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Date:   Thu, 21 May 2020 12:41:16 -0700
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Suman Anna <s-anna@...com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Clement Leger <cleger@...ray.eu>,
        Loic Pallardy <loic.pallardy@...com>,
        Arnaud Pouliquen <arnaud.pouliquen@...com>,
        Lokesh Vutla <lokeshvutla@...com>,
        linux-remoteproc@...r.kernel.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] remoteproc: introduce version element into resource
 type field

On Thu 21 May 12:29 PDT 2020, Suman Anna wrote:

> On 5/21/20 2:21 PM, Bjorn Andersson wrote:
> > On Thu 21 May 12:06 PDT 2020, Suman Anna wrote:
> > 
> > > Hi Bjorn,
> > > 
> > > On 5/21/20 12:54 PM, Bjorn Andersson wrote:
> > > > On Wed 25 Mar 13:46 PDT 2020, Suman Anna wrote:
> > > > 
> > > > > The current remoteproc core has supported only 32-bit remote
> > > > > processors and as such some of the current resource structures
> > > > > may not scale well for 64-bit remote processors, and would
> > > > > require new versions of resource types. Each resource is currently
> > > > > identified by a 32-bit type field. Introduce the concept of version
> > > > > for these resource types by overloading this 32-bit type field
> > > > > into two 16-bit version and type fields with the existing resources
> > > > > behaving as version 0 thereby providing backward compatibility.
> > > > > 
> > > > > The version field is passed as an additional argument to each of
> > > > > the handler functions, and all the existing handlers are updated
> > > > > accordingly. Each specific handler will be updated on a need basis
> > > > > when a new version of the resource type is added.
> > > > > 
> > > > 
> > > > I really would prefer that we add additional types for the new
> > > > structures, neither side will be compatible with new versions without
> > > > enhancements to their respective implementations anyways.
> > > 
> > > OK.
> > > 
> > > > 
> > > > > An alternate way would be to introduce the new types as completely
> > > > > new resource types which would require additional customization of
> > > > > the resource handlers based on the 32-bit or 64-bit mode of a remote
> > > > > processor, and introduction of an additional mode flag to the rproc
> > > > > structure.
> > > > > 
> > > > 
> > > > What would this "mode" indicate? If it's version 0 or 1?
> > > 
> > > No, for indicating if the remoteproc is 32-bit or 64-bit and adjust the
> > > loading handlers if the resource types need to be segregated accordingly.
> > > 
> > 
> > Sorry, I think I'm misunderstanding something. Wouldn't your 64-bit
> > remote processor need different firmware from your 32-bit processor
> > anyways, if you want to support the wider resource? And you would pack
> > your firmware with the appropriate resource types?
> 
> Yes, that's correct.
> 
> > 
> > Afaict the bit width of your remote processor, busses or memory is
> > unrelated to the choice of number of bits used to express things in the
> > resource table.
> 
> I would have to add the new resource type to the loading_handlers right, so
> it is a question of whether we want to impose any restrictions in remoteproc
> core or not from supporting a certain resource type (eg: I don't expect
> RSC_TRACE entries on 64-bit processors).
> 

Right, but either you add support for new resource types to the
loading_handlers, or you add the version checks within each handler,
either way you will have to do some work to be compatible with new
versions.

Regarding what resources would be fit for a 64-bit processor probably
relates to many things, in particular the question of what we actually
mean when we say that a coprocessor is 64-bit. So I don't really see a
need for the remoteproc core to prevent someone to design their
system/firmware to have a 64-bit CPU being passed 32-bit addresses.

Regards,
Bjorn

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