speed-bmc-misc driver

[Andrew Jeffery]

Hi Vijay,

On Thu, 10 Oct 2019, at 04:17, Vijay Khemka wrote:
>  
> Hi Andrew,
> 
> I saw this driver in LF aspeed Linux 

What do you mean by "LF aspeed Linux"? The only place this driver lives is
in the OpenBMC kernel tree (openbmc/linux on github).

> and was wondering how to use. Can 
> you please suggest some usage example like device tree entry as well as 
> sysfs interface.

Honestly, I wouldn't recommend using (yet). It can't be upstreamed in its
current form (I've tried), and so using it as is comes with userspace-breaking
changes in the future. I reserve the right to break your machines if you do
make use of it when I get the time to rework the patches.

Having said that, its purpose is to expose arbitrary fields in arbitrary registers
on the BMC to userspace via sysfs. This is useful when the field's value is
entirely determined by userspace policy and there's no need for additional
kernel infrastructure around the configuration.

Originally this was intended to expose to userspace the bits that control the
state of the ASPEED hardware backdoors, but we changed tack on the
solution to CVE-2019-6260 before the bmc-misc idea got very far.

However you can find some slightly abusive uses if you search the dtsis:

https://github.com/openbmc/linux/blob/dev-5.3/arch/arm/boot/dts/aspeed-g5.dtsi#L1682

In that instance we're exposing the SuperIO scratch registers to userspace
using this mechanism. The attributes can be found in sysfs associated with
the devicetree node. I did have a hack to add a sysfs class for them, but that
was even more controversial than the general concept of the "driver" so
you're going to have to cope with changes to the devicetree potentially
breaking userspace unless you're willing to rework the patches yourself.

Hope that helps.

Andrew