Re: [PATCH] Open Firmware device tree virtual filesystem

From: Jan Engelhardt
Date: Sun Dec 31 2006 - 09:23:04 EST

On Dec 30 2006 15:38, Mitch Bradley wrote:
> Request for comments.
> It is similar in some respect to fs/proc/proc_devtree.c , but does
> not use procfs, nor does it require an intermediate layer of code
> to create a flattened representation of the device tree.

NB: openpromfs does not use procfs either. It just happens to be
mounted somewhere down there.

> +++ b/Documentation/filesystems/ofwfs.txt
> +== Property Value Encoding ==
> +
> +A regular file in ofwfs contains the exact byte sequence that
> +comprises the OFW property value. Properties are not reformatted
> +into text form, so numeric property values appear as binary
> +integers. While this is inconvenient for viewing, it is generally
> +easier for programs that read property values, and it means that
> +Open Firmware documentation about property values applies
> +directly, without having to separately document an ASCII
> +transformation (which would have to separately specified for
> +different kinds of properties).

I appreciate the ASCII formatting that openpromfs currently does.
Perhaps, should OFWFS be used, it could offer both?

> +== Environment Assumptions ==
> +
> +The ofwfs code assumes that the Open Firmware client interface
> +callback can be executed during Linux kernel startup
> +(specifically, at "core_initcall" time). When ofwfs is
> +initialized, it copies out the property values, so that subsequent
> +accesses to the tree do not require callbacks into OFW.

openpromfs also has many parts read-only, only one object,
/options/security-password, is writable. Hence caching it once and
forever seems ok.

BUT, the eeprom utility may be used to modify values, and if used, I
would like to see ofwfs show the updated value. openpromfs does it

15:09 ares:/proc/openprom/options # cat oem-banner?
15:09 ares:/proc/openprom/options # eeprom 'oem-banner?=true'
15:09 ares:/proc/openprom/options # cat oem-banner?

(BTW, why does not openpromfs have it rw?)

> +== Recommended Mount Point ==
> +
> +The recommended mount point for ofwfs is /ofw. (TBD: Should it be
> +mounted somewhere under /sys instead?)

Somewhere in /sys/firmware perhaps.

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