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From:   Xiongwei Song <sxwjean@gmail.com>
Date:   Fri, 6 Aug 2021 21:22:52 +0800
Message-ID: <CAEVVKH8wjaR3JqgkWX=X6n00nmPEjcUCHU=_5w_xxjsAWHFcUg@mail.gmail.com>
Subject: Re: [RFC PATCH 1/4] powerpc: Optimize register usage for esr register
To:     Christophe Leroy <christophe.leroy@csgroup.eu>
Cc:     Xiongwei Song <sxwjean@me.com>,
        PowerPC <linuxppc-dev@lists.ozlabs.org>, oleg@redhat.com,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>, ravi.bangoria@linux.ibm.com,
        npiggin@gmail.com, aneesh.kumar@linux.ibm.com,
        sandipan@linux.ibm.com, efremov@linux.com, peterx@redhat.com,
        akpm@linux-foundation.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Fri, Aug 6, 2021 at 3:32 PM Christophe Leroy
<christophe.leroy@csgroup.eu> wrote:
>
>
>
> Le 06/08/2021 =C3=A0 05:16, Xiongwei Song a =C3=A9crit :
> > On Thu, Aug 5, 2021 at 6:06 PM Christophe Leroy
> > <christophe.leroy@csgroup.eu> wrote:
> >>
> >>
> >>
> >> Le 26/07/2021 =C3=A0 16:30, sxwjean@me.com a =C3=A9crit :
> >>> From: Xiongwei Song <sxwjean@gmail.com>
> >>>
> >>> Create an anonymous union for dsisr and esr regsiters, we can referen=
ce
> >>> esr to get the exception detail when CONFIG_4xx=3Dy or CONFIG_BOOKE=
=3Dy.
> >>> Otherwise, reference dsisr. This makes code more clear.
> >>
> >> I'm not sure it is worth doing that.
> > Why don't we use "esr" as reference manauls mentioned?
> >
> >>
> >> What is the point in doing the following when you know that regs->esr =
and regs->dsisr are exactly
> >> the same:
> >>
> >>   > -    err =3D ___do_page_fault(regs, regs->dar, regs->dsisr);
> >>   > +    if (IS_ENABLED(CONFIG_4xx) || IS_ENABLED(CONFIG_BOOKE))
> >>   > +            err =3D ___do_page_fault(regs, regs->dar, regs->esr);
> >>   > +    else
> >>   > +            err =3D ___do_page_fault(regs, regs->dar, regs->dsisr=
);
> >>   > +
> > Yes, we can drop this. But it's a bit vague.
> >
> >> Or even
> >>
> >>   > -    int is_write =3D page_fault_is_write(regs->dsisr);
> >>   > +    unsigned long err_reg;
> >>   > +    int is_write;
> >>   > +
> >>   > +    if (IS_ENABLED(CONFIG_4xx) || IS_ENABLED(CONFIG_BOOKE))
> >>   > +            err_reg =3D regs->esr;
> >>   > +    else
> >>   > +            err_reg =3D regs->dsisr;
> >>   > +
> >>   > +    is_write =3D page_fault_is_write(err_reg);
> >>
> >>
> >> Artificially growing the code for that makes no sense to me.
> >
> > We can drop this too.
> >>
> >>
> >> To avoid anbiguity, maybe the best would be to rename regs->dsisr to s=
omething like regs->sr , so
> >> that we know it represents the status register, which is DSISR or ESR =
depending on the platform.
> >
> > If so, this would make other people more confused. My consideration is
> > to follow what the reference
> > manuals represent.
>
> Maybe then we could rename the fields as regs->dsisr_esr and regs->dar_de=
ar

I still prefer my method.

>
> That would be more explicit for everyone.
>
> The UAPI header however should remain as is because anonymous unions are =
not supported by old
> compilers as mentioned by Michael.

Sure. Will update in v2.

>
> But nevertheless, there are also situations where was is stored in regs->=
dsisr is not what we have
> in DSISR register. For instance on an ISI exception, we store a subset of=
 the content of SRR1
> register into regs->dsisr.

Can I think my method has better expansibility here;-)?
Let me finish esr and dear first. Thank you for the reminder.

Regards,
Xiongwei
>
> Christophe