> -----Original Message-----
> From: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> Sent: Monday, October 14, 2024 11:21 AM
> To: Biju Das <biju.das.jz@xxxxxxxxxxxxxx>
> Cc: Michael Turquette <mturquette@xxxxxxxxxxxx>; Stephen Boyd <sboyd@xxxxxxxxxx>; linux-renesas-
> soc@xxxxxxxxxxxxxxx; linux-clk@xxxxxxxxxxxxxxx; Prabhakar Mahadev Lad <prabhakar.mahadev-
> lad.rj@xxxxxxxxxxxxxx>; biju.das.au <biju.das.au@xxxxxxxxx>; Hien Huynh <hien.huynh.px@xxxxxxxxxxx>
> Subject: Re: [PATCH v2] clk: renesas: rzg2l: Fix FOUTPOSTDIV clk
>
> Hi Biju,
>
> On Mon, Oct 14, 2024 at 11:55 AM Biju Das <biju.das.jz@xxxxxxxxxxxxxx> wrote:
> > > From: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> On Fri, Oct 11, 2024
> > > at 6:20 PM Biju Das <biju.das.jz@xxxxxxxxxxxxxx> wrote:
> > > > While computing foutpostdiv_rate, the value of params->pl5_fracin
> > > > is discarded, which results in the wrong refresh rate. Fix the
> > > > formula for computing foutpostdiv_rate.
> > > >
> > > > Fixes: 1561380ee72f ("clk: renesas: rzg2l: Add FOUTPOSTDIV clk
> > > > support")
> > > > Signed-off-by: Hien Huynh <hien.huynh.px@xxxxxxxxxxx>
> > > > Signed-off-by: Biju Das <biju.das.jz@xxxxxxxxxxxxxx>
> > > > ---
> > > > v1->v2:
> > > > * Improved the precision by division of params->pl5_refdiv
> > > > done after all multiplication.
> > > > ---
> > > > drivers/clk/renesas/rzg2l-cpg.c | 12 +++++++-----
> > > > 1 file changed, 7 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/drivers/clk/renesas/rzg2l-cpg.c
> > > > b/drivers/clk/renesas/rzg2l-cpg.c index 88bf39e8c79c..a1e22d353689
> > > > 100644
> > > > --- a/drivers/clk/renesas/rzg2l-cpg.c
> > > > +++ b/drivers/clk/renesas/rzg2l-cpg.c
> > > > @@ -548,7 +548,7 @@ static unsigned long
> > > > rzg2l_cpg_get_foutpostdiv_rate(struct rzg2l_pll5_param *params,
> > > > unsigned long rate) {
> > > > - unsigned long foutpostdiv_rate;
> > > > + unsigned long foutpostdiv_rate, foutvco_rate;
> > >
> > > While the resulting 64-bit value fits in foutvco_rate because
> > > unsigned long is 64-bit on the target platform, I'd rather play it
> > > safe
> > > (reuse!) and use u64 explicitly.
> >
> > OK will use u64.
> >
> > >
> > > >
> > > > params->pl5_intin = rate / MEGA;
> > > > params->pl5_fracin = div_u64(((u64)rate % MEGA) << 24,
> > > > MEGA); @@ -557,10 +557,12 @@ rzg2l_cpg_get_foutpostdiv_rate(struct rzg2l_pll5_param *params,
> > > > params->pl5_postdiv2 = 1;
> > > > params->pl5_spread = 0x16;
> > > >
> > > > - foutpostdiv_rate =
> > > > - EXTAL_FREQ_IN_MEGA_HZ * MEGA / params->pl5_refdiv *
> > > > - ((((params->pl5_intin << 24) + params->pl5_fracin)) >> 24) /
> > > > - (params->pl5_postdiv1 * params->pl5_postdiv2);
> > > > + foutvco_rate =
> > > > + (EXTAL_FREQ_IN_MEGA_HZ * MEGA *
> > > > + ((params->pl5_intin << 24) + params->pl5_fracin) /
> > > > + params->pl5_refdiv) >> 24;
> > >
> > > Shouldn't this use mul_u32_u32(EXTAL_FREQ_IN_MEGA_HZ * MEGA,
> > > ((params->pl5_intin << 24) + params->pl5_fracin)) instead of a plain
> > > multiplication?
> > > See also the comment for mul_u32_u32() in <linux/math64.h>.
> >
> > OK. Will use mul_u32_u32().
> >
> > >
> > > > + foutpostdiv_rate = DIV_ROUND_CLOSEST_ULL(foutvco_rate,
> > > > +
> > > > + params->pl5_postdiv1 * params->pl5_postdiv2);
> > >
> > > Unfortunately we don't have a helper macro yet to round the result
> > > of div_u64(), so you will have to open-code that (for now).
> >
> > As per [1], round_closest(x,y) where x is u64 and y is u32
> >
> > In this case max value of x is 3000MHz < 2^32
>
> But that is not obvious from the code (and foutvco_rate is u64 soon?).
> Also, is that guaranteed? What if the user plugs in a 4K or 8K HDMI display?
>
> >
> > and
> >
> > y < 50
> >
> >
> > So, do we need open-code? Am I missing anything here?
> >
> >
> > [1]
> > https://elixir.bootlin.com/linux/v6.0-rc4/source/include/linux/math.h#
> > L101
>
> You mean
> https://elixir.bootlin.com/linux/v6.0-rc4/source/drivers/gpu/ipu-v3/ipu-image-convert.c#L477
> ?
Sorry, I meant for DIV_ROUND_CLOSEST_ULL, I incorrectly wrote it as round_closest(x,y)
https://elixir.bootlin.com/linux/v6.0-rc4/source/include/linux/math.h#L101
Cheers,
Biju
