Re: [PATCH v29 9/9] Documentation: dt: chosen properties for arm64 kdump

[Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org>]

On Fri, Jan 13, 2017 at 06:13:49PM +0900, AKASHI Takahiro wrote:
> On Thu, Jan 12, 2017 at 03:39:45PM +0000, Mark Rutland wrote:
> > On Wed, Dec 28, 2016 at 01:37:34PM +0900, AKASHI Takahiro wrote:
> > > +linux,crashkernel-base
> > > +linux,crashkernel-size
> > > +----------------------
> > > +
> > > +These properties (currently used on PowerPC and arm64) indicates
> > > +the base address and the size, respectively, of the reserved memory
> > > +range for crash dump kernel.
> > 
> > From this description, it's not clear to me what the (expected)
> > consumers of this property are, nor what is expected to provide it.
> > 
> > In previous rounds of review, I had assumed that this was used to
> > describe a preference to the first kernel as to what region of memory
> > should be used for a subsequent kdump kernel. Looking around, I'm not
> > sure if I was correct in that assessment.
> > 
> > I see that arch/powerpc seems to consume this property to configure
> > crashk_res, but it also rewrites it based on crashk_res, presumably for
> > the benefit of userspace. It's not clear to me how on powerpc the kdump
> > kernel knows its memory range -- is more DT modification done in the
> > kernel and/or userspace?
> 
> I don't believe that powerpc will rewrite the property any way.
> As far as I know from *the source code*, powerpc kernel retrieves
> the memory range for crash dump kernel from a kernel command line, i.e.
> crashkernel=, and then exposes it through DT to userspace (assuming
> kexec-tools).

The rewriting I describe is in export_crashk_values() in
arch/powerpc/kernel/machine_kexec.c, where the code deletes existing the
properties, and adds new ones, to the DT exposed to userspace.

So I think we're just quibbling over the definition of "rewrite".

> > arm64 we should either ensure that /proc/iomem is consistently usable
> > (and have userspace consistently use it), or we should expose a new file
> > specifically to expose this information.
> 
> The thing that I had in my mind when adding this property is that
> /proc/iomem would be obsolete in the future, then we should have
> an alternative in hand.

Ok.

My disagreement is with using the DT as a channel to convey information
from the kernel to userspace.

I'm more than happy for a new file or other mechanism to express this
information. For example, we could add
/sys/kernel/kexec_crash_{base,size} or similar.


> > Further, I do not think we need this property. It makes more sense to me
> > for the preference of a a region to be described to the *first* kernel
> > using the command line consistently.
> > 
> > So I think we should drop this property, and not use it on arm64. Please
> > document this as powerpc only.
> 
> OK, but if we drop the property from arm64 code, we have no reason
> to leave its description in this patch.
> (In fact, there are a few more (undocumented) properties that only ppc
> uses for kdump.)

I'm happy to drop it, then.

> > > +linux,usable-memory-range
> > > +-------------------------
> > > +
> > > +This property (currently used only on arm64) holds the memory range,
> > > +the base address and the size, which can be used as system ram on
> > > +the *current* kernel. Note that, if this property is present, any memory
> > > +regions under "memory" nodes in DT blob or ones marked as "conventional
> > > +memory" in EFI memory map should be ignored.
> > 
> > Could you please replace this with:
> > 
> >   This property (arm64 only) holds a base address and size, describing a
> >   limited region in which memory may be considered available for use by
> >   the kernel. Memory outside of this range is not available for use.
> >   
> >   This property describes a limitation: memory within this range is only
> >   valid when also described through another mechanism that the kernel
> >   would otherwise use to determine available memory (e.g. memory nodes
> >   or the EFI memory map). Valid memory may be sparse within the range.
> 
> Sure.

Cheers!

Thanks,
Mark.
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