Re: [PATCH] clk: tegra: fix HOST1X clock divider on Tegra20 and Tegra30

[Luca Ceresoli]

Hello Dmitry,

On Mon, 31 Oct 2022 03:34:07 +0300
Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote:

> On 10/28/22 10:48, luca.ceresoli@xxxxxxxxxxx wrote:
> > From: Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>
> > 
> > On Tegra20 and Tegra30 the HOST1X clock is a fractional clock divider with
> > 7 integer bits + 1 decimal bit. This has been verified on both
> > documentation and real hardware for Tegra20 an on the documentation I was
> > able to find for Tegra30.
> > 
> > However in the kernel code this clock is declared as an integer divider. A
> > consequence of this is that requesting 144 MHz for HOST1X which is fed by
> > pll_p running at 216 MHz would result in 108 MHz (216 / 2) instead of 144
> > MHz (216 / 1.5).
> > 
> > Fix by replacing the INT() macro with the MUX() macro which, despite the
> > name, defines a fractional divider. The only difference between the two
> > macros is the former does not have the TEGRA_DIVIDER_INT flag.
> > 
> > Also move the line together with the other MUX*() ones to keep the existing
> > file organization.
> > 
> > Fixes: 76ebc134d45d ("clk: tegra: move periph clocks to common file")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Cc: Peter De Schrijver <pdeschrijver@xxxxxxxxxx>
> > Signed-off-by: Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>
> > ---
> >  drivers/clk/tegra/clk-tegra-periph.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/clk/tegra/clk-tegra-periph.c b/drivers/clk/tegra/clk-tegra-periph.c
> > index 4dcf7f7cb8a0..806d835ca0d2 100644
> > --- a/drivers/clk/tegra/clk-tegra-periph.c
> > +++ b/drivers/clk/tegra/clk-tegra-periph.c
> > @@ -615,7 +615,6 @@ static struct tegra_periph_init_data periph_clks[] = {
> >  	INT("vde", mux_pllp_pllc_pllm_clkm, CLK_SOURCE_VDE, 61, 0, tegra_clk_vde),
> >  	INT("vi", mux_pllm_pllc_pllp_plla, CLK_SOURCE_VI, 20, 0, tegra_clk_vi),
> >  	INT("epp", mux_pllm_pllc_pllp_plla, CLK_SOURCE_EPP, 19, 0, tegra_clk_epp),
> > -	INT("host1x", mux_pllm_pllc_pllp_plla, CLK_SOURCE_HOST1X, 28, 0, tegra_clk_host1x),
> >  	INT("mpe", mux_pllm_pllc_pllp_plla, CLK_SOURCE_MPE, 60, 0, tegra_clk_mpe),
> >  	INT("2d", mux_pllm_pllc_pllp_plla, CLK_SOURCE_2D, 21, 0, tegra_clk_gr2d),
> >  	INT("3d", mux_pllm_pllc_pllp_plla, CLK_SOURCE_3D, 24, 0, tegra_clk_gr3d),
> > @@ -664,6 +663,7 @@ static struct tegra_periph_init_data periph_clks[] = {
> >  	MUX("owr", mux_pllp_pllc_clkm, CLK_SOURCE_OWR, 71, TEGRA_PERIPH_ON_APB, tegra_clk_owr_8),
> >  	MUX("nor", mux_pllp_pllc_pllm_clkm, CLK_SOURCE_NOR, 42, 0, tegra_clk_nor),
> >  	MUX("mipi", mux_pllp_pllc_pllm_clkm, CLK_SOURCE_MIPI, 50, TEGRA_PERIPH_ON_APB, tegra_clk_mipi),
> > +	MUX("host1x", mux_pllm_pllc_pllp_plla, CLK_SOURCE_HOST1X, 28, 0, tegra_clk_host1x),
> >  	MUX("vi_sensor", mux_pllm_pllc_pllp_plla, CLK_SOURCE_VI_SENSOR, 20, TEGRA_PERIPH_NO_RESET, tegra_clk_vi_sensor),
> >  	MUX("vi_sensor", mux_pllc_pllp_plla, CLK_SOURCE_VI_SENSOR, 20, TEGRA_PERIPH_NO_RESET, tegra_clk_vi_sensor_9),
> >  	MUX("cilab", mux_pllp_pllc_clkm, CLK_SOURCE_CILAB, 144, 0, tegra_clk_cilab),  
> 
> This was attempted in the past
> https://lore.kernel.org/all/20180723085010.GK1636@xxxxxxxxxxxxxxxxxxxxxxxxx/
> 
> I assume here you're also porting the downstream patches to upstream.
> This one is too questionable. The host1x clock shouldn't affect overall
> performance to begin with. It doesn't make sense to use fractional clock
> just for getting extra KHz.

Thank you for the review and for the pointer!

Indeed I'm not sure this patch brings an actual improvement to my use
case, however I reached it by trying to replicate the configuration on
a known-working kernel 3.1, which uses a 1.5 divider. This seems to be
the same reason that led to the 2018 patch that also got rejected.

I'll be OK with dropping this patch after I have a 100% working setup
with an integer divider, which is very likely given your reply. But it
took time before I found the root cause of this issue, and I would like
to avoid other people waste time in the future, so what about adding a
comment there?

What about:

  /*
   * The host1x clock shouldn't affect overall performance. It doesn't
   * make sense to use fractional clock just for getting extra KHz, so
   * let's pretend it's an integer divider
   */

?

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com