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

[Roman Stratiienko]

Hi,

DVFS was tested as DVFS using devfreq driver, not the script.

The following OPP table was used:
https://github.com/clementperon/linux/commit/add3ef683238095d2721de03601d5b01f2d9ce22

As is already mentioned in the commit message, P causes the issues as well.

Regards,
Roman

сб, 25 июн. 2022 г. в 13:43, Jernej Škrabec <jernej.skrabec at gmail.com>:

>
> Hi Roman,
>
> Dne petek, 24. junij 2022 ob 18:52:11 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   |
> >
> > Once bits that caused system failure disabled (kept default 0),
> > it was discovered that GPU_CLK.MUX was used during DFS for some
> > reason and was causing the failure too.
> >
> > After disabling GPU_PLL.OUTDIV the system started to fail during
> > booting for some reason until the maximum frequency of GPU_PLL
> > clock was limited to 756MHz.
> >
> > After all the changes made DVFS started to work seamlessly.
>
> I appreciate testing effort, but I don't think userspace approach is good way
> for testing DVFS. I see 2 issues:
> - As name already suggest, voltage also plays crucial role for stability. You
> didn't say on which board you tested this, but I assume it has PMIC. Did you
> make sure GPU voltage regulator is always at 1.04 V, which is needed for 756
> MHz?
> - Kernel clock driver always goes through proper procedure for clock rate
> change, which involves several steps. Bypassing them might also cause some
> stability problems.
>
> I agree that GPU PLL should be limited to 756 MHz max. This seems to be
> maximum operating point specified at vendor DT. But I managed to extract some
> more information from vendor GPU driver. More specifically, from this snippet,
> located in modules/gpu/mali-midgard/kernel_mode/driver/drivers/gpu/arm/
> midgard/platform/sunxi/mali_kbase_config_sunxi.c:
>
> pll_freq = target->freq;
> while (pll_freq < 288000000)
>         pll_freq *= 2;
>
> err = clk_set_rate(sunxi_mali->gpu_pll_clk, pll_freq);
> <...>
> err = clk_set_rate(kbdev->clock, target->freq);
> <...>
>
> Apparently, minimum stable PLL frequency is 288 MHz (this should be added) and
> divider in peripheral clock can really be used, although preferably not.
> Vendor GPU operating points specify only 2 lower than 288 MHz points - at 264
> MHz and 216 MHz. I'm fully aware that they may not be really stable and given
> that these two and next two all share minimum voltage of 810 mV, power and
> thermal savings are probably not that great, so we can skip them and pin
> peripheral divider to 1, as you already did.
>
> Another discrepancy I see is that vendor DT has two operating points, at 336
> MHz and 384 MHz, which also use factor P (also known as d2 in vendor clock
> source). This can be again an oversight or alternatively, it can be that P
> factor can actually be used, but just with lower frequencies.
>
> Can you please make another test with GPU operating points specified in DT and
> check if it works with P factor left in?
>
> For reference, vendor DT has following operating points (kHz, uV):
> 756000 1040000
> 624000 950000
> 576000 930000
> 540000 910000
> 504000 890000
> 456000 870000
> 432000 860000
> 420000 850000
> 408000 840000
> 384000 830000
> 360000 820000
> 336000 810000
> 312000 810000
> 264000 810000
> 216000 810000
>
> Best regards,
> Jernej
>
> >
> > Signed-off-by: Roman Stratiienko <r.stratiienko at gmail.com>
> > ---
> >  drivers/clk/sunxi-ng/ccu-sun50i-h6.c | 12 +++++-------
> >  1 file changed, 5 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/clk/sunxi-ng/ccu-sun50i-h6.c
> > b/drivers/clk/sunxi-ng/ccu-sun50i-h6.c index 2ddf0a0da526f..d941238cd178a
> > 100644
> > --- a/drivers/clk/sunxi-ng/ccu-sun50i-h6.c
> > +++ b/drivers/clk/sunxi-ng/ccu-sun50i-h6.c
> > @@ -95,13 +95,14 @@ 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
> */
> > +     .max_rate       = 756000000UL,
> >       .common         = {
> >               .reg            = 0x030,
> >               .hw.init        = CLK_HW_INIT("pll-gpu", "osc24M",
> > @@ -294,12 +295,9 @@ 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);
> >
> > -static const char * const gpu_parents[] = { "pll-gpu" };
> > -static SUNXI_CCU_M_WITH_MUX_GATE(gpu_clk, "gpu", gpu_parents, 0x670,
> > -                                    0, 3,    /* M */
> > -                                    24, 1,   /* mux */
> > -                                    BIT(31), /* gate */
> > -                                    CLK_SET_RATE_PARENT);
> > +/* GPU_CLK divider kept disabled to avoid interferences with DFS */
> > +static SUNXI_CCU_GATE(gpu_clk, "gpu", "pll-gpu", 0x670,
> > +                   BIT(31), CLK_SET_RATE_PARENT);
> >
> >  static SUNXI_CCU_GATE(bus_gpu_clk, "bus-gpu", "psi-ahb1-ahb2",
> >                     0x67c, BIT(0), 0);
>
>
>
>