linux-kernel - Re: [PATCH 03/13] dt-bindings: mmc: vt8500-sdmmc: Convert to YAML

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Message-ID: <14de236b-e2a7-4bde-986d-1e5ffddd01b4@kernel.org>
Date: Tue, 22 Apr 2025 10:08:43 +0200
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Alexey Charkov <alchark@...il.com>, Rob Herring <robh@...nel.org>
Cc: Andi Shyti <andi.shyti@...nel.org>, Conor Dooley <conor+dt@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Ulf Hansson <ulf.hansson@...aro.org>, Andrew Lunn <andrew+netdev@...n.ch>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Uwe Kleine-König <ukleinek@...nel.org>,
 Daniel Lezcano <daniel.lezcano@...aro.org>,
 linux-arm-kernel@...ts.infradead.org, linux-i2c@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-mmc@...r.kernel.org, netdev@...r.kernel.org, linux-pwm@...r.kernel.org
Subject: Re: [PATCH 03/13] dt-bindings: mmc: vt8500-sdmmc: Convert to YAML

On 18/04/2025 14:38, Alexey Charkov wrote:
>>
>>>> +
>>>> +  reg:
>>>> +    maxItems: 1
>>>> +
>>>> +  clocks:
>>>> +    maxItems: 1
>>>> +
>>>> +  interrupts:
>>>> +    items:
>>>> +      - description: SDMMC controller interrupt
>>>> +      - description: SDMMC controller DMA interrupt
>>>> +
>>>> +  sdon-inverted:
>>>> +    type: boolean
>>>> +    description: SD_ON bit is inverted on the controller
>>>
>>> This implies I know what the non-inverted state is. If you know, please
>>> state that here.
>>
>> This is a tricky one. The only answer I have is "it's inverted in
>> later versions vs. the first version I saw in the wild, and I'm not
>> sure if it's board related or IP version related - nor if the original
>> was active low or high". No docs, no schematics, no vendor left around
>> to chase for answers.
>>
>> Will dig around some more and update the description if I succeed in
>> uncovering any further clues :)
> 
> I've found some extra clues and would like to consult on the best way forward.
> 
> It turns out (if my understanding of the decompiled binary-only WM8505
> vendor driver is correct) that all chips before (not including) WM8505
> rev. A2 treated their "clock stop" bit (register offset 0x08 a.k.a.
> SDMMC_BUSMODE, bit 0x10 a.k.a. BM_CST in vendor sources, BM_SD_OFF in
> mainline) as "set 1 to disable SD clock", while all the later versions
> treated it as "set 0 to disable SD clock". Which means that there are
> WM8505 based systems that rely on either of those behaviours, while
> any later chips need "set 0 to disable". This is not a board related
> quirk but an on-chip SDMMC controller revision related quirk.
> 
> I'd love to switch to a compatible-based logic and drop the
> "sdon-inverted" flag altogether from the binding I'm writing, but here
> are my doubts where I'd love to consult.
> 
> * Looks like WM8505 rev. A2 needs a separate compatible string vs.
> prior WM8505. Can we have something like "wm,wm8505a2-sdhc" and
> "wm,wm8505-sdhc" respectively? WM8505a2 not being an actual chip name,
> but something discoverable by reading its hardware ID from a system
> configuration register at runtime

Then maybe it can be fully detected runtime? E.g. via soc_id driver (see
drivers/soc/)?

> * If I introduce new compatible strings for "wm,wm8650-sdhc",
> "wm,wm8750-sdhc", "wm,wm8850-sdhc" and "wm,wm8880-sdhc" in bindings,
> DTS and driver code, then the new driver and new DTB should work fine,
> and the DTS should pass schema checks. New driver code won't work with
> older DTB unless I keep the logic to parse "sdon-inverted" which
> wouldn't be part of the binding. Old driver code would not work with
> newer DTB except for pre-A2 versions of WM8505. Is that acceptable?
> * Existing DTS doesn't differentiate between pre-A2 vs. post-A2
> revisions of WM8505 and is bound to fail on the latter

That's an old platform, so we should not break the ABI, thus drivers
must fully support old DTBs.

> 
> I realize that breaking backward/forward compatibility is undesirable,
> but frankly these systems seem to have few mainline users, and those
> people who do run mainline on them ought to be compiling the kernel
> and its DTB at the same time, because the firmware doesn't know

There might be other users of DTS and anyway what would be exactly the
benefit? This hardware aspect is already documented via sdon-inverted
property.

> anything about DT and any modern kernel can only be booted in
> "appended DTB" mode. I also don't know of any non-Linux code that
> might be using these device trees.
> 
> Any guidance would be much appreciated.
> 


Best regards,
Krzysztof