STM32MP1 how to setup the SDMMC_CK clock frequency in Linux

[Emmanuel COMBETTE DE RYMON]

In the below sdmmc info command, we see  timing spec is "mmc DDR52",
whereas clock frequency on SDMMC_CK pin is only at 12.5 MHz
why ?
 
root@stm32mp151c-oeswa63:~# cat /sys/kernel/debug/mmc0/ios
clock:          25000000 Hz  >>>> is SDMMC kernel clock
actual clock:   12500000 Hz >>>> is clock frequency on SDMMC_CK pin chosen by the driver
vdd:            21 (3.3 ~ 3.4 V)
bus mode:       2 (push-pull)
chip select:    0 (don't care)
power mode:     2 (on)
bus width:      3 (8 bits)
timing spec:    8 (mmc DDR52)
signal voltage: 0 (3.30 V)
driver type:    0 (driver type B)
 The Linux SDMMC driver decides to takes the highest frequency for the SDMMC_CK pin possible
within the max frequency defined by the SDMMC mode (in device tree binding file and table below)
 
The frequency on SDMMC_CK pin depends SDMMC kernel clock and SDMMC divider.
This divider is at minimum = 2 when the SDMMC is in DDR mode or 1 when in SDR mode
(RM0436rv5 Figure 693) 
 

• Let's take the RCC setup of the command above where:

SDMMC is connected to eMMC in  DDR mode " eMMC HighSpeed" (see table below) with SDMMC kernel clock source = PLL4P.

 
PLL4P settings in TF-A device tree :
 
cfg = < 1 42 20 7 7 PQR(1,0,1) >;
frac = < 0x1800 >;
 
Let's decode the PLL settings from this two device tree lines,
see https://wiki.st.com/stm32mpu/wiki/Clock_device_tree_configuration_-_Bootloader_specific#Defining_peripheral_PLL_frequencies_with_st-2Cpll_property
 
HSE=24Mhz
PLL4 freq= HSE/2 * (42+1 + 0x1800/8192) = 12 * 43.75 = 525MHz
 
20=>DIVP=21 ->525/21=25MHz
0x1800 = 6144
8192 value from RefMan FRACV definition
FracN provide decimal value, between 0 min and almost 1 (8191/8192))

So in current situation, PLL4P is 25MHz.

The driver sets a DDR mode " eMMC HighSpeed" ("mmc-ddr-3_3v" is in sdmmc device tree node)
and selects the highest clock below 52 MHz freq with div=2. It sets SDMMC_CK to PLL4P/2 =12.5 MHz.
 

• Choice of PLL frequency for the SDMMC kernel clock  

Let's assume it was intended that PLL3R(104.5 MHz) is for SDMMC and PLL4P(89.57 MHz) is for Ethernet. Indeed, using for SDMMC PLL3R at 104.5 MHz is worse than PLL4 at 89.5 MHz
 
SDMMC Mode in device Tree : mmc-ddr-3_3v (cf binding and tables below) =>max SDMMC_CK = 52 MHz  ​
Better to set PLL4P=89.57MHz -> SDMMC_CK=44.785
Because if we set 104.5 /2 = 52.25   > 52 MHz so driver will not use div=2.
It will use div=4,  104.5/4 = 26.125 MHz
 
The list of possible  "parent clock" for SDMMC kernel clock
See  https://github.com/STMicroelectronics/u-boot/blob/v2020.10-stm32mp/include/dt-bindings/clock/stm32mp1-clksrc.h
 
The parent clock is selected by the device tree "st,pkcs" property
See https://wiki.st.com/stm32mpu/wiki/Clock_device_tree_configuration_-_Bootloader_specific#Defining_peripheral_kernel_clock_tree_distribution_with_st-2Cpkcs_property
 

Background information

SMMDC mode is set according to the flash device and the expected speed .
All mode are not supported by HW (e.g. HS400 on eMMC)
 
The driver modes are defined in device tree file:
https://github.com/STMicroelectronics/linux/blob/v5.10-stm32mp/Documentation/devicetree/bindings/mmc/mmc-controller.yaml#L181
 
SDMMC_CK clock max frequency is in tables of RM0436rv5  "SDMMC operation modes"
 It is a maximum, so for example SDMMC in "eMMC High speed DDR" mode can have whatever SDMMC_CK frequency value between 0 and 52MHz.


In RefMan0436 tables present HS200, 200MHz is the max clock is defined here is from the JDEC standard .
On STM32MP1 there is limitation on SDMMC_CK for eMMC and SDCard: Max is not 130MHz but 100MHZ. 
I/O max speed is the limitation.