Hi James, Rob,


On 2020/5/30 0:11, James Morse wrote:

Hi guys,

On 26/05/2020 22:18, Rob Herring wrote:

quoted

On Fri, May 22, 2020 at 11:24:11AM +0800, chenzhou wrote:

quoted

On 2020/5/21 21:29, Rob Herring wrote:

quoted

On Thu, May 21, 2020 at 3:35 AM Chen Zhou [off-list ref] wrote:

quoted

Add documentation for DT property used by arm64 kdump:
linux,low-memory-range.
"linux,low-memory-range" is an another memory region used for crash
dump kernel devices.

diff --git a/Documentation/devicetree/bindings/chosen.txt b/Documentation/devicetree/bindings/chosen.txt
index 45e79172a646..bfe6fb6976e6 100644
--- a/Documentation/devicetree/bindings/chosen.txt
+++ b/Documentation/devicetree/bindings/chosen.txt
+linux,low-memory-range
+----------------------
+This property (arm64 only) holds a base address and size, describing a
+limited region below 4G. Similar to "linux,usable-memory-range", it is
+an another memory range which may be considered available for use by the
+kernel.

Why can't you just add a range to "linux,usable-memory-range"? It
shouldn't be hard to figure out which part is below 4G.

The comments from James:
Won't this break if your kdump kernel doesn't know what the extra parameters are?
Or if it expects two ranges, but only gets one? These DT properties should be treated as
ABI between kernel versions, we can't really change it like this.

I think the 'low' region is an optional-extra, that is never mapped by the first kernel. I
think the simplest thing to do is to add an 'linux,low-memory-range' that we
memblock_add() after memblock_cap_memory_range() has been called.
If its missing, or the new kernel doesn't know what its for, everything keeps working.

I don't think there's a compatibility issue here though. The current 
kernel doesn't care if the property is longer than 1 base+size. It only 
checks if the size is less than 1 base+size.

Aha! I missed that.

quoted

And yes, we can rely on 
that implementation detail. It's only an ABI if an existing user 
notices.

Now, if the low memory is listed first, then an older kdump kernel 
would get a different memory range. If that's a problem, then define 
that low memory goes last. 

This first entry would need to be the 'crashkernel' range where the kdump kernel is
placed, otherwise an older kernel won't boot. The rest can be optional extras, as long as
we are tolerant of it being missing...

How about like this:

1. The low memory region remained as "Crash kernel (low)".
2. Userspace will find "Crash kernel" and "Crash kernel (low)" region in /proc/iomem,
and add "Crash kernel (low)" as the last range of property "linux,usable-memory-range".

Thanks,
Chen Zhou

I'll try and look at the rest of this series on Monday,


Thanks,

James

.

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