[RFC/PATCH] of: Mark property::value as const

On Thu, Feb 23, 2017 at 5:08 PM, Frank Rowand [off-list ref] wrote:

On 02/13/17 18:50, Stephen Boyd wrote:

quoted

The 'blob' we pass into populate_properties() is marked as const,
but we cast that const away when we assign the result of
fdt_getprop_by_offset() to pp->value. Let's mark value as const
instead, so that code can't mistakenly write to the value of the
property that we've so far advertised as const.

Instead of struct property field value being a pointer into the
FDT, I would rather copy the data to newly allocated memory and
have value be a pointer to that memory.  This is required if we
want to make /sys/firmware/fdt optional, which would allow us to
free the memory containing the initial boot FDT.

Making a copy would simplify several things.

I also do not want overlay live subtrees to have any pointers
into the FDT that was used to populate the overlay, so copying
the data solves that problem also.

quoted

Unfortunately, this exposes a problem with the fdt resolver code,
where we overwrite the value member of properties of phandles to
update them with their final value. Add a comment for now to
indicate where we're potentially writing over const data.

Yes, the resolver code needs to adjust phandle values.

I think I can get rid of the resolver modifying the various phandle
values, and instead just modify the phandle value in struct
device_node.  At the same time, I think I can also remove all
instances of the phandle properties ('linux,phandle', 'ibm,phandle',
'phandle') in the live tree.  These properties should not be
accessed directly by any code outside of the device tree framework
since the phandle is located in the struct device_node.  A quick
grep does not show any such accesses of the phandle properties,
but I want to look more closely.

Good idea.

BTW, I recently noticed that dtc by default generates both
linux,phandle and phandle properties. I would think the new one has
been around long enough that we can turn off the old one by default
now.

Rob