On 10/7/21 3:03 PM, Rob Herring wrote: > On Thu, Oct 7, 2021 at 10:41 AM Zev Weiss <zev@xxxxxxxxxxxxxxxxx> wrote: >> >> On Thu, Oct 07, 2021 at 03:31:39AM PDT, Greg Kroah-Hartman wrote: >>> On Thu, Oct 07, 2021 at 02:05:41AM -0700, Zev Weiss wrote: >>>> On Thu, Oct 07, 2021 at 12:04:41AM PDT, Andy Shevchenko wrote: >>>>> On Thu, Oct 7, 2021 at 3:10 AM Zev Weiss <zev@xxxxxxxxxxxxxxxxx> wrote: >>>>>> This patch series is in some ways kind of a v2 for the "Dynamic >>>>>> aspeed-smc flash chips via 'reserved' DT status" series I posted >>>>>> previously [0], but takes a fairly different approach suggested by Rob >>>>>> Herring [1] and doesn't actually touch the aspeed-smc driver or >>>>>> anything in the MTD subsystem, so I haven't marked it as such. >>>>>> >>>>>> To recap a bit of the context from that series, in OpenBMC there's a >>>>>> need for certain devices (described by device-tree nodes) to be able >>>>>> to be attached and detached at runtime (for example the SPI flash for >>>>>> the host's firmware, which is shared between the BMC and the host but >>>>>> can only be accessed by one or the other at a time). >>>>> >>>>> This seems quite dangerous. Why do you need that? >>>> >>>> Sometimes the host needs access to the flash (it's the host's firmware, >>>> after all), sometimes the BMC needs access to it (e.g. to perform an >>>> out-of-band update to the host's firmware). To achieve the latter, the >>>> flash needs to be attached to the BMC, but that requires some careful >>>> coordination with the host to arbitrate which one actually has access to it >>>> (that coordination is handled by userspace, which then tells the kernel >>>> explicitly when the flash should be attached and detached). >>>> >>>> What seems dangerous? >>>> >>>>> Why can't device tree overlays be used? >>>> >>>> I'm hoping to stay closer to mainline. The OpenBMC kernel has a documented >>>> policy strongly encouraging upstream-first development: >>>> https://github.com/openbmc/docs/blob/master/kernel-development.md >>>> >>>> I doubt Joel (the OpenBMC kernel maintainer) would be eager to start >>>> carrying the DT overlay patches; I'd likewise strongly prefer to avoid >>>> carrying them myself as additional downstream patches. Hence the attempt at >>>> getting a solution to the problem upstream. >>> >>> Then why not work to get device tree overlays to be merged properly? > > TBC, it's 'just' the general purpose userspace interface to apply > overlays that's missing. A fuller view of what is missing is at: https://elinux.org/Frank%27s_Evolving_Overlay_Thoughts#issues_and_what_needs_to_be_completed_--_Not_an_exhaustive_list > > I did suggest what's done here as overlays are kind of an overkill for > this usecase. Much easier to write to a sysfs file than write an > overlay, compile it with dtc, and provide it to the kernel all just to > enable a device. > > Perhaps this could also be supported in the driver model directly. > Given the "what about ACPI question", that is probably what should be > done unless the answer is we don't care. I think we'd just need a flag > to create devices, but not bind automatically. Or maybe abusing > driver_override can accomplish that. > >>> Don't work on a half-of-a-solution when the real solution is already >>> here. >>> >> >> I had been under the impression that the overlay patches had very dim >> prospects of ever being accepted and that this might be a more tractable >> alternative, but apparently was mistaken -- I'll look into what the >> outstanding issues were with that and perhaps take a stab at addressing >> them. > > What's dim is the patches allowing any modification to any part of the > DT. Any changes to a DT need to work (i.e. have some effect). For > example, randomly changing/adding/removing properties wouldn't have > any effect because they've probably already be read and used. Yes, that is a good description. > > What I've suggested before is some sort of registration of nodes > allowed to apply child nodes and properties to. That would serve the > add-on board usecases which have been the main driver of this to date. > That also got hung up on defining interface nodes to add-on boards. > Your scope is more limited and can be limited to that scope while > using the same configfs interface. > > Rob >
