MMC/SD Introduction

The multimedia card high-speed/SDIO (MMC/SDIO) host controller provides an interface between a local host (LH) such as a microprocessor unit (MPU) or digital signal processor (DSP) and either MMC, SD® memory cards, or SDIO cards and handles MMC/SDIO transactions with minimal LH intervention.


MMC/SD Driver Architecture Acronyms & Definitions

Acronym Definition
MMC Multimedia Card
HS-MMC High Speed MMC
SD Secure Digital
SDHC SD High Capacity
SDIO SD Input/Output

Table: MMC/SD: Acronyms Features Supported High Speed Modes Driver Configuration Steps for working around SD card issues

In some cases failure can be seen while using some SD cards,

  1. Kernel fails to enumerate SD, thus failing to mount the root file system. This is the case when kernel hangs during boot with a message similar to the following,

    [    2.563279] Waiting for root device PARTUUID=835b171b-02...
  2. A lot of SDHCI register dumps logs getting printed continuously,

    [   10.811723] mmc1: Got data interrupt 0x00000002 even though no data operation was in progress.
    [   10.820321] mmc1: sdhci: ============ SDHCI REGISTER DUMP ===========
    [   10.826745] mmc1: sdhci: Sys addr:  0x00000080 | Version:  0x00001004
    [   10.833169] mmc1: sdhci: Blk size:  0x00007200 | Blk cnt:  0x00000080
    [   10.839593] mmc1: sdhci: Argument:  0x00000000 | Trn mode: 0x00000033
    [   10.846016] mmc1: sdhci: Present:   0x01f70000 | Host ctl: 0x0000001f
    [   10.852440] mmc1: sdhci: Power:     0x0000000f | Blk gap:  0x00000080
    [   10.858864] mmc1: sdhci: Wake-up:   0x00000000 | Clock:    0x00000007
    [   10.865287] mmc1: sdhci: Timeout:   0x00000000 | Int stat: 0x00000000
    [   10.871711] mmc1: sdhci: Int enab:  0x03ff008b | Sig enab: 0x03ff008b
    [   10.878134] mmc1: sdhci: ACmd stat: 0x00000000 | Slot int: 0x00000000
    [   10.884557] mmc1: sdhci: Caps:      0x3de8c801 | Caps_1:   0x18002407
    [   10.890981] mmc1: sdhci: Cmd:       0x00000c1a | Max curr: 0x00000000
    [   10.897404] mmc1: sdhci: Resp[0]:   0x00000b00 | Resp[1]:  0x0075cf7f
    [   10.903828] mmc1: sdhci: Resp[2]:   0x32db7900 | Resp[3]:  0x00000900
    [   10.910251] mmc1: sdhci: Host ctl2: 0x0000000b
    [   10.914682] mmc1: sdhci: ADMA Err:  0x00000000 | ADMA Ptr: 0x00000000a2e90200

Given below are the list of various workarounds that can be done in the device tree node to get SD card working. The workarounds are ordered in increasing order of reducing performance.

All the changes mentioned below, are to be done in the MMCSD device tree node corresponding to the SD instance. This is usually the first (index starting from zero) instance.

  1. Restricting to a given speed mode

    • By default the kernel driver tries to enumerate a SD card in the highest supported speed mode. Given below is the order in which the driver tries to enumerate a SD card

      • SDR104
      • DDR50
      • SDR50
      • SD HS
      • SD legacy
    • These speed capabilites can be masked using device tree property sdhci-caps-mask.

      • Limit to DDR50: sdhci-caps-mask = <0x00000002 0x00000000>

      • Limit to SDR50: sdhci-caps-mask = <0x00000006 0x00000000>

      • Limit to SD HS: sdhci-caps-mask = <0x00000007 0x00000000>

      • Limit to SD legacy: sdhci-caps-mask = <0x00000007 0x00200000>

        &sdhci1 {
            /* SD/MMC */
           vmmc-supply = <&vdd_mmc1>;
           vqmmc-supply = <&vdd_sd_dv>;
           pinctrl-names = "default";
           pinctrl-0 = <&main_mmc1_pins_default>;
           ti,driver-strength-ohm = <50>;
           sdhci-caps-mask = <0x00000006 0x00000000>; /* Limiting to SDR50 speed mode */
      • Limiting to SD HS speed mode can also be done by using the property no-1-8-v. This disable switching to 1.8V which is required for UHS speed modes(SDR104, DDR50, SDR50)

        &sdhci1 {
            /* SD/MMC */
            vmmc-supply = <&vdd_mmc1>;
            vqmmc-supply = <&vdd_sd_dv>;
            pinctrl-names = "default";
            pinctrl-0 = <&main_mmc1_pins_default>;
            ti,driver-strength-ohm = <50>;
            no-1-8-v; /* disabling all the UHS modes */
  2. Reduce the bus width

  • The SD interface supports a bus width of 4. It can be reduced to 1 by changing the bus-width device tree property from 4 to 1.

    diff --git a/arch/arm64/boot/dts/ti/k3-am62-main.dtsi b/arch/arm64/boot/dts/ti/k3-am62-main.dtsi
    index 7bbfcb158842..2ef974f7206f 100644
    --- a/arch/arm64/boot/dts/ti/k3-am62-main.dtsi
    +++ b/arch/arm64/boot/dts/ti/k3-am62-main.dtsi
    @@ -424,7 +424,7 @@
                    ti,itap-del-sel-sdr12 = <0x0>;
                    ti,itap-del-sel-sdr25 = <0x0>;
                    ti,clkbuf-sel = <0x7>;
    -               bus-width = <4>;
    +               bus-width = <1>;
     sdhci2: mmc@fa20000 {