On Fri, 10 May 2024 at 12:19, Tomi Valkeinen <tomi.valkeinen@xxxxxxxxxxxxxxxx> wrote: > > Hi, > > On 03/05/2024 16:45, Ulf Hansson wrote: > > + Abel, Saravanna, Stephen > > > > On Mon, 15 Apr 2024 at 19:17, Tomi Valkeinen > > <tomi.valkeinen@xxxxxxxxxxxxxxxx> wrote: > >> > >> On 15/04/2024 19:00, Tomi Valkeinen wrote: > >>> Add a new flag, TI_SCI_PD_KEEP_BOOT_STATE, which can be set in the dts > >>> when referring to power domains. When this flag is set, the ti-sci > >>> driver will check if the PD is currently enabled in the HW, and if so, > >>> set the GENPD_FLAG_ALWAYS_ON flag so that the PD will stay enabled. > >>> > >>> The main issue I'm trying to solve here is this: > >>> > >>> If the Display Subsystem (DSS) has been enabled by the bootloader, the > >>> related PD has also been enabled in the HW. When the tidss driver > >>> probes, the driver framework will automatically enable the PD. While > >>> executing the probe function it is very common for the probe to return > >>> EPROBE_DEFER, and, in rarer cases, an actual error. When this happens > >>> (probe() returns an error), the driver framework will automatically > >>> disable the related PD. > >>> > >>> Powering off the PD while the DSS is enabled and displaying a picture > >>> will cause the DSS HW to enter a bad state, from which (afaik) it can't > >>> be woken up except with full power-cycle. Trying to access the DSS in > >>> this state (e.g. when retrying the probe) will usually cause the board > >>> to hang sooner or later. > >>> > >>> Even if we wouldn't have this board-hangs issue, it's nice to be able to > >>> keep the DSS PD enabled: we want to keep the DSS enabled when the > >>> bootloader has enabled the screen. If, instead, we disable the PD at the > >>> first EPROBE_DEFER, the screen will (probably) go black. > >> > >> A few things occurred to me. The driver is supposed to clear the > >> GENPD_FLAG_ALWAYS_ON when the driver has probed successfully. There are > >> two possible issues with that: > >> > >> - Afaics, there's no API to do that, and currently I just clear the bit > >> in genpd->flags. There's a clear race there, so some locking would be > >> required. > >> > >> - This uses the GENPD_FLAG_ALWAYS_ON flag to say "PD is always on, until > >> the driver has started". If the PD would have GENPD_FLAG_ALWAYS_ON set > >> for other reasons, the driver would still go and clear the flag, which > >> might break things. > >> > >> Also, unrelated to the above and not a problem in practice at the very > >> moment, but I think clocks should also be dealt with somehow. Something, > >> at early-ish boot stage, should mark the relevant clocks as in use, so > >> that there's no chance they would be turned off when the main kernel has > >> started (the main display driver is often a module). > >> > >> It would be nice to deal with all the above in a single place. I wonder > >> if the tidss driver itself could somehow be split into two parts, an > >> early part that would probe with minimal dependencies, mainly to reserve > >> the core resources without doing any kind of DRM init. And a main part > >> which would (somehow) finish the initialization at a later point, when > >> we have the filesystem (for firmware) and the other bridge/panel drivers > >> have probed. > >> > >> That can be somewhat achieved with simplefb or simpledrm, though, but we > >> can't do any TI DSS specific things there, and it also creates a > >> requirement to have either of those drivers built-in, and the related DT > >> nodes to be added. > > > > Without going into too much detail, this and similar problems have > > been discussed in the past. With the fw_devlink and the ->sync_state() > > callback we are getting closer to a solution, but for genpd a solution > > is still pending. > > > > If you want to read up on earlier discussions and join us moving > > forward, that would be great. The last attempt for genpd to move this > > forward was posted by Abel Vesa: > > https://lore.kernel.org/linux-pm/20230621144019.3219858-1-abel.vesa@xxxxxxxxxx/ > > > > Beyond that, we have also discussed various solutions at the last LPC > > in Richmond. I think the consensus at that point was that Saravana > > targeted to post something for clocks - and when that was done, we > > should do the similar thing for genpd. Anyway, I have looped them into > > this thread, so they can share any updates on their side of the > > matter. > > If I understand the series correctly, it has an issue at least for this > case/platform. > > The devlinks are between the consumer devices and the PD provider > device. TI SCI PD provider has quite a lot of PDs, and all the consumers > would have to be probed before any of the PDs could be disabled. So, to > get the display PD disabled, I would have to load, e.g., the GPU driver > (which I don't even have). > > I believe this is the case for the clocks also. > > Perhaps that can be considered a feature, but I fear that in practice it > would mean that most of the time for most users all the boot-time > enabled powerdomains would be always on. > > Nevertheless, I believe the series would fix the issue mentioned in this > patch, so I'll see if I can get the series working on the TI platform to > get a bit more experience on this whole issue. [...] Just to share a brief update on this matter. I have had an offlist chat with Saravana and some Qcom guys about this topic. In particular we looked closer at some implementation details. Allow me a few weeks, then I will post a series for genpd to implement the above. I will keep you posted and would appreciate reviews and tests, of course. Kind regards Uffe
