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

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Date:   Tue, 22 Mar 2022 18:51:35 +0100
From:   Jonathan Neuschäfer <j.neuschaefer@....net>
To:     Avi Fishman <avifishman70@...il.com>
Cc:     Jonathan Neuschäfer <j.neuschaefer@....net>,
        Tomer Maimon <tmaimon77@...il.com>, KWLIU@...oton.com,
        Tali Perry <tali.perry1@...il.com>,
        Linux I2C <linux-i2c@...r.kernel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Benjamin Fair <benjaminfair@...gle.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>,
        OpenBMC Maillist <openbmc@...ts.ozlabs.org>,
        JJLIU0@...oton.com, Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Tomer Maimon <tomer.maimon@...oton.com>,
        devicetree <devicetree@...r.kernel.org>, bence98@....bme.hu,
        Arnd Bergmann <arnd@...db.de>, sven@...npeter.dev,
        Rob Herring <robh+dt@...nel.org>,
        Avi Fishman <Avi.Fishman@...oton.com>,
        Tyrone Ting <warp5tw@...il.com>, yangyicong@...ilicon.com,
        semen.protsenko@...aro.org, jie.deng@...el.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

On Tue, Mar 22, 2022 at 07:18:34PM +0200, Avi Fishman wrote:
> On Fri, Mar 4, 2022 at 10:42 PM Jonathan Neuschäfer
> <j.neuschaefer@....net> wrote:
> >
> > 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).
> >
> 
> Hi Andy,
> After this clarification can you please acknowledge this specific patch?
> If you think there is a better way to describe this, can you propose one?

To be honest, I think it's probably best to include all the necessary
explanations in the next version of this patch, i.e.:

 - That the register was always defined as 8-bit in the datasheets,
   and so the 32-bit access was always incorrect, but simply didn't
   cause a visible error
 - How the 32-bit access caused an error now, perhaps with a trimmed
   Oops log as Andy suggested


Jonathan

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