Re: [PATCH for-7.2 v2 10/20] hw/ppc: set machine->fdt in spapr machine

[David Gibson]

On Fri, Aug 19, 2022 at 06:42:34AM -0300, Daniel Henrique Barboza wrote:
> 
> 
> On 8/18/22 23:11, Alexey Kardashevskiy wrote:
> > 
> > 
> > On 05/08/2022 19:39, Daniel Henrique Barboza wrote:
> > > The pSeries machine never bothered with the common machine->fdt
> > > attribute. We do all the FDT related work using spapr->fdt_blob.
> > > 
> > > We're going to introduce HMP commands to read and save the FDT, which
> > > will rely on setting machine->fdt properly to work across all machine
> > > archs/types.
> > 
> > 
> > Out of curiosity - why new HMP command, is not QOM'ing this ms::fdt 
> > property enough?
> 
> I tried to do the minimal changes needed for the commands to work. ms::fdt is
> one of the few MachineState fields that hasn't been QOMified by
> machine_class_init() yet. All pre-existing code that uses ms::fdt are using 
> the
> pointer directly. To make a QOMified use of it would require extra patches
> in machine.c to QOMify the property first.
> 
> There's also the issue with how each machine is creating the FDT. Most are 
> using
> helpers from device_tree.c, some are creating it from scratch, others required
> a .dtb file, most of them are not doing a fdt_pack() and so on. To really 
> QOMify
> the use of ms::fdt we would need some machine hooks that standardize all that.
> I believe it's worth the trouble, but it would be too much to do
> right now.

Hmm.. I think this depends on what you mean by "QOM"ify exactly.  If
you're meaning make the full DT representation QOM objects, that you
can look into in detail, then, yes, that's pretty complicated.

I suspect what Alexey was suggesting though, was merely to make
ms::fdt accessible as a single bytestring property on the machine QOM
object.  Effectively it's just "dumpdtb" but as a property get.

I'm not 100% certain if QOM can safely represent arbitrary bytestrings
as QOM properties, which would need checking.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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