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

[Rob Herring <robh+dt@kernel.org>]

On Thu, Oct 21, 2021 at 9:00 PM Zev Weiss <zev@bewilderbeest.net> 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.

I skimmed this, and overall I like the approach.

> 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.
>
> One shortcoming of this series is that it doesn't automatically apply
> universally across all busses and drivers -- patch 5 enables support
> for platform devices, but similar changes would be required for
> support in other busses (e.g. in of_register_spi_devices(),
> of_i2c_register_devices(), etc.) and drivers that instantiate DT
> devices.  Since at present a "reserved" status is treated as
> equivalent to "disabled" and this series preserves that status quo in
> those cases I'd hope this wouldn't be considered a deal-breaker, but
> a thing to be aware of at least.
>
> Greg: I know on [1] you had commented nack-ing the addition of boolean
> function parameters; patch 4 adds a flags mask instead in an analogous
> situation.  I'm not certain how much of an improvement you'd consider
> that (hopefully at least slightly better, in that the arguments passed
> at the call site are more self-explanatory); if that's still
> unsatisfactory I'd welcome any suggested alternatives.

Can't we add a flag bit in struct device to reflect manual binding?
bind will set it and unbind clears it.

Rob