On Thu, Oct 7, 2021 at 3:10 AM Zev Weiss [off-list ref] 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? Why can't device
tree overlays be used?

To provide that
ability, this series adds support for a new common device-tree
property, a boolean "dynamic" that indicates that the device may come
and go at runtime.  When present on a node, the sysfs file for that
node's "status" property is made writable, allowing userspace to do
things like:

  $ echo okay > /sys/firmware/devicetree/.../status
  $ echo reserved > /sys/firmware/devicetree/.../status

to activate and deactivate a dynamic device.

Because it leans on the OF_DYNAMIC machinery internally, this
functionality will only work on busses that register for OF reconfig
notifications and handle them appropriately (presently platform, i2c,
and spi).  This series does not attempt to solve the "dynamic devices
further down the tree" problem [2]; my hope is that handling for OF
reconfig notifications can be extended to other families of devices
(e.g. individual MTD spi-nor flash chips) in the future.

What about ACPI and software nodes?
How will all this affect the host?

[0] https://lore.kernel.org/openbmc/20210929115409.21254-1-zev at bewilderbeest.net/
[1] https://lore.kernel.org/openbmc/CAL_JsqJH+b5oFuSP+KBLBsN5QTA6xASuqXJWXUaDkHhugXPpnQ at mail.gmail.com/
[2] https://lore.kernel.org/openbmc/20210929220038.GS17315 at packtop/
[3] https://git.kernel.org/pub/scm/linux/kernel/git/robh/linux.git/commit/?id=6663ae07d995f5fbe2988a19858b2f87e68cf929

-- 
With Best Regards,
Andy Shevchenko