[PATCH 1/8] i3c: renesas: Fix zero I2C SCL rate on pure I3C buses

[Geert Uytterhoeven]

Hi Diogo,

On Tue, 20 Oct 2026 at 11:15, Diogo Ivo <diogo.ivo.sx at renesas.com> wrote:
> On a pure I3C bus there are no I2C devices, so the I3C core leaves
> bus->scl_rate.i2c at zero if i2c-scl-hz is not specified in DT.
> Deriving i2c_total_ticks from that zero rate programs a zero-width
> open-drain SCL into the STDBR open-drain fields, which are still used
> in pure I3C mode for example in DAA.
>
> Fall back to the 1 MHz FM+ limit instead, the fastest rate an I2C
> device could have, so that a sane open-drain timing is programmed.
>
> Fixes: d028219a9f14 ("i3c: master: Add basic driver for the Renesas I3C controller")
> Cc: stable at vger.kernel.org
> Signed-off-by: Diogo Ivo <diogo.ivo.sx at renesas.com>

Thanks for your patch!

> --- a/drivers/i3c/master/renesas-i3c.c
> +++ b/drivers/i3c/master/renesas-i3c.c
> @@ -579,7 +579,18 @@ static int renesas_i3c_bus_init(struct i3c_master_controller *m)
>         if (!i3c->rate)
>                 return -EINVAL;
>
> -       i2c_total_ticks = DIV_ROUND_UP(i3c->rate, bus->scl_rate.i2c);
> +       /*
> +        * A pure I3C bus has no I2C devices, so the I3C core leaves
> +        * scl_rate.i2c at zero if not explicitly defined in DT. The
> +        * open-drain SCL fields of STDBR, which are used for the DAA
> +        * pending read, are derived from it though, so fall back to
> +        * the 1 MHz FM+ limit instead of programming a zero-width clock.
> +        */
> +       if (!bus->scl_rate.i2c)
> +               i2c_total_ticks = DIV_ROUND_UP(i3c->rate, I3C_BUS_I2C_FM_PLUS_SCL_MAX_RATE);
> +       else
> +               i2c_total_ticks = DIV_ROUND_UP(i3c->rate, bus->scl_rate.i2c);
> +
>         i3c_total_ticks = DIV_ROUND_UP(i3c->rate, bus->scl_rate.i3c);

This looks like a good opportunity to use the shortened tertiary ?:-operator:

    bus->scl_rate.i2c ?: I3C_BUS_I2C_FM_PLUS_SCL_MAX_RATE

>
>         i2c_parse_fw_timings(&m->dev, &t, true);

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-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