Hi Andy,
Thanks for your reply!
> From: Andy Shevchenko <andy.shevchenko@xxxxxxxxx>
> 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@xxxxxxxxxxx> 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?
32766 is the correct value.
Clock "csiclk" gets divided by 2 * CSI_CLKSEL_CKS in order to generate the
serial clock (output from master), with CSI_CLKSEL_CKS ranging from 0x1 (that
means "csiclk" is divided by 2) to 0x3FFF ("csiclk" is divided by 32766).
>
> > #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.
To cater for a wider audience (and not just for those who have read the
HW manual), I think perhaps the below would probably be the best compromise:
/*
* Clock "csiclk" gets divided by 2 * CSI_CLKSEL_CKS in order to generate the
* serial clock (output from master), with CSI_CLKSEL_CKS ranging from 0x1 (that
* means "csiclk" is divided by 2) to 0x3FFF ("csiclk" is divided by 32766).
*/
#define CSI_CKS_MAX (BIT(14)-1)
>
> ...
>
> >
> > 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.
The above is actually equivalent to ilog2()
>
> > 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().
rounddown_pow_of_two().
I have tested the driver with s/x_trg/ilog2 and
s/x_trg_words/roundup_pow_of_two and it looks like I am losing tiny bit of
performance (probably down to the use of ternary operators in both macros)
but I think it's okay, let's not reinvent the wheel and let's keep it more
readable, I'll switch to using the above macros.
>
> ...
>
> > > > + /* 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.
That's the plan.
Thanks for your help Andy.
Cheers,
Fab
>
> ...
>
> > > > + 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
