Re: [RFC] ccu-sun50i-h6: Bump-up DDR0 PLL to 1800MHz

From: Jernej Škrabec
Date: Sun Jul 10 2022 - 14:48:03 EST

Hi Roman,

Dne nedelja, 10. julij 2022 ob 19:52:14 CEST je Roman Stratiienko napisal(a):
> While debugging complex animated composition cases I noticed a glitch
> which as it turned-out eventually was caused by lack of memory bandwidth.
> I can't find a DRAMC manual to check what input frequency must be
> supplied to avoid exceeding the 800MHz LPDDR3 which is installed on
> my orangepi3 board. But the system is running stable so far.

this is hard topic. Generally, max. supported DRAM frequency is board specific
and is set by SPL (part of U-Boot), so we can't assume every board supports
max. frequency for its memory type. In fact, this assumption was proven wrong.
Some boards were unstable until DRAM frequency was lowered. So in short, Linux
clock driver certainly shouldn't set DRAM frequency. There is, however, Linux
MBUS driver for H5 and A64, which can throttle DRAM frequency when system is
idle, but max. frequency is still that one which was programmed by SPL.

There is no definitive DRAM documentation, because Allwinner usually tries to
hide all details. DRAM drivers are, except in rare cases, all reverse
engineered by community. Register values (mostly) match those produced by
vendor driver. Fortunately, at least H6 DRAM controller matches pretty good
with one other memory controller (one in Zynq UltraScale+ MPSoC), which is
fully documented (see comments in U-Boot driver). MBUS is also pretty standard
and it's not a mystery.

Note that it's hard to convince maintainers to either lower or increase DRAM
frequency as it was common issue in the past. You have to do a lot of memory
intensive tests, both CPU and GPU, running at the same time for extended
period of time. Initial board bring ups usually just use same frequency as in
BSP based images. If I understand procedure correctly, boards are sent to
Allwinner, so their engineers determine max. DRAM frequency and timings, so
these values are pretty trustwothy.

There is other possibility. Along with DRAM controller and PHY, there is MBUS
core, which is memory arbiter and responsible for bandwidth allocation. You
can change DE priority and bandwith there (again, in U-Boot DRAM driver).

I hope this explains why things are the way they are.

Best regards,

> Signed-off-by: Roman Stratiienko <r.stratiienko@xxxxxxxxx>
> ---
> drivers/clk/sunxi-ng/ccu-sun50i-h6.c | 11 +++++++++++
> 1 file changed, 11 insertions(+)
> diff --git a/drivers/clk/sunxi-ng/ccu-sun50i-h6.c
> b/drivers/clk/sunxi-ng/ccu-sun50i-h6.c index 750368a86b8b6..abdde80307993
> 100644
> --- a/drivers/clk/sunxi-ng/ccu-sun50i-h6.c
> +++ b/drivers/clk/sunxi-ng/ccu-sun50i-h6.c
> @@ -1203,6 +1203,17 @@ static int sun50i_h6_ccu_probe(struct platform_device
> *pdev) val &= ~GENMASK(3, 0);
> writel(val, reg + gpu_clk.common.reg);
> + /*
> + * Increase DDR0 PLL from 1488(default) to 1800MHz.
> + * (DE3.0 require higher memory bandwidth while displaying
> + * complex composition at 1920x1080@60FPS)
> + */
> +
> + val = readl(reg + SUN50I_H6_PLL_DDR0_REG);
> + val &= ~GENMASK(15, 0);
> + val |= 74 << 8;
> + writel(val, reg + SUN50I_H6_PLL_DDR0_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