linux-kernel - Re: Re: [PATCH v3] clk: sunxi-ng: sun50i: h6: Modify GPU clock configuration to support DFS

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Date:   Tue, 05 Jul 2022 20:06:59 +0200
From:   Jernej Škrabec <jernej.skrabec@...il.com>
To:     Roman Stratiienko <r.stratiienko@...il.com>
Cc:     Samuel Holland <samuel@...lland.org>,
        Clément Péron <peron.clem@...il.com>,
        Michael Turquette <mturquette@...libre.com>, sboyd@...nel.org,
        mripard@...nel.org, wens@...e.org, linux-clk@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-sunxi@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: Re: [PATCH v3] clk: sunxi-ng: sun50i: h6: Modify GPU clock configuration to support DFS

Dne torek, 05. julij 2022 ob 18:29:39 CEST je Roman Stratiienko napisal(a):
> Hi Jernej,
> 
> вт, 5 июл. 2022 г. в 19:07, Jernej Škrabec <jernej.skrabec@...il.com>:
> > Hi Roman,
> > 
> > Dne torek, 05. julij 2022 ob 09:52:26 CEST je Roman Stratiienko 
napisal(a):
> > > Using simple bash script it was discovered that not all CCU registers
> > > 
> > > can be safely used for DFS, e.g.:
> > >     while true
> > >     do
> > >     
> > >         devmem 0x3001030 4 0xb0003e02
> > >         devmem 0x3001030 4 0xb0001e02
> > >     
> > >     done
> > > 
> > > Script above changes the GPU_PLL multiplier register value. While the
> > > script is running, the user should interact with the user interface.
> > > 
> > > Using this method the following results were obtained:
> > > | Register  | Name           | Bits  | Values | Result |
> > > | --        | --             | --    | --     | --     |
> > > | 0x3001030 | GPU_PLL.MULT   | 15..8 | 20-62  | OK     |
> > > | 0x3001030 | GPU_PLL.INDIV  |     1 | 0-1    | OK     |
> > > | 0x3001030 | GPU_PLL.OUTDIV |     0 | 0-1    | FAIL   |
> > > | 0x3001670 | GPU_CLK.DIV    |  3..0 | ANY    | FAIL   |
> > > 
> > > DVFS started to work seamlessly once dividers which caused the
> > > glitches were set to fixed values.
> > > 
> > > Signed-off-by: Roman Stratiienko <r.stratiienko@...il.com>
> > > 
> > > ---
> > > 
> > > Changelog:
> > > 
> > > V2:
> > > - Drop changes related to mux
> > > - Drop frequency limiting
> > > - Add unused dividers initialization
> > > 
> > > V3:
> > > - Adjust comments
> > 
> > I don't see any comment fixed, at least not to "1", as we discussed. Did I
> > miss anything?
> 
> I've added the "bits" word, so now it should sound correct.

Technically it's correct, but this would be third form of comments for fixed 
bits. Let's stick to the form which is most informative ("Force PLL_GPU output 
divider to 1"). Ideally, comment would also point to gpu_clk comment for 
reason why, like it's done for video PLL block already.

> 
> > Also, please add min and max.
> 
> What is the rationale for additional limits?

If limits are specified in whatever form, they should be added. As I said 
several times already, vendor code limits PLL frequency to 288 MHz minimum and 
lists maximum. As experienced a few times before with video PLLs, these are 
important, otherwise PLL is unstable. For example, OPP table in vendor DT has 
two operating points lower than 288 MHz, which means it would either lock up 
or be unstable. In such cases, vendor code actually sets GPU_CLK divider to 2, 
but we can skip them, because GPU_CLK divider will be hardcoded to 1 with this 
patch.

> CPU_PLL doesn't have these limits. I don't want to make them different.

Why CPU_PLL? Why not video PLL? In any case, it doesn't matter if struct looks 
similar to some other or is unique. Only important thing is that struct 
describes PLL as best as possible.

> 
> > I also consent to R-B, which you
> > didn't include.
> 
> I was expecting an explicit 'review-by' line. Anyway I can add it and
> resend v4 if it's necessary.

