no. > For "hard", I means that we need two interleaved recursion to make the order correct. Otherwise, I think it is a bug or limitation. > 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. > Yeah, I agree that iterating over device tree may hurt performance. I guess the iterating will not cost the majority of the suspend time, comparing to the device_suspend(), which causes hardware's sync. But data is more persuasive. Besides the performance, do you have other concern till now? > Now, if we have it for suspend and resume, it can also be used for shutdown. > Yes, I do think so. Thanks and regards, Pingfan
