Re: [PATCH 0/9] Dynamic DT device nodes

[Zev Weiss]

On Thu, Oct 07, 2021 at 03:31:39AM PDT, Greg Kroah-Hartman wrote:
> On Thu, Oct 07, 2021 at 02:05:41AM -0700, Zev Weiss wrote:
> > On Thu, Oct 07, 2021 at 12:04:41AM PDT, Andy Shevchenko wrote:
> > > On Thu, Oct 7, 2021 at 3:10 AM Zev Weiss <zev@xxxxxxxxxxxxxxxxx> wrote:
> > > > This patch series is in some ways kind of a v2 for the "Dynamic
> > > > aspeed-smc flash chips via 'reserved' DT status" series I posted
> > > > previously [0], but takes a fairly different approach suggested by Rob
> > > > Herring [1] and doesn't actually touch the aspeed-smc driver or
> > > > anything in the MTD subsystem, so I haven't marked it as such.
> > > >
> > > > To recap a bit of the context from that series, in OpenBMC there's a
> > > > need for certain devices (described by device-tree nodes) to be able
> > > > to be attached and detached at runtime (for example the SPI flash for
> > > > the host's firmware, which is shared between the BMC and the host but
> > > > can only be accessed by one or the other at a time).
> > >
> > > This seems quite dangerous. Why do you need that?
> >
> > Sometimes the host needs access to the flash (it's the host's firmware,
> > after all), sometimes the BMC needs access to it (e.g. to perform an
> > out-of-band update to the host's firmware).  To achieve the latter, the
> > flash needs to be attached to the BMC, but that requires some careful
> > coordination with the host to arbitrate which one actually has access to it
> > (that coordination is handled by userspace, which then tells the kernel
> > explicitly when the flash should be attached and detached).
> >
> > What seems dangerous?
> >
> > > Why can't device tree overlays be used?
> >
> > I'm hoping to stay closer to mainline.  The OpenBMC kernel has a documented
> > policy strongly encouraging upstream-first development:
> > https://github.com/openbmc/docs/blob/master/kernel-development.md
> >
> > I doubt Joel (the OpenBMC kernel maintainer) would be eager to start
> > carrying the DT overlay patches; I'd likewise strongly prefer to avoid
> > carrying them myself as additional downstream patches.  Hence the attempt at
> > getting a solution to the problem upstream.
>
> Then why not work to get device tree overlays to be merged properly?
> Don't work on a half-of-a-solution when the real solution is already
> here.

I had been under the impression that the overlay patches had very dim 
prospects of ever being accepted and that this might be a more tractable 
alternative, but apparently was mistaken -- I'll look into what the 
outstanding issues were with that and perhaps take a stab at addressing 
them.


Zev