linux-kernel - Re: [PATCH v4 05/20] dt-bindings: memory: factorise LPDDR props into SDRAM props

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Message-ID: <CAODwPW_kex5Agqxg_i-XC308scEpUJU0me55G7iZ8nB9LC0acg@mail.gmail.com>
Date: Wed, 23 Jul 2025 14:48:21 -0700
From: Julius Werner <jwerner@...omium.org>
To: Clément Le Goffic <clement.legoffic@...s.st.com>
Cc: Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Maxime Coquelin <mcoquelin.stm32@...il.com>, Alexandre Torgue <alexandre.torgue@...s.st.com>, 
	Philipp Zabel <p.zabel@...gutronix.de>, Jonathan Corbet <corbet@....net>, 
	Gatien Chevallier <gatien.chevallier@...s.st.com>, Michael Turquette <mturquette@...libre.com>, 
	Stephen Boyd <sboyd@...nel.org>, Gabriel Fernandez <gabriel.fernandez@...s.st.com>, 
	Krzysztof Kozlowski <krzk@...nel.org>, Le Goffic <legoffic.clement@...il.com>, 
	Julius Werner <jwerner@...omium.org>, linux-arm-kernel@...ts.infradead.org, 
	linux-perf-users@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-stm32@...md-mailman.stormreply.com, linux-kernel@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-clk@...r.kernel.org
Subject: Re: [PATCH v4 05/20] dt-bindings: memory: factorise LPDDR props into
 SDRAM props

> +      Compatible strings can be either explicit vendor names and part numbers
> +      (e.g. elpida,ECB240ABACN), or generated strings of the form
> +      (lp)?ddrX-Y,Z where X, Y and Z are in lower case hexadecimal with leading
> +      zeroes and :
> +        - X is the SDRAM version (2, 3, 4, etc.)
> +        - for LPDDR :
> +          - Y is the manufacturer ID (from MR5), 2 bytes
> +          - Z is the revision ID (from MR6 and MR7), 4 bytes

It's actually one byte manufacturer, two bytes revision. The YY,ZZZZ
is supposed to represent the amount of hex digits needed.

> +        - for DDR4 with SPD, according to JEDEC SPD4.1.2.L-6 :
> +          - Y is the manufacturer ID, 2 bytes, from bytes 320 and 321
> +          - Z is the revision ID, 1 byte, from byte 349

I don't think this will identify a part unambiguously, I would expect
the DDR revision ID to be specific to the part number. (In fact, we're
also not sure whether manufacturer+revision identifies LPDDR parts
unambiguously for every vendor, we just didn't have anything more to
work with there.) I would suggest to use either `ddrX-YYYY,AAA...,ZZ`
or `ddrX-YYYY,ZZ,AAA...` (where AAA... is the part number string from
SPD 329-348 without the trailing spaces). The first version looks a
bit more natural but it might get confusing on the off chance that
someone uses a comma in a part number string.

> +      The latter form can be useful when SDRAM nodes are created at runtime by
> +      boot firmware that doesn't have access to static part number information.

nit: This text slightly doesn't make sense anymore when in the DDR
case we do actually have the part number. I guess the real thing the
bootloader wouldn't have access to is the JEDEC manufacturer ID to
name mapping.

> +      SDRAM revision ID:
> +        - LPDDR SDRAM, decoded from Mode Register 6 and 7.
> +        - DDR4 SDRAM, decoded from the SPD from bytes 349 according to
> +          JEDEC SPD4.1.2.L-6.

nit: Clarify that this is always two bytes for LPDDR and always one
byte for DDR.

> +      Density of SDRAM chip in megabits:
> +        - LPDDR SDRAM, decoded from Mode Register 8.
> +        - DDR4 SDRAM, decoded from the SPD from bytes 322 to 325 according to
> +          JEDEC SPD4.1.2.L-6.

Are these numbers correct? I downloaded SPD4.1.2.L-6 now and it looks
like 322 is manufacturing location and 323-324 are manufacturing date.
(Also, I think all of these are specific to DDR4 (and possibly 5?),
but not to earlier versions. I don't think we need to list it for
every version, but we should at least be specific what it applies to.)