linux-kernel - Re: [PATCH v2 3/5] spi: Add support for Renesas CSI

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Message-ID: <CAHp75VcJvRq7BotoODW_BOh84+TD_1Q3vbXSQv3FCiJfnBx8Vw@mail.gmail.com>
Date:   Thu, 13 Jul 2023 19:37:20 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Fabrizio Castro <fabrizio.castro.jz@...esas.com>
Cc:     Mark Brown <broonie@...nel.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Magnus Damm <magnus.damm@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
        "linux-renesas-soc@...r.kernel.org" 
        <linux-renesas-soc@...r.kernel.org>,
        Chris Paterson <Chris.Paterson2@...esas.com>,
        Biju Das <biju.das@...renesas.com>
Subject: Re: [PATCH v2 3/5] spi: Add support for Renesas CSI

On Thu, Jul 13, 2023 at 6:52 PM Fabrizio Castro
<fabrizio.castro.jz@...esas.com> wrote:

...

> > > +#define CSI_CKS_MAX                0x3FFF
> >
> > If it's limited by number of bits, i would explicitly use that information
> > as
> > (BIT(14) - 1).
>
> That value represents the register setting for the maximum clock divider.
> The maximum divider and corresponding register setting are plainly stated
> in the HW User Manual, therefore I would like to use either (plain) value
> to make it easier for the reader.
>
> I think perhaps the below makes this clearer:
> #define CSI_CKS_MAX_DIV_RATIO   32766

Hmm... To me it's a bit confusing now. Shouldn't it be 32767?

> #define CSI_CKS_MAX             (CSI_CKS_MAX_DIV_RATIO >> 1)

Whatever you choose it would be better to add a comment to explain
this. Because the above is more clear to me with BIT(14)-1 if the
register field is 14-bit long.
With this value(s) I'm lost. Definitely needs a comment.

...

> > > +static const unsigned char x_trg[] = {
> > > +   0, 1, 1, 2, 2, 2, 2, 3,
> > > +   3, 3, 3, 3, 3, 3, 3, 4,
> > > +   4, 4, 4, 4, 4, 4, 4, 4,
> > > +   4, 4, 4, 4, 4, 4, 4, 5
> > > +};
> > > +
> > > +static const unsigned char x_trg_words[] = {
> > > +   1,  2,  2,  4,  4,  4,  4,  8,
> > > +   8,  8,  8,  8,  8,  8,  8,  16,
> > > +   16, 16, 16, 16, 16, 16, 16, 16,
> > > +   16, 16, 16, 16, 16, 16, 16, 32
> > > +};
> >
> > Why do you need tables? At the first glance these values can be easily
> > calculated from indexes.
>
> I think I am going to replace those tables with the below (and of course
> adjust the callers accordingly since the argument is not an index anymore):
>
> static inline unsigned int x_trg(unsigned int words)
> {
>         return fls(words) - 1;
> }

OK, but I think you can use it just inplace, no need to have such as a
standalone function.

> static inline unsigned int x_trg_words(unsigned int words)
> {
>         return 1 << x_trg(words);
> }

Besides a better form of BIT(...) this looks to me like NIH
roundup_pow_of_two().

...

> > > +   /* Setup clock polarity and phase timing */
> > > +   rzv2m_csi_reg_write_bit(csi, CSI_CLKSEL, CSI_CLKSEL_CKP,
> > > +                           !(spi->mode & SPI_CPOL));
> > > +   rzv2m_csi_reg_write_bit(csi, CSI_CLKSEL, CSI_CLKSEL_DAP,
> > > +                           !(spi->mode & SPI_CPHA));
> >
> > Is it a must to do in a sequential writes?
>
> It's not a must, I'll combine those 2 statements into 1.

If so, you can use SPI_MODE_X_MASK.

...

> > > +   controller->mode_bits = SPI_CPOL | SPI_CPHA | SPI_LSB_FIRST;
> >
> > SPI_MODE_X_MASK
>
> This statement sets the mode_bits. Using a macro meant to be used as a
> mask in this context is something I would want to avoid if possible.

Hmm... not a big deal, but I think that's what covers all mode_bits,
and mode_bits by nature _is_ a mask.

-- 
With Best Regards,
Andy Shevchenko