If you at least add min and max limits, you can add following tag:
Reviewed-by: Jernej Skrabec <jernej.skrabec@...il.com>

If you send it before Friday, it will be in 5.20.

Best regards,
Jernej

> 
> Regards,
> Roman
> 
> > Did you resend v2 instead of v3?
> > 
> > Best regards,
> > Jernej
> > 
> > > ---
> > > 
> > >  drivers/clk/sunxi-ng/ccu-sun50i-h6.c | 16 +++++++++++++---
> > >  1 file changed, 13 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/drivers/clk/sunxi-ng/ccu-sun50i-h6.c
> > > b/drivers/clk/sunxi-ng/ccu-sun50i-h6.c index
> > > 2ddf0a0da526f..068d1a6b2ebf3
> > > 100644
> > > --- a/drivers/clk/sunxi-ng/ccu-sun50i-h6.c
> > > +++ b/drivers/clk/sunxi-ng/ccu-sun50i-h6.c
> > > @@ -95,13 +95,13 @@ static struct ccu_nkmp pll_periph1_clk = {
> > > 
> > >       },
> > >  
> > >  };
> > > 
> > > +/* For GPU PLL, using an output divider for DFS causes system to fail
> > > */
> > > 
> > >  #define SUN50I_H6_PLL_GPU_REG                0x030
> > >  static struct ccu_nkmp pll_gpu_clk = {
> > >  
> > >       .enable         = BIT(31),
> > >       .lock           = BIT(28),
> > >       .n              = _SUNXI_CCU_MULT_MIN(8, 8, 12),
> > >       .m              = _SUNXI_CCU_DIV(1, 1), /* input divider */
> > > 
> > > -     .p              = _SUNXI_CCU_DIV(0, 1), /* output divider
> > 
> > */
> > 
> > >       .common         = {
> > >       
> > >               .reg            = 0x030,
> > >               .hw.init        = CLK_HW_INIT("pll-gpu", "osc24M",
> > > 
> > > @@ -294,9 +294,9 @@ static SUNXI_CCU_M_WITH_MUX_GATE(deinterlace_clk,
> > > "deinterlace", static SUNXI_CCU_GATE(bus_deinterlace_clk,
> > > "bus-deinterlace", "psi-ahb1-ahb2", 0x62c, BIT(0), 0);
> > > 
> > > +/* Keep GPU_CLK divider const to avoid DFS instability. */
> > > 
> > >  static const char * const gpu_parents[] = { "pll-gpu" };
> > > 
> > > -static SUNXI_CCU_M_WITH_MUX_GATE(gpu_clk, "gpu", gpu_parents, 0x670,
> > > -                                    0, 3,    /* M */
> > > +static SUNXI_CCU_MUX_WITH_GATE(gpu_clk, "gpu", gpu_parents, 0x670,
> > > 
> > >                                      24, 1,   /* mux */
> > >                                      BIT(31), /* gate */
> > >                                      CLK_SET_RATE_PARENT);
> > > 
> > > @@ -1193,6 +1193,16 @@ static int sun50i_h6_ccu_probe(struct
> > > platform_device *pdev) if (IS_ERR(reg))
> > > 
> > >               return PTR_ERR(reg);
> > > 
> > > +     /* Force PLL_GPU output divider bits to 0 */
> > > +     val = readl(reg + SUN50I_H6_PLL_GPU_REG);
> > > +     val &= ~BIT(0);
> > > +     writel(val, reg + SUN50I_H6_PLL_GPU_REG);
> > > +
> > > +     /* Force GPU_CLK divider bits to 0 */
> > > +     val = readl(reg + gpu_clk.common.reg);
> > > +     val &= ~GENMASK(3, 0);
> > > +     writel(val, reg + gpu_clk.common.reg);
> > > +
> > > 
> > >       /* Enable the lock bits on all PLLs */
> > >       for (i = 0; i < ARRAY_SIZE(pll_regs); i++) {
> > >       
> > >               val = readl(reg + pll_regs[i]);
> > > 
> > > --
> > > 2.34.1