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. 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. It is hard to extract high dimension info and pack them into one dimension linked-list. 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. Thanks and regards, Pingfan
