[PATCH v5.5] of/fdt: export fdt blob as /sys/firmware/fdt

[grant.likely@linaro.org (Grant Likely)]

On Wed, 19 Nov 2014 09:24:41 +0100
, Ard Biesheuvel <ard.biesheuvel@linaro.org>
 wrote:
> On 19 November 2014 00:11, Rob Herring <rob.herring@linaro.org> wrote:
> > On Tue, Nov 18, 2014 at 4:11 PM, Grant Likely <grant.likely@linaro.org> wrote:
> >> On Tue, 18 Nov 2014 17:25:45 +0000
> >> , Mark Rutland <mark.rutland@arm.com>
> >>  wrote:
> >>> On Tue, Nov 18, 2014 at 04:51:45PM +0000, Grant Likely wrote:
> >>> > On Fri, 14 Nov 2014 18:05:35 +0100
> >>> > , Ard Biesheuvel <ard.biesheuvel@linaro.org>
> >>> >  wrote:
> >>> > > Create a new /sys entry '/sys/firmware/fdt' to export the FDT blob
> >>> > > that was passed to the kernel by the bootloader. This allows userland
> >>> > > applications such as kexec to access the raw binary.
> >
> > [...]
> >
> >>> > * It also helps with exposing the reserved map to userspace, but kexec
> >>> >   has done without that feature for years, and it is in the process of
> >>> >   being deprecated in favour of /reserved-memory anyway.
> >>>
> >>> This is the first I'd heard of the reserve map being deprecated, and
> >>> we're going to have DTs with reserved map entries for a long time going
> >>> forwards.
> >>
> >> Deprecated, not removed or disabled. It will still work pretty much
> >> forever, but users should be encouraged to move to the reserve-memory
> >> tree.
> >
> > I thought you had said reserve map was still the right way for memory
> > the kernel should never touch.
> >
> >>> Can't we expose the header fields under something like
> >>> /sys/firmware/devicetree/dtb-header/, parallel to the usual
> >>> /sys/firmware/devicetree/base for nodes?
> >>
> >> We could do that too.
> >>
> >> Honestly though, I'm just unsure of what the best thing to do is. If you
> >> and a few others tell me that, "no, exporting the raw dtb is the right
> >> thing to do", then I'll be okay, merge the patch and sleep properly.
> >
> > I always sleep better when others can take the blame.
> >
> > What happens when we rev the dtb format? Is the ABI the blob or the
> > format of the blob?
> >
> > I lean towards we should add this. This is providing what is "in the
> > firmware" while /proc/devicetree provides the live tree state
> > including overlays.
> >
> 
> Well, my pov is that FDT != devicetree ever since we started (ab)using
> the FDT container format to pass just the UEFI entry points to the
> kernel.
> The boot protocol describes what should be passed in x0, and /that/ is
> what we expose in /sys/firmware/fdt, regardless of how the kernel
> decided to configure itself.
> (perhaps we need to fix the wording in the document to refer to FDT not dtb)
> 
> So that also means I don't care about memreserve vs reserved-memory or
> other DT specific details: as long as x0 points to something libfdt
> understands at boot, we expose it and not interpret it any further.

Alright, my doubt is quelled. Merged.

g.