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

[Luca Ceresoli <luca.ceresoli@bootlin.com>]

Hi Dmitry,

On Fri, 4 Nov 2022 00:44:16 +0300
Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:

> On 11/2/22 11:32, Luca Ceresoli wrote:
> > Hello Dmitry,
> > 
> > On Mon, 31 Oct 2022 03:34:07 +0300
> > Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:
> >   
> >> On 10/28/22 10:48, luca.ceresoli@bootlin.com wrote:  
> >>> From: Luca Ceresoli <luca.ceresoli@bootlin.com>
> >>>
> >>> 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@vger.kernel.org
> >>> Cc: Peter De Schrijver <pdeschrijver@nvidia.com>
> >>> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
> >>> ---
> >>>  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@tbergstrom-lnx.Nvidia.com/
> >>
> >> 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
> >    */  
> 
> If host1x isn't the only clock like that, then comment shouldn't be
> directed to host1x. Have you checked other clocks?

No, apologies, I don't know enough about this SoC to be able to put
into a comment anything interesting other than what you wrote in your
previous reply.

> I'm curious who made that change originally in your downstream, was it
> coming from NVIDIA?

It is coming from our customer, not sure where they got it initially,
but this is the commit where it was added, with a DIV_U71 flag:

https://osdn.net/projects/android-x86/scm/git/kernel/commits/d861196163e30c07add471562b45dce38517c9b2

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