Re: [PATCH v8 5/5] dt-bindings: chosen: Document linux,low-memory-range for arm64 kdump

[chenzhou <chenzhou10@huawei.com>]

Hi James, Rob,


On 2020/5/30 0:11, James Morse wrote:
> Hi guys,
>
> On 26/05/2020 22:18, Rob Herring wrote:
>> On Fri, May 22, 2020 at 11:24:11AM +0800, chenzhou wrote:
>>> On 2020/5/21 21:29, Rob Herring wrote:
>>>> On Thu, May 21, 2020 at 3:35 AM Chen Zhou <chenzhou10@huawei.com> wrote:
>>>>> 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.
>
>
>> 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
>
> .
>