arm64: dts: rockchip: Fix eMMC Data Strobe PD on rk3588

JEDEC standard JESD84-B51 defines the eMMC Data Strobe line, which is
currently used only in HS400 mode, as a device->host clock signal that
"is used only in read operation. The Data Strobe is always High-Z (not
driven by the device and pulled down by RDS) or Driven Low in write
operation, except during CRC status response." RDS is a pull-down
resistor specified in the 10K-100K ohm range. Thus per the standard, the
Data Strobe is always pulled to ground (by the eMMC and/or RDS) during
write operations.

Evidently, the eMMC host controller in the RK3588 considers an active
voltage on the eMMC-DS line during a write to be an error.

The default (i.e. hardware reset, and Rockchip BSP) behavior for the
RK3588 is to activate the eMMC-DS pin's builtin pull-down. As a result,
many RK3588 board designers do not bother adding a dedicated RDS
resistor, instead relying on the RK3588's internal bias. The current
devicetree, however, disables this bias (`pcfg_pull_none`), breaking
HS400-mode writes for boards without a dedicated RDS, but with an eMMC
chip that chooses to High-Z (instead of drive-low) the eMMC-DS line.
(The Turing RK1 is one such board.)

Fix this by changing the bias in the (common) emmc_data_strobe case to
reflect the expected hardware/BSP behavior. This is unlikely to cause
regressions elsewhere: the pull-down is only relevant for High-Z eMMCs,
and if this is redundant with a (dedicated) RDS resistor, the effective
result is only a lower resistance to ground -- where the range of
tolerance is quite high. If it does, it's better fixed in the specific
devicetrees.

Fixes: d85f8a5 ("arm64: dts: rockchip: Add rk3588 pinctrl data")
Signed-off-by: Sam Edwards <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Heiko Stuebner <[email protected]>

Author: CFSworks

arch/arm64/boot/dts/rockchip/rk3588s-pinctrl.dtsi

@@ -369,7 +369,7 @@
 		emmc_data_strobe: emmc-data-strobe {
 			rockchip,pins =
 				/* emmc_data_strobe */
-				<2 RK_PA2 1 &pcfg_pull_none>;
+				<2 RK_PA2 1 &pcfg_pull_down>;
 		};
 	};