Re: [PATCH 05/10] arm64: Convert dts to use reserved-memory nodes

[Mark Rutland <mark.rutland@arm.com>]

On Fri, Oct 31, 2014 at 11:44:52PM +0000, Geoff Levand wrote:
> Hi,
> 
> On Fri, 2014-10-24 at 13:27 +0100, Mark Rutland wrote:
> > On Fri, Oct 24, 2014 at 11:59:38AM +0100, Grant Likely wrote:
> > > On Fri, Oct 24, 2014 at 11:51 AM, Mark Rutland <mark.rutland@arm.com> wrote:
> > > > On Fri, Oct 24, 2014 at 12:10:58AM +0100, Geoff Levand wrote:
> > > >> Device tree regions described by /memreserve/ entries are not available in
> > > >> /proc/device-tree, and hence are not available to user space utilities that use
> > > >> /proc/device-tree.  In order for these regions to be available, convert any
> > > >> arm64 DTS files using /memreserve/ entries to use reserved-memory nodes.
> > > >
> > > > The limitation here is in the kernel (and a partially in userspace), so
> > > > modifying the dts files is a workaround rather than a fix.
> > > >
> > > > It's perfectly valid for people to remain using /memreserve/, so this
> > > > isn't sufficient. There are also existing DTBs using /memreserve/ which
> > > > we can't rely on being modified to use reserved-memory.
> > > >
> > > > I think we need to expose memreserves to userspace somehow, potentially
> > > > along with other DTB header fields. Grant, ideas?
> > > 
> > > Yes, I suggested the same thing to Geoff in a separate thread. Here's
> > > what I wrote:
> > > 
> > > >> Geoff Levand wrote:
> > > >>> I did some work on this and I think we just need to convert all the
> > > >>> arm64 dts from /memreserve/ to reserved-memory nodes and update the
> > > >>> arm64 booting.txt to specify using reserved-memory.  I'll prepare a
> > > >>> patch for it.
> > > >>
> > > >> I don't think that is going to be entirely sufficient. There will be
> > > >> platforms that don't get converted over, and this is a generic problem
> > > >> that covers all architectures using DT, not just aarch64. The solution
> > > >> probably needs to include exposing the /memreserve/ sections to
> > > >> userspace. I can see two ways to do this:
> > > >>
> > > >> 1) Create a new property in /sysfs with all the memreserve sctions
> > > >> 2) Live-modify the device tree to put the memreserve data into a node
> > > >> at boot time.
> > > >>
> > > >> Option 2 is probably the most generic solution, but it will require
> > > >> some care to make sure there aren't any overlaps if a reserved-memory
> > > >> node already exists.
> > 
> > I would prefer the former currently. While I currently believe that
> > modifying the tree is something we're going to have to do for stateful
> > properties, it's not someting I want to have to do unless absolutely
> > necessary.
> 
> Current user space kexec utilities use /proc/device-tree and nothing
> else.  The intension of the device tree is to describe the system
> sufficiently for a kernel to boot, so I think we should put the
> /memreserve/ info into /proc/device-tree.

I don't follow. The intention of device tree generically has precisely
nothing to do with the Linux-specific filesystem hierarchy for exposing
device tree to userspace.

The memreserve entries aren't the same as other elements in
/proc/device-tree. They're a flat table pointed to by the header rather
than a hierarchy of nodes and properties. There's no way to expose that
difference under /proc/device-tree, so exposing that as a parallel
hierarchy makes the most sense to me.

> We could put the /memreserve/ entries in there directly, or convert
> to reserved-memory nodes.  At the moment I like the idea to convert to
> reserved-memory nodes.

I think we need a parallel hierarchy to expose header information
(though we likely only need the memreserve entries). I'm not keen on
rewriting the device tree to work around a historical mistake.

> > > > For reference, here is an old conversation about this exact thing.
> > > > Reading through it, the opinions I expressed then don't necessarily
> > > > match what I think now. I still don't think it is a good idea to
> > > > expose the physical address of the old .dtb blob, but I do agree that
> > > > the memreserve sections need to be exposed.
> > > >
> > > > https://lists.ozlabs.org/pipermail/linuxppc-dev/2010-July/083993.html
> > 
> > Another option would be to expose the original DTB as a (read-only)
> > binary file somewhere. That might interact poorly with live tree
> > modification in future, however.
> 
> I don't like the idea of having two interfaces to get essentially the same
> info.  I think it will be a maintenance problem over time.

Given Grant's comments, I'm perfectly happy to not expose the raw DTB
for situations otehr than debugging.

Thanks,
Mark.

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