Re: [PATCH 0/5] driver core, of: support for reserved devices

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Oct 22, 2021 at 02:00:57AM -0700, Zev Weiss wrote:
> On Thu, Oct 21, 2021 at 11:50:07PM PDT, Greg Kroah-Hartman wrote:
> > On Thu, Oct 21, 2021 at 07:00:27PM -0700, Zev Weiss wrote:
> > > Hello all,
> > > 
> > > This series is another incarnation of a couple other patchsets I've
> > > posted recently [0, 1], but again different enough in overall
> > > structure that I'm not sure it's exactly a v2 (or v3).
> > > 
> > > As compared to [1], it abandons the writable binary sysfs files and at
> > > Frank's suggestion returns to an approach more akin to [0], though
> > > without any driver-specific (aspeed-smc) changes, which I figure might
> > > as well be done later in a separate series once appropriate
> > > infrastructure is in place.
> > > 
> > > The basic idea is to implement support for a status property value
> > > that's documented in the DT spec [2], but thus far not used at all in
> > > the kernel (or anywhere else I'm aware of): "reserved".  According to
> > > the spec (section 2.3.4, Table 2.4), this status:
> > > 
> > >   Indicates that the device is operational, but should not be used.
> > >   Typically this is used for devices that are controlled by another
> > >   software component, such as platform firmware.
> > > 
> > > With these changes, devices marked as reserved are (at least in some
> > > cases, more on this later) instantiated, but will not have drivers
> > > bound to them unless and until userspace explicitly requests it by
> > > writing the device's name to the driver's sysfs 'bind' file.  This
> > > enables appropriate handling of hardware arrangements that can arise
> > > in contexts like OpenBMC, where a device may be shared with another
> > > external controller not under the kernel's control (for example, the
> > > flash chip storing the host CPU's firmware, shared by the BMC and the
> > > host CPU and exclusively under the control of the latter by default).
> > > Such a device can be marked as reserved so that the kernel refrains
> > > from touching it until appropriate preparatory steps have been taken
> > > (e.g. BMC userspace coordinating with the host CPU to arbitrate which
> > > processor has control of the firmware flash).
> > > 
> > > Patches 1-3 provide some basic plumbing for checking the "reserved"
> > > status of a device, patch 4 is the main driver-core change, and patch
> > > 5 tweaks the OF platform code to not skip reserved devices so that
> > > they can actually be instantiated.
> > 
> > Again, the driver core should not care about this, that is up to the bus
> > that wants to read these "reserved" values and do something with them or
> > not (remember the bus is the thing that does the binding, not the driver
> > core).
> > 
> > But are you sure you are using the "reserved" field properly?
> 
> Well, thus far both Rob Herring and Oliver O'Halloran (originator of the
> "reserved" status in the DT spec, whom I probably should have CCed earlier,
> sorry) have seemed receptive to this interpretation of it, which I'd hope
> would lend it some credence.

Ok, that's up to the DT people, I'll let you all fight it out with the
platform creators :)

Good luck!

greg k-h