RE: [PATCH v4] mmc: renesas_sdhi: Fix rounding errors

[Biju Das <biju.das.jz@bp.renesas.com>]

Hi Geert,

Thanks for the testing.

> Subject: Re: [PATCH v4] mmc: renesas_sdhi: Fix rounding errors
> 
> Hi Biju,
> 
> On Mon, Sep 26, 2022 at 7:10 PM Biju Das <biju.das.jz@bp.renesas.com>
> wrote:
> > Due to clk rounding errors on RZ/G2L platforms, it selects a clock
> > source with a lower clock rate compared to a higher one.
> > For eg: The rounding error (533333333 Hz / 4 * 4 = 533333332 Hz <
> > 5333333
> > 33 Hz) selects a clk source of 400 MHz instead of 533.333333 MHz.
> >
> > This patch fixes this issue by adding a margin of (1/1024) higher to
> > the clock rate.
> >
> > Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
> > Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
> > ---
> > v3->v4:
> >  * Added Tested-by tag from Wolfram.
> >  * Updated commit description and code comment with real example.
> 
> Thanks for the update!
> 
> Unfortunately this patch causes a change in the clock frequencies used
> on R-Car M2-W:
> 
>     -clk_summary:          sd0                 97500000
>     +clk_summary:          sd0                 32500000
>     -clk_summary:             sdhi0            97500000
>     +clk_summary:             sdhi0            32500000
> 
>     -clk_summary:             sd3              12786885
>     +clk_summary:             sd3              12187500
>     -clk_summary:                sdhi3         12786885
>     +clk_summary:                sdhi3         12187500
> 
>     -clk_summary:             sd2              12786885
>     +clk_summary:             sd2              12187500
>     -clk_summary:                sdhi2         12786885
>     +clk_summary:                sdhi2         12187500

That is not good.

> 
> > --- a/drivers/mmc/host/renesas_sdhi_core.c
> > +++ b/drivers/mmc/host/renesas_sdhi_core.c
> 
> > @@ -153,10 +154,17 @@ static unsigned int
> renesas_sdhi_clk_update(struct tmio_mmc_host *host,
> >          * greater than, new_clock.  As we can divide by 1 << i for
> >          * any i in [0, 9] we want the input clock to be as close as
> >          * possible, but no greater than, new_clock << i.
> > +        *
> > +        * Add an upper limit of 1/1024 rate higher to the clock
> rate to fix
> > +        * clk rate jumping to lower rate due to rounding error (eg:
> RZ/G2L has
> > +        * 3 clk sources 533.333333 MHz, 400 MHz and 266.666666 MHz.
> The request
> > +        * for 533.333333 MHz will selects a slower 400 MHz due to
> rounding
> > +        * error (533333333 Hz / 4 * 4 = 533333332 Hz < 533333333
> Hz)).
> >          */
> > +       new_upper_limit = (new_clock << i) + ((new_clock << i) >>
> 10);
> 
> Mea culpa: while new_clock is a constant inside the loop, i is not!
> So it should be moved back inside the loop below.
> With that change, R-Car M2-W is happy again, and I noticed no
> regression on R-Car H3 ES2.0.

OK.

> 
> >         for (i = min(9, ilog2(UINT_MAX / new_clock)); i >= 0; i--) {
> >                 freq = clk_round_rate(ref_clk, new_clock << i);
> > -               if (freq > (new_clock << i)) {
> > +               if (freq > new_upper_limit) {
> >                         /* Too fast; look for a slightly slower
> option */
> >                         freq = clk_round_rate(ref_clk, (new_clock <<
> i) / 4 * 3);
> >                         if (freq > (new_clock << i))
>                                      ^^^^^^^^^^^^^^^^ Probably this
> should become new_upper_limit too, for consistency?
> It doesn't seem to matter in my testing, though.

OK. Will do the below change in next version.

-	new_upper_limit = (new_clock << i) + ((new_clock << i) >> 10);
 	for (i = min(9, ilog2(UINT_MAX / new_clock)); i >= 0; i--) {
 		freq = clk_round_rate(ref_clk, new_clock << i);
+		new_upper_limit = (new_clock << i) + ((new_clock << i) >> 10);
 		if (freq > new_upper_limit) {
 			/* Too fast; look for a slightly slower option */
 			freq = clk_round_rate(ref_clk, (new_clock << i) / 4 * 3);
-			if (freq > (new_clock << i))
+			if (freq > new_upper_limit)
 				continue;
 		}

Cheers,
Biju