Re: [PATCH 6/6] doc/devicetree: NVDIMM region documentation

[Rob Herring <robh@kernel.org>]

On Tue, Mar 27, 2018 at 9:53 AM, Oliver <oohall@gmail.com> wrote:
> On Tue, Mar 27, 2018 at 9:24 AM, Rob Herring <robh@kernel.org> wrote:
>> On Fri, Mar 23, 2018 at 07:12:09PM +1100, Oliver O'Halloran wrote:
>>> Add device-tree binding documentation for the nvdimm region driver.
>>>
>>> Cc: devicetree@vger.kernel.org
>>> Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
>>> ---
>>>  .../devicetree/bindings/nvdimm/nvdimm-region.txt   | 45 ++++++++++++++++++++++
>>>  1 file changed, 45 insertions(+)
>>>  create mode 100644 Documentation/devicetree/bindings/nvdimm/nvdimm-region.txt
>>>
>>> diff --git a/Documentation/devicetree/bindings/nvdimm/nvdimm-region.txt b/Documentation/devicetree/bindings/nvdimm/nvdimm-region.txt
>>> new file mode 100644
>>> index 000000000000..02091117ff16
>>> --- /dev/null
>>> +++ b/Documentation/devicetree/bindings/nvdimm/nvdimm-region.txt
>>> @@ -0,0 +1,45 @@
>>> +Device-tree bindings for NVDIMM memory regions
>>> +-----------------------------------------------------
>>> +
>>> +Non-volatile DIMMs are memory modules used to provide (cacheable) main memory
>>
>> Are DIMMs always going to be the only form factor for NV memory?
>>
>> And if you have multiple DIMMs, does each DT node correspond to a DIMM?
>
> A nvdimm-region might correspond to a single NVDIMM, a set of
> interleaved NVDIMMs, or it might just be a chunk of normal memory that
> you want treated as a NVDIMM for some reason. The last case is useful
> for provisioning install media on servers since it allows you do
> download a DVD image, turn it into an nvdimm-region, and kexec into
> the installer which can use it as a root disk. That may seem a little
> esoteric, but it's handy and we're using a full linux environment for
> our boot loader so it's easy to make use of.

I'm really just asking if we should drop the "dimm" name because it is
not always a DIMM. Maybe pmem instead? I don't know, naming is
hard(TM).

>> If not, then what if we want/need to provide power control to a DIMM?
>
> That would require a DIMM (and probably memory controller) specific
> driver. I've deliberately left out how regions are mapped back to
> DIMMs from the binding since it's not really clear to me how that
> should work. A phandle array pointing to each DIMM device (which could
> be anything) would do the trick, but I've found that a bit awkward to
> plumb into the model that libnvdimm expects.
>
>>> +that retains its contents across power cycles. In more practical terms, they
>>> +are kind of storage device where the contents can be accessed by the CPU
>>> +directly, rather than indirectly via a storage controller or similar. The an
>>> +nvdimm-region specifies a physical address range that is hosted on an NVDIMM
>>> +device.
>>> +
>>> +Bindings for the region nodes:
>>> +-----------------------------
>>> +
>>> +Required properties:
>>> +     - compatible = "nvdimm-region"
>>> +
>>> +     - reg = <base, size>;
>>> +             The system physical address range of this nvdimm region.
>>> +
>>> +Optional properties:
>>> +     - Any relevant NUMA assocativity properties for the target platform.
>>> +     - A "volatile" property indicating that this region is actually in
>>> +       normal DRAM and does not require cache flushes after each write.
>>> +
>>> +A complete example:
>>> +--------------------
>>> +
>>> +/ {
>>> +     #size-cells = <2>;
>>> +     #address-cells = <2>;
>>> +
>>> +     platform {
>>
>> Perhaps we need a more well defined node here. Like we have 'memory' for
>> memory nodes.
>
> I think treating it as a platform device is fine. Memory nodes are

Platform device is a Linux term...

> special since the OS needs to know where it can allocate early in boot
> and I don't see non-volatile memory as being similarly significant.
> Fundamentally an NVDIMM is just a memory mapped storage device so we
> should be able to defer looking at them until later in boot.

It's not clear if 'platform' is just an example or random name or what
the node is required to be called. In the latter case, we should be
much more specific because 'platform' could be anything. In the former
case, then we have no way to find or validate the node because the
name could be anything and there's no compatible property either.

"region" is pretty generic too.

>
> That said you might have problems with XIP kernels and what not. I
> think that problem is better solved through other means though.
>
>>> +             region@5000 {
>>> +                     compatible = "nvdimm-region;
>>> +                     reg = <0x00000001 0x00000000 0x00000000 0x40000000>
>>> +
>>> +             };
>>> +
>>> +             region@6000 {
>>> +                     compatible = "nvdimm-region";
>>> +                     reg = <0x00000001 0x00000000 0x00000000 0x40000000>

Thinking about this some more, the 2 levels of nodes is pointless.
Just follow memory nodes structure.

nv-memory@100000000 {
  compatible = "nvdimm-region";
  reg = <0x00000001 0x00000000 0x00000000 0x40000000>;
};

nv-memory@200000000 {
  compatible = "nvdimm-region";
  reg = <0x00000002 0x00000000 0x00000000 0x40000000>;
};

or:

nv-memory@100000000 {
  compatible = "nvdimm-region";
  reg = <0x00000001 0x00000000 0x00000000 0x40000000>
    <0x00000002 0x00000000 0x00000000 0x40000000>;
};

Both forms should be allowed.

Rob
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