linux-kernel - Re: [PATCH v3 08/11] i2c: npcm: Correct register access width

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Message-ID: <YiJ5unrCb82ZMV4Z@latitude>
Date:   Fri, 4 Mar 2022 21:42:34 +0100
From:   Jonathan Neuschäfer <j.neuschaefer@....net>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Tali Perry <tali.perry1@...il.com>,
        Tomer Maimon <tmaimon77@...il.com>,
        devicetree <devicetree@...r.kernel.org>,
        yangyicong@...ilicon.com, Linux I2C <linux-i2c@...r.kernel.org>,
        Benjamin Fair <benjaminfair@...gle.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>,
        OpenBMC Maillist <openbmc@...ts.ozlabs.org>,
        JJLIU0@...oton.com, lukas.bulwahn@...il.com,
        tomer.maimon@...oton.com, KWLIU@...oton.com, bence98@....bme.hu,
        arnd@...db.de, sven@...npeter.dev,
        Rob Herring <robh+dt@...nel.org>,
        Avi Fishman <Avi.Fishman@...oton.com>,
        Tyrone Ting <warp5tw@...il.com>, semen.protsenko@...aro.org,
        jie.deng@...el.com, avifishman70@...il.com,
        Patrick Venture <venture@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Wolfram Sang <wsa@...nel.org>, kfting@...oton.com,
        Tali Perry <tali.perry@...oton.com>, olof@...om.net
Subject: Re: [PATCH v3 08/11] i2c: npcm: Correct register access width

Hello,

On Thu, Mar 03, 2022 at 04:15:18PM +0200, Andy Shevchenko wrote:
> On Thu, Mar 03, 2022 at 02:54:27PM +0200, Tali Perry wrote:
> > > On Thu, Mar 03, 2022 at 04:31:38PM +0800, Tyrone Ting wrote:
> > > > From: Tyrone Ting <kfting@...oton.com>
> > > >
> > > > Use ioread8 instead of ioread32 to access the SMBnCTL3 register since
> > > > the register is only 8-bit wide.
> > >
> > > > Fixes: 56a1485b102e ("i2c: npcm7xx: Add Nuvoton NPCM I2C controller driver")
> > >
> > > No, this is bad commit message, since you have bitwise masks and there is
> > > nothing to fix from functional point of view. So, why is this a fix?
> > >
> > 
> > The next gen of this device is a 64 bit cpu.
> > The module is and was 8 bit.
> > 
> > The ioread32 that seemed to work smoothly on a 32 bit machine
> > was causing a panic on a 64 bit machine.
> > since the module is 8 bit we changed to ioread8.
> > This is working both for the 32 and 64 CPUs with no issue.
> 
> Then the commit message is completely wrong here.

I disagree: The commit message is perhaps incomplete, but not wrong.
The SMBnCTL3 register was specified as 8 bits wide in the datasheets of
multiple chip generations, as far as I can tell, but the driver wrongly
made a 32-bit access, which just happened not to blow up.

So, indeed, "since the register is only 8-bit wide" seems to be a
correct claim.

> And provide necessary (no need to have noisy commit messages)
> bits of the oops to show what's going on

I guess it's blowing up now because SMBnCTL3 isn't 32-bit aligned
(being at offset 0x0e in the controller).


Jonathan

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