Re: [PATCH v24 9/9] Documentation: dt: chosen properties for arm64 kdump

Rob,

On Wed, Aug 31, 2016 at 02:02:16PM +0900, AKASHI Takahiro wrote:

On Wed, Aug 31, 2016 at 08:45:46AM +0900, AKASHI Takahiro wrote:

quoted

Rob,

On Tue, Aug 30, 2016 at 11:34:10AM -0500, Rob Herring wrote:

quoted

On Sun, Aug 21, 2016 at 11:28 PM, AKASHI Takahiro
[off-list ref] wrote:

quoted

Rob,
[Cc: Mark]

On Fri, Aug 19, 2016 at 08:26:41AM -0500, Rob Herring wrote:

quoted

On Tue, Aug 09, 2016 at 10:57:47AM +0900, AKASHI Takahiro wrote:

quoted

From: James Morse <redacted>

Add documentation for
    linux,crashkernel-base and crashkernel-size,
    linux,usable-memory-range, and
    linux,elfcorehdr
used by arm64 kexec/kdump to decribe the kdump reserved area, and
the elfcorehdr's location within it.

Signed-off-by: James Morse <redacted>
[takahiro.akashi-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org:
    renamed "usable-memory" to "usable-memory-range",
    added "linux,crashkernel-base" and "-size" ]
Signed-off-by: AKASHI Takahiro <redacted>
---
 Documentation/devicetree/bindings/chosen.txt | 50 ++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/Documentation/devicetree/bindings/chosen.txt b/Documentation/devicetree/bindings/chosen.txt
index 6ae9d82..236188a 100644
--- a/Documentation/devicetree/bindings/chosen.txt
+++ b/Documentation/devicetree/bindings/chosen.txt

@@ -52,3 +52,53 @@ This property is set (currently only on PowerPC, and only needed on
 book3e) by some versions of kexec-tools to tell the new kernel that it
 is being booted by kexec, as the booting environment may differ (e.g.
 a different secondary CPU release mechanism)
+
+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.
+e.g.
+
+/ {
+   chosen {
+           linux,crashkernel-base = <0x9 0xf0000000>;
+           linux,crashkernel-size = <0x0 0x10000000>;
+   };
+};
+
+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.
+e.g.
+
+/ {
+   chosen {
+           linux,usable-memory-range = <0x9 0xf0000000 0x0 0x10000000>;
+   };
+};

I've read the discussion on this. I think you should use the existing
linux,usable-memory property in the memory nodes. If UEFI systems don't
have memory nodes, then you can find an UEFI way to describe this. DT is
not the dumping ground for what doesn't fit in UEFI. How do x86 systems
work?

Kdump for x86 passes the range of usable memory to crash dump kernel
either via:
(a) "memmap=nn@ss" command line parameter, or
(b) faked BIOS e820 map (a sort of memory map table)

(a) was rejected by Mark [1], and (b) is x86-specific.

I believe that my approach is better because it works in the same way
either on UEFI systems or DT-based systems.

So we have a new way for both UEFI and DT. If UEFI folks can't come up
with a standard way to do things in the UEFI standard, then we just
dump them into DT?

No, I don't think so.
According to "Documentation/devicetree/bindings/chosen.txt,
  "The chosen node does not represent a real device, but serves as a place
   for passing data between firmware and the operating system, like boot
   arguments."

Since kexec/dump is some sort of boot loader, it all makes sense to
add a new property as an interface from the old kernel(kexec/dump)
to the new kernel. So my approach doesn't break any DT rules.

Please note that, even on UEFI/APCI systems, there are already a few
properties used under /chosen, like "linux,uefi-system-table" and 
"linux,mmap-*."
(Those properties are currently *not* defined in this document, though.)

quoted

I'd rather see alignment with existing DT systems
(PPC) and in particular the tools.

I'm not sure what you mean here, but I guess:
1) On DT-only systems, we should follow the way that PPC does.
   (That is, adding "usable-memory" property to each "memory" node.)
2) On UEFI/ACPI system, we must invent a new own way for our purpose
   because, as I said before, there is no standard, but x86-specific way. 

Is this what you want to do?

Yes?

quoted

If so,
do you agree to my approach of adding "linux,usable-memory-range" property
to /chosen *for UEFI/ACPI use*?

I've got another idea here: using "/memreserve/" field.

We'd better to use "reserved-memory" node:
  reserved-memory {
    #address-cells = <2>;
    #size-cells = <2>;
    ranges;

    crash_dump@80000000 {
      reg = <0x0 0x80000000 0x0 0x8000000>;
      no-map;
    }
    ...
  }
so that we can clearly state that those regions are for kdump.
Unfortunately, we can't place "reserved-memory" under /chosen.

All the memory regions used by the crashed kernel would be added
as "/memreserve/" and be memblock_reserve'd later in crash dump kernel.
Consequently, they will be excluded from the usable memory.

Obviously,
- this approach will work both on UEFI/ACPI and DT-only systems, and
- patch#2 ("memblock: add memblock_cap_memory_range()"), as well as
  "linux,usable-memory-range" property, is no longer necessary.
  (So nothing new, except for "linux.elfcorehdr," will be invented.)

But on the other hand,
- arm64 and ppc do the different ways even though both are DT systems,
- those reserved regions are still *in* linear mapping and appear as
  "System RAM" in /proc/iomem, and
- this may imply that we cannot do kexec on crash dump kernel.

If you never accept my approach of adding "linux,usable-memory-range,"
I will send a new version, v26, based on "reserved-memory" next week.

(The only change is to remove patch#2/#3 from v25, though.)

-Takahiro AKASHI

-Takahiro AKASHI

quoted

Thanks,
-Takahiro AKASHI

quoted

Rob

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