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SubjectRe: [RFC] Correct memory layout reporting for "jedec,lpddr2" and related bindings
Hi,

On Wed, Jun 15, 2022 at 2:27 PM Julius Werner <jwerner@chromium.org> wrote:
>
> > Two comments about the above:
> >
> > 1. It seems like the top-level node should have a compatible of some
> > type. Without that I guess you're just relying on people to find it
> > based on the name of the node?
> >
> > 2. Why not put the `channel-io-width` property in the channel? Then
> > you don't need to repeat it for each rank that's under the channel?
>
> Yes, we could do it that way. That seemed a bit more complicated to
> me, but if there's precedent for that in other devices it's probably
> the right thing.
>
> > 1. In the above the two ranks are in series, right? ...with a chip
> > select to select rank0 vs rank1? From how SPI works I'd expect that to
> > be represented using "reg", AKA:
>
> I wouldn't call it "in series" (rank is just a separate dimension of
> its own, in my mental model) but yes, if you think they should also be
> named with a property inside the node (and not just distinguished by
> node name), we can do that. Using "reg" for this feels a bit odd to
> me, but if that's common device tree practice we can do it that way.

Definitely should listen to real DT maintainers and not just me--it
just sounded like it matched how SPI did things where the chip select
was the "reg".


> > 2. I guess if you had two things in parallel you'd want to know how?
> > Maybe if you had 4 8-bit chips connected to a 32-bit channel maybe
> > it'd look like this: [...]
>
> I think the channel-io-width mechanism is sufficient to distinguish
> this (by dividing by io-width), so I don't think there's anything to
> gain from listing each of these parallel chips separately. This also
> more closely reflects the way the memory training firmware that's
> writing these entries actually sees the system. The way I understand
> it, from the memory controller's perspective there's actually no
> difference between talking to a single 32-bit chip or two 16-bit chips
> in parallel -- there's no difference in register settings or anything,
> both software and hardware are totally unaware of this. This is all
> just implemented by wiring the respective components together
> correctly in the board layout (split the DQ pins between the two
> chips, and short all the other pins like clock and chip select
> together). When reading the mode register value, the controller only
> reads the first chip's register (which is connected to DQ[0:7]). When
> writing a mode register, the one Write Mode Register cycle will write
> all chips at once (because the written value is transferred through
> the column address pins which are shorted together between all chips).
> So if we were to pretend in the FDT that we had separate density and
> io-width values for each chip, that's kinda disingenuous, because the
> firmware can only read one of them and just assumes that it applies to
> all chips in parallel on that channel. The only way the firmware could
> know how many chips there are in parallel would also be by dividing
> the width of its channel by the io-width reported by the chip -- so I
> think it would be more honest there to just report those two "original
> source" values to the kernel / userspace and let them make that
> deduction themselves if they care to.
>
> > ...and I guess you could have things that include serial and parallel hookups...
>
> Sorry, just to avoid having more confusion here: there is no "serial"
> dimension to this as far as I'm aware (in my original email I called
> the "several chips per channel" thing "in series", but you are right
> that it would really be more accurate to call it "in parallel").
> There's only three dimensions: a) multiple channels (totally separate
> sets of pins coming out of the controller), b) multiple chips per
> channel (splitting e.g. 32 pins from the controller onto two 16-pin
> parts), and c) multiple ranks within each chip (which chip select pin
> is asserted in each access cycle).
>
> > > > This would be describing a dual-channel, dual-rank layout where each
> > > > 32-bit channel is connected to two 16-bit LPDDR chips in series. The
> > > > total capacity would be (2048 Mbits * (32/16) chips + 1024 Mbits *
> > > > (32/16) chips) * 2 channels = 12Gbits.
> >
> > Just to make sure I'm understanding things: in your hypothetical
> > example we're effectively wasting half of the SDRAM bandwidth of the
> > controller, right? So while what you describe is legal you'd get a
> > much more performant system by hooking the two big chips in parallel
> > on one channel and the two small chips in parallel on the other
> > channel. That would effectively give you a 64-bit wide bus as opposed
> > to the 32-bit wide bus that you describe.
>
> No, I don't think you're wasting bandwidth. In my example the
> controller has two 32-bit channels, so it always uses 64 bits of
> bandwidth in total. There's no asymmetry in the "chips per channel"
> dimension in my example (maybe that was a misunderstanding due to our
> different use of "in series" vs "in parallel") -- in fact, there can
> never be asymmetry in that dimension, when you split a channel onto
> more than one chip then those chips always must be exactly equal in
> geometry and timings (because, as mentioned above, they all get
> initialized the same way with parallel Write Mode Register commands).
> Asymmetry can only come in at the rank or channel dimension.
> (Asymmetry there may have a minor performance penalty since you'd be
> limiting the amount of rank or channel interleaving the controller can
> do, but it would be an indirect penalty that depends on the access
> pattern and not be anywhere near as bad as "half the bandwidth".)
>
> Anyway, whether it's a good idea or not, these parts definitely do
> exist and get sold that way. I can't find an example with a public
> datasheet right now, but e.g. the MT53E1536M32DDNQ-046 WT:A part
> offers two 16-bit channels that have two ranks each, where rank 0 is 8
> Gbits and rank 1 is 16 Gbits for each channel (6 GB part in total).

Ah, got it. I re-read your email and I see my confusion. I thought in
your example there were a total of 4 chips in the system, but there
were actually 8 chips. You were definitely explicit about it but I
still got confused. :( I was somehow assuming that you were saying
that each channel was 32-bits wide but that we were only connecting
16-bits of it.

OK, then what you have seems OK. Personally I guess I'd find it a
little less confusing if we described it as "num-chips" or something
like that. I guess that would make me feel like the io-width could
stay where it is and describe the full width, like maybe for your
original example:

lpddr2-channel0 {
compatible = "jdec,lpddr2-channel";
#address-cells = <1>;
#size-cells = <0>;

rank@0 {
reg = <0>;
compatible = "jedec,lpddr2";
density = <2048>;
io-width = <32>;
num-chips = <2>;
};
rank@1 {
reg = <1>;
compatible = "jedec,lpddr2";
density = <1024>;
io-width = <32>;
num-chips = <2>;
};
};


> > I'm happy to let others chime in, but one way to do this would be to
> > put the super common properties (density, width, etc) in a common file
> > and have it "included" by everyone else. See
> > `bindings/spi/spi-controller.yaml` and then see how all the SPI
> > controllers "reference" that.
>
> Okay, that should work. I don't think there would be any differences
> other than the compatible strings right now (and maybe which values
> are valid for each property... not sure if that can be distinguished
> while still including shared definitions?), but I can write them as
> three dummy binding files that contain nothing but a compatible string
> and an include.

They do have different sets of values valid for each property. The
properties are annoyingly not sorted consistently with each other, but
I think there are also different sets of properties aren't there? Like
I only see tRASmin-min-tck in the LPDDR2 one and not LPDDR3.

I was suggesting keeping most of the current stuff separate between
DDR2 / DDR3 / DDR4 and only putting the bare minimum "this is clearly
something you'd describe for any memory chip" in a common place...

-Doug

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