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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Wed, 9 Feb 2022 23:56:59 +0100
Message-ID: <CAK8P3a3wOxRdGJfDx86WiNwFvbK=wXYCtPoS-DQKqyYfHZiMjw@mail.gmail.com>
Subject: Re: [PATCH 2/3] i2c: sh_mobile: Use platform_get_irq_optional() to
 get the interrupt
To:     Sergei Shtylyov <sergei.shtylyov@gmail.com>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>,
        Rob Herring <robh+dt@kernel.org>,
        Nicolas Saenz Julienne <nsaenz@kernel.org>,
        Florian Fainelli <f.fainelli@gmail.com>,
        Ray Jui <rjui@broadcom.com>,
        Scott Branden <sbranden@broadcom.com>,
        bcm-kernel-feedback-list <bcm-kernel-feedback-list@broadcom.com>,
        Chris Brandt <chris.brandt@renesas.com>,
        Wolfram Sang <wsa+renesas@sang-engineering.com>,
        Linux I2C <linux-i2c@vger.kernel.org>,
        linux-rpi-kernel <linux-rpi-kernel@lists.infradead.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
        Prabhakar <prabhakar.csengg@gmail.com>,
        Linux-sh list <linux-sh@vger.kernel.org>
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On Wed, Feb 9, 2022 at 5:08 PM Sergei Shtylyov
<sergei.shtylyov@gmail.com> wrote:
> On 2/9/22 7:02 PM, Arnd Bergmann wrote:
> >
> > +       shlr2   r4
> > +       shlr    r4
> > +       mov     r4, r0          ! save vector->jmp table offset for later
> > +
> > +       shlr2   r4              ! vector to IRQ# conversion
> > +       add     #-0x10, r4
> > +
> > +       cmp/pz  r4              ! is it a valid IRQ?
> > +       bt      10f
> >
> > gets the vector (0x200 for this device), shifts it five bits to 0x10,
> > and subtracts 0x10,
> > then branches to do_IRQ if the interrupt number is non-zero, otherwise it goes
> > through the exception_handling_table.
>
>    The SH4 manual I found on my disk (have it from MontaVista times) tells me cmp/pz
> sets T if Rn is >= 0, then bt branches if T = 1. So I do think the code is correct.
>    One more thing: the board code for those boards was added in 2011, we can assume
> it was working back then, right? :-_

Indeed, this does make more sense, I had not realized that the numbers could get
negative here.

         Arnd