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

On Thu, Mar 29, 2018 at 4:06 AM, Rob Herring [off-list ref] wrote:

On Tue, Mar 27, 2018 at 9:53 AM, Oliver [off-list ref] wrote:

quoted

On Tue, Mar 27, 2018 at 9:24 AM, Rob Herring [off-list ref] wrote:

quoted

On Fri, Mar 23, 2018 at 07:12:09PM +1100, Oliver O'Halloran wrote:

quoted

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).

pmem is probably a better name. I'll fix that up.

quoted

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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.

quoted

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+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...

quoted

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.

Sorry, the platform node is just there as an example. I'll remove it.

"region" is pretty generic too.

It is, but I didn't see a compelling reason to call it something else.

quoted

That said you might have problems with XIP kernels and what not. I
think that problem is better solved through other means though.

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+             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.

In the example you need two separate nodes since one has the
"volatile" property to indicate it's backed by normal memory while the
other doesn't. That detail is important since the OS can skip doing
cache flushes when writing to a region that it knows is volatile.

Anyway, the usefulness of having multiple ranges in the reg is a bit
dubious since you should never see dis-contiguous ranges of memory
backed by the same devices. Keep in mind that this binding here is
deliberately skeletal and leaves out the parts required to map the
region to the backing devices, once that is added there's not going to
be a whole lot of room for coalescing nodes. That said, I'll add
support for it anyway since it might be nice to have for hand-written
DTs (ours are mostly generated by FW).

Thanks,
Oliver