linux-kernel - Re: [PATCH 2/2] i2c: i2c-stm32f7: allows for any bus frequency

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Date:   Thu, 26 Mar 2020 11:12:19 +0100
From:   Alain Volmat <alain.volmat@...com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
CC:     Wolfram Sang <wsa@...-dreams.de>, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Pierre Yves MORDRET <pierre-yves.mordret@...com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre TORGUE <alexandre.torgue@...com>,
        linux-i2c <linux-i2c@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        "linux-stm32@...md-mailman.stormreply.com" 
        <linux-stm32@...md-mailman.stormreply.com>,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Fabrice GASNIER <fabrice.gasnier@...com>
Subject: Re: [PATCH 2/2] i2c: i2c-stm32f7: allows for any bus frequency

Hi Andy,

Thanks for the review.

On Wed, Mar 25, 2020 at 06:53:45PM +0000, Andy Shevchenko wrote:
> On Wed, Mar 25, 2020 at 8:38 PM Alain Volmat <alain.volmat@...com> wrote:
> >
> > Do not limitate to the 3 (100KHz, 400KHz, 1MHz) bus frequency but
> > instead allows any frequency (if it matches timing requirements).
> > Depending on the requested frequency, use the spec data from either
> > Standard, Fast or Fast Plus mode.
> >
> > Hardcoding of min/max bus frequencies is removed and is instead computed.
> >
> > The driver do not use anymore speed identifier but instead handle
> > directly the frequency and figure out the spec data (necessary
> > for the computation of the timing register) based on the frequency.
> 
> ...
> 
> > +static struct stm32f7_i2c_spec *get_specs(u32 rate)
> > +{
> > +       int i;
> > +
> > +       for (i = 0; i < ARRAY_SIZE(i2c_specs); i++)
> > +               if (rate <= i2c_specs[i].rate)
> > +                       return &i2c_specs[i];
> > +
> 
> > +       /* NOT REACHED */
> > +       return ERR_PTR(-EINVAL);
> 
> WARN_ONCE() ?

The comment should actually be removed. get_specs return value is
properly checked in stm32f7_i2c_compute_timing and an error message
is displayed in case of an error.

> 
> > +}
> 
> ...
> 
> > -                       if ((tscl_l < i2c_specs[setup->speed].l_min) ||
> > +                       if ((tscl_l < specs->l_min) ||
> 
> >                             (i2cclk >=
> >                              ((tscl_l - af_delay_min - dnf_delay) / 4))) {
> 
> Perhaps squash above two to one line at the same time?

I agree that this is not very pretty to read now but that would lead to
a line exceeding 80 characters. To fix that it'd be better to rework the code
but in such case that should be done at a separate time to keep this commit
as small / simpler to understand as possible. So I'd prefer leave this code
for the time being.

> 
> ...
> 
> > +       int i;
> > +
> > +       for (i = ARRAY_SIZE(i2c_specs) - 1; i >= 0; i--)
> 
> 
> Perhaps
> 
>        int i = ARRAY_SIZE(i2c_specs);
> 
>        while(i--)
> 
> ?

I propose the following code to make it a bit easier to read/understand:

static u32 get_lower_rate(u32 rate)
{
        int i = ARRAY_SIZE(i2c_specs);

        while (i--)
                if (i2c_specs[i].rate < rate)
                        break;

        return i2c_specs[i].rate;
}

If you agree with that I'll push a v2.

> 
> > +               if (i2c_specs[i].rate < rate)
> > +                       return i2c_specs[i].rate;
> 
> -- 
> With Best Regards,
> Andy Shevchenko