linux-kernel - Re: [PATCH 8/9] i2c: riic: Add `riic_bus_barrier()` to check bus availability

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Message-ID: <CAMuHMdXOaphnU=caTuCx3yme_hi-_zdn5PN5P1zhrOSA3duBWg@mail.gmail.com>
Date: Mon, 16 Dec 2024 20:12:50 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: "Lad, Prabhakar" <prabhakar.csengg@...il.com>
Cc: Chris Brandt <chris.brandt@...esas.com>, Andi Shyti <andi.shyti@...nel.org>, 
	Philipp Zabel <p.zabel@...gutronix.de>, Wolfram Sang <wsa@...nel.org>, 
	linux-renesas-soc@...r.kernel.org, linux-i2c@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Biju Das <biju.das.jz@...renesas.com>, 
	Fabrizio Castro <fabrizio.castro.jz@...esas.com>, 
	Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
Subject: Re: [PATCH 8/9] i2c: riic: Add `riic_bus_barrier()` to check bus availability

Hi Prabhakar,

On Mon, Dec 16, 2024 at 7:19 PM Lad, Prabhakar
<prabhakar.csengg@...il.com> wrote:
> On Mon, Dec 16, 2024 at 4:09 PM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> > On Fri, Dec 13, 2024 at 6:58 PM Prabhakar <prabhakar.csengg@...il.com> wrote:
> > > From: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
> > >
> > > Introduce a new `riic_bus_barrier()` function to verify bus availability
> > > before initiating an I2C transfer. This function enhances the bus
> > > arbitration check by ensuring that the SDA and SCL lines are not held low,
> > > in addition to checking the BBSY flag using `readb_poll_timeout()`.
> > >
> > > Previously, only the BBSY flag was checked to determine bus availability.
> > > However, it is possible for the SDA line to remain low even when BBSY = 0.
> > > This new implementation performs an additional check on the SDA and SCL
> > > lines to avoid potential bus contention issues.
> > >
> > > Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
> >
> > Thanks for your patch!
> >
> > > --- a/drivers/i2c/busses/i2c-riic.c
> > > +++ b/drivers/i2c/busses/i2c-riic.c
> >
> > > @@ -135,6 +138,27 @@ static inline void riic_clear_set_bit(struct riic_dev *riic, u8 clear, u8 set, u
> > >         riic_writeb(riic, (riic_readb(riic, reg) & ~clear) | set, reg);
> > >  }
> > >
> > > +static int riic_bus_barrier(struct riic_dev *riic)
> > > +{
> > > +       int ret;
> > > +       u8 val;
> > > +
> > > +       /*
> > > +        * The SDA line can still be low even when BBSY = 0. Therefore, after checking
> > > +        * the BBSY flag, also verify that the SDA and SCL lines are not being held low.
> > > +        */
> > > +       ret = readb_poll_timeout(riic->base + riic->info->regs[RIIC_ICCR2], val,
> > > +                                !(val & ICCR2_BBSY), 10, riic->adapter.timeout);
> > > +       if (ret)
> > > +               return -EBUSY;
> > > +
> > > +       if (!(riic_readb(riic, RIIC_ICCR1) & ICCR1_SDAI) ||
> > > +           !(riic_readb(riic, RIIC_ICCR1) & ICCR1_SCLI))
> >
> > Surely you can read the register once, and check both bits?
> >
> Agreed, I will add a helper func for this something like below, as
> this can be re-used in patch 9/9
>
> static inline bool riic_lines_ok(struct riic_dev *riic)
> {
>     u8 reg = riic_readb(riic, RIIC_ICCR1);
>
>     if (!(reg & ICCR1_SDAI) || !(reg & ICCR1_SCLI))
>         return false;
>
>     return true;
> }

A helper might be overkill...

    if ((riic_readb(riic, RIIC_ICCR1) & (ICCR1_SDAI | ICCR1_SCLI)) !=
        ICCR1_SDAI | ICCR1_SCLI)
            ...

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds