linux-kernel - Re: [PATCH v2 2/2] iio: adc: Add rtq6056 support

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Message-ID: <CAHp75VfyKtFQbcJJxW8TNSHMZppM6Qgj4hxiUzLB1bSmeWBBAg@mail.gmail.com>
Date:   Tue, 5 Jul 2022 11:04:32 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     ChiYuan Huang <u0084500@...il.com>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Lars-Peter Clausen <lars@...afoo.de>,
        cy_huang <cy_huang@...htek.com>,
        linux-iio <linux-iio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] iio: adc: Add rtq6056 support

On Tue, Jul 5, 2022 at 3:41 AM ChiYuan Huang <u0084500@...il.com> wrote:
> Andy Shevchenko <andy.shevchenko@...il.com> 於 2022年7月5日 週二 清晨5:52寫道：
> > On Mon, Jul 4, 2022 at 9:27 AM ChiYuan Huang <u0084500@...il.com> wrote:
> > > ChiYuan Huang <u0084500@...il.com> 於 2022年7月4日 週一 上午11:16寫道：
> > > > Andy Shevchenko <andy.shevchenko@...il.com> 於 2022年7月1日 週五 下午6:05寫道：
> > > > > On Wed, Jun 29, 2022 at 4:23 AM cy_huang <u0084500@...il.com> wrote:

...

> > > > > > +       struct {
> > > > > > +               u16 vals[RTQ6056_MAX_CHANNEL];
> > > > > > +               int64_t timestamp;
> > > > > > +       } data __aligned(8);
> > > > >
> > > > > Hmm... alignment of this struct will be at least 4 bytes, but
> > > > > shouldn't we rather be sure that the timestamp member is aligned
> > > > > properly? Otherwise this seems fragile and dependent on
> > > > > RTQ6056_MAX_CHANNEL % 4 == 0.
> > > > >
> > > > Yap, from the 'max channel', it already guarantee this struct will be
> > > > aligned at lease 4.
> > > > Actually, It can be removed.
> >
> > I think for the safest side it should be given to the timestamp member. No?
> >
> Sorry, following your comment, Why to use 'align' for the timestamp member?
> the data member already guarantee 2 * 4 = 8 byte, then timestamp will
> be 8 byte aligned, right?

Today it's true, tomorrow it might be different. Imagine if this
driver will cover a new (version of) hardware and needs an additional
channel, how do you guarantee alignment in that case? So, current
approach is working, but fragile.

> what you mentioned is to put __aligned(8) only for timestamp.

Yes.

> I try to put aligned in two ways ( one is only for timestamp, another
> is the whole struct). the result is the same.
> From my thinking, in this case, the struct is already 8 byte aligned
> for timestamp member. don't you think to put 'aligned' is redundant?

No.

-- 
With Best Regards,
Andy Shevchenko