Re: [PATCH 3/3] sunxi: H616: dram: fix LPDDR3 mode register settings

["Jernej Škrabec" <jernej.skrabec@gmail.com>]

Dne ponedeljek, 11. avgust 2025 ob 18:08:16 Srednjeevropski poletni čas je Andre Przywara napisal(a):
> On Mon, 11 Aug 2025 17:49:06 +0200
> Jernej Škrabec <jernej.skrabec@gmail.com> wrote:
> 
> Hi Jernej,
> 
> many thanks for having a look!
> 
> > Dne sobota, 2. avgust 2025 ob 01:49:18 Srednjeevropski poletni čas je Andre Przywara napisal(a):
> > > The JEDEC LPDDR3 spec defines mode register 0 (MR0) as being read-only,
> > > so there is no point in trying to set its value.
> > > Also the H616 memory controller encodes the mode register index to be
> > > written starting from bit 8 in MRCTRL1 (for LPDDR3 and LPDDR4 chips), so
> > > we need to OR in that number to tell the controller which MR to program.
> > > 
> > > On top of that, the mode registers between DDR3 and LPDDR3 are
> > > completely different, so writing values crafted for DDR3 into a LPDDR3
> > > chip is just wrong. Due to the above mentioned bugs the writes for
> > > MR0-MR2 did not have any effect (as they were all trying to set the
> > > read-only MR0), so the mode registers just stayed unchanged.  
> > 
> > Nice catch! Looking at BSP DRAM code, it only sets MR1, MR2 and MR3.
> > 
> > > 
> > > Looking at the LPDDR3 spec and the BSP code, let's write the proper MR
> > > values into LPDDR3 chips, using the proper addressing mode.  
> > 
> > Please explain how you find those values. Are they always set in this way
> > for all boards using LPDDR3?
> 
> Yeah, that's a good question! At first, I tried using what JEDEC describes
> as the default settings, but that didn't work. Then I looked into some
> parameters found in some BSP image dumps, but that didn't work either.
> JEDEC describes those MRs as write-only, and I haven't tried whether
> reading them would work nevertheless, not sure about the exact algorithm
> for reading MRs anyway.
> What eventually worked was to use the BSP values written by exactly the
> boot0 on the eMMC that boots an LPDDR3 system - which is actually only one
> in mainline: the Tanix TX1.
> 
> So yeah, those values work for me (TM), and there is only one LPDDR3 board
> supported, so fingers crossed. But actually I wonder if those MR values
> really belong into the timing routine (mctl_set_timing_params(), since
> parts depend on the speed bin. I see some other SoC/DRAM type combinations
> doing that already (H6 DDR3/LPDDR3 and A523 LPDDR4), maybe we should
> follow suit here?

Historically, I checked several boards and if MR value was the same in all
configs, I just hardcoded the value. While this is a bit naive, it works most
of the time. There are some cases where it is set dynamically in BSP code. Those
cases can be a bit annoying to find. Ideally, MR values shout be Kconfig
option with default values.

It's up to you to decide which approach to take. Since it affects just one
board and changes were tested:

Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>

Best regards,
Jernej

> 
> Cheers,
> Andre
> 
> 
> > 
> > Best regards,
> > Jernej
> > 
> > > Use the opportunity to document the LPDDR3 mode register bits written.
> > > 
> > > Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> > > ---
> > >  arch/arm/mach-sunxi/dram_sun50i_h616.c | 12 ++++++------
> > >  1 file changed, 6 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/arch/arm/mach-sunxi/dram_sun50i_h616.c b/arch/arm/mach-sunxi/dram_sun50i_h616.c
> > > index 877181016f3..3345c9b8e82 100644
> > > --- a/arch/arm/mach-sunxi/dram_sun50i_h616.c
> > > +++ b/arch/arm/mach-sunxi/dram_sun50i_h616.c
> > > @@ -1078,18 +1078,18 @@ static bool mctl_phy_init(const struct dram_para *para,
> > >  		mctl_await_completion(&mctl_ctl->mrctrl0, BIT(31), 0);
> > >  		break;
> > >  	case SUNXI_DRAM_TYPE_LPDDR3:
> > > -		writel(mr0, &mctl_ctl->mrctrl1);
> > > -		writel(0x800000f0, &mctl_ctl->mrctrl0);
> > > -		mctl_await_completion(&mctl_ctl->mrctrl0, BIT(31), 0);
> > > -
> > > -		writel(4, &mctl_ctl->mrctrl1);
> > > +		/* MR0 is read-only */
> > > +		/* MR1: nWR=14, BL8 */
> > > +		writel(0x183, &mctl_ctl->mrctrl1);
> > >  		writel(0x800000f0, &mctl_ctl->mrctrl0);
> > >  		mctl_await_completion(&mctl_ctl->mrctrl0, BIT(31), 0);
> > >  
> > > -		writel(mr2, &mctl_ctl->mrctrl1);
> > > +		/* MR2: no WR leveling, WL set A, use nWR>9, nRL=14/nWL=8 */
> > > +		writel(0x21c, &mctl_ctl->mrctrl1);
> > >  		writel(0x800000f0, &mctl_ctl->mrctrl0);
> > >  		mctl_await_completion(&mctl_ctl->mrctrl0, BIT(31), 0);
> > >  
> > > +		/* MR3: 34.3 Ohm pull-up/pull-down resistor */
> > >  		writel(0x301, &mctl_ctl->mrctrl1);
> > >  		writel(0x800000f0, &mctl_ctl->mrctrl0);
> > >  		mctl_await_completion(&mctl_ctl->mrctrl0, BIT(31), 0);
> > >   
> > 
> > 
> > 
> > 
> 
>