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From:   Andy Shevchenko <andy.shevchenko@gmail.com>
Date:   Tue, 5 Jul 2022 11:04:32 +0200
Message-ID: <CAHp75VfyKtFQbcJJxW8TNSHMZppM6Qgj4hxiUzLB1bSmeWBBAg@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] iio: adc: Add rtq6056 support
To:     ChiYuan Huang <u0084500@gmail.com>
Cc:     Jonathan Cameron <jic23@kernel.org>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Lars-Peter Clausen <lars@metafoo.de>,
        cy_huang <cy_huang@richtek.com>,
        linux-iio <linux-iio@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        devicetree <devicetree@vger.kernel.org>
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On Tue, Jul 5, 2022 at 3:41 AM ChiYuan Huang <u0084500@gmail.com> wrote:
> Andy Shevchenko <andy.shevchenko@gmail.com> =E6=96=BC 2022=E5=B9=B47=E6=
=9C=885=E6=97=A5 =E9=80=B1=E4=BA=8C =E6=B8=85=E6=99=A85:52=E5=AF=AB=E9=81=
=93=EF=BC=9A
> > On Mon, Jul 4, 2022 at 9:27 AM ChiYuan Huang <u0084500@gmail.com> wrote=
:
> > > ChiYuan Huang <u0084500@gmail.com> =E6=96=BC 2022=E5=B9=B47=E6=9C=884=
=E6=97=A5 =E9=80=B1=E4=B8=80 =E4=B8=8A=E5=8D=8811:16=E5=AF=AB=E9=81=93=EF=
=BC=9A
> > > > Andy Shevchenko <andy.shevchenko@gmail.com> =E6=96=BC 2022=E5=B9=B4=
7=E6=9C=881=E6=97=A5 =E9=80=B1=E4=BA=94 =E4=B8=8B=E5=8D=886:05=E5=AF=AB=E9=
=81=93=EF=BC=9A
> > > > > On Wed, Jun 29, 2022 at 4:23 AM cy_huang <u0084500@gmail.com> wro=
te:

...

> > > > > > +       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 =3D=3D 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 membe=
r?
> the data member already guarantee 2 * 4 =3D 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.

--=20
With Best Regards,
Andy Shevchenko