On Thu, Oct 07, 2021 at 03:03:43PM -0500, 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. > > 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. The driver model already allows devices to be bound/unbound from drivers, but no, it does not allow new devices to be "created" from userspace as that is a very bus-specific thing to have happen. If this is "just" a platform device, perhaps add that logic to the platform bus code? thanks, greg k-h
