SDHCI MMC Driver

This driver project is a part of GSoC’18 and is aimed at providing support for PCI devices with class 8 and subclass 5 over x86 architecture. This document will make you familiar with the code produced during GSoC, loading and testing the driver(including hardware emulation), insight into the code and future tasks.

For detailed explanations about the project, you can refer the weekly reports and comment issues if any. For this project we have referred SD Host Controller Spec Version 1.00 and Physical Layer Spec Version 1.10.

Loading and testing the driver

Emulating the hardware

We will emulate a SDHC device using qemu as all system may not have the device. These days systems provide transfer to SD/ MMC card over USB. The document will not instruct you on how to build haiku but you can refer the link to compile and build the haiku images or the week #1 and #2 project report will also work.

After building the image, we will emulate the hardware and host haiku on top of that.

Emulation

For emulating a sdhci-pci device

qemu-img create sd-card.img 32M
qemu-system-x86_64 -drive index=0,file=haiku-nightly-anyboot.iso,format=raw \
    -device sdhci-pci -device sd-card,drive=mydrive \
    -drive if=sd,index=1,file=sd-card.img,format=raw,id=mydrive
    -m 512M -enable-kvm -usbdevice tablet -machine q35

This does the following: - Create an SD card image of 32MB - Run qemu with a bootable image in an IDE disk, and an SDHCI bus with an SD card in it - Have enough memory to boot Haiku, use KVM mode for speed, and a tablet for ease of use - Use the Q35 chipset so the mouse and SDHCI controllers don’t share an interrupt (not strictly required, but it avoids calls to the SDHCI interrupt handler on every mouse move).

Tracing of SD operations can also be added to see how qemu is interpreting our commands:

-trace sdhci* -trace sdbus* -trace sdcard*

Testing and loading the driver

The code is merged and part of the default Haiku build.

Insight into the code and future tasks

Bus, bus manager, and drivers

The MMC stack is a device manager based “new style” driver. This requires splitting the driver in different parts but allow easy reuse of each part (for example to support eMMC or SDIO with a large part of the code in common with plain SD/MMC).

MMC Bus drivers (src/add-ons/kernel/busses/mmc)

The bus driver provides the low level aspects: interrupts management, DMA transfer, accessing the hardware registers. It acts as a platform abstraction layer so that the bus manager and disk driver can be written independently of the underlying hardare.

Currently there is a single implementation for SDHCI (MMC bus over PCI). Later on, other drivers will be added for other ways to access the MMC bus (for example on ARM devices where it does not live on a PCI bus, and may have a different register layout).

For this reason, the bus drivers should only do the most low-level things, trying to keep as much code as possible in the upper layers.

One slightly confusing thing about SDHCI is that it allows a single PCI device to implement multiple separate MMC busses (each of which could have multiple devices attached). For this reason there is an SDHCI “device” that attaches to the PCI device node for the controller, and then publishes multiple device nodes for each available bus. The nodes then work independently of each other.

The Bus Manager (src/add-ons/kernel/bus_managers/mmc)

The bus manager is responsible for enumerating devices on the bus, assigning them addresses, and keeping track of which card is active at any given time.

Essentially it has everything that requires collaboration between multiple MMC devices, as well as things that are not specific to a device type (common to SDIO, SD and MMC cards, for example)

Disk Driver (src/add-ons/kernel/drivers/disk/mmc)

This is a mass storage driver for MMC, SD and SDHC cards. Currently only SD and SDHC are tested, MMC and eMMC will have to be added (they are similar but there are some differences).

Wiring the driver in the device manager (src/system/kernel/device_manager/device_manager.cpp)

(note: possibly not accurate documentation, I did not check how things in the device manager are actually implemented, but this is my understanding of it).

The device manager attempts to implement lazy, on-demand scanning of the devices. The idea is to speed up booting by not spending a lot of time scanning everything first, and only scanning small parts of the device tree as they are needed.

The trigger is accesses to the devfs. For example, when an application opens /dev/disk, the device manager will start looking for disks so it can populate it. This means the device manager needs to know which branches of the device tree to explore. Currently this knowledge is hardcoded into the device tree sourcecode, and there’s a TODO item about moving that knowledge to drivers instead. But it’s tricky, since the whole point is to avoid loading all the drivers.

Anyway, currently, the device manager is hardcoded to look for mass storage devices under SDHCI busses, both standard ones and some non-standard ones (for example, Ricoh provides SDHCI implenentations that are conform to the spec, except they don’t have the right device type in the PCI registers).

Insight into the code

MMC Bus management overview

The device tree for MMC support looks like this:

  • PCI bus manager

    • (other PCI devices)

    • SDHCI controller

      • SDHCI bus

        • MMC bus manager

          • MMC device

            • mmc_disk device

          • MMC device

            • (other SDIO driver)

        • MMC bus manager (second MMC bus)

          • MMC device

            • mmc_disk device

At the first level, the PCI bus manager publishes a device node for each device found. One of them is our SDHCI controller, identified either by the PCI device class and subclass, or for not completely SDHCI compatible device, by the device and vendor IDs.

The SDHCI bus driver attaches to this device and publishes his own node. It then scans the device and publishes an MMC bus node for each slot (there may be multiple SD slots attached to a single PCI controller).

The MMC bus manager then attach to each of these slots, and send the appropriate commands for enumerating the SD cards (there may be multiple cards in a “slot”), and publishes a device node for each of them. Finally, the mmc_disk driver can bind itself to one of these device nodes, and publish the corresponding disk node, which is also be made available in /dev/disk/mmc.

Currently the mmc bus does not publish anything in the devfs, but this could be added if sending raw SD/MMC commands to SD cards from userland is considered desirable.

SDHCI driver

The SDHCI driver is the lowest level of the MMC stack. It provides abstraction of the SDHCI device. Later on, different way to access an SD bus may be added, for example for ARM devices which decided to use a different register interface.

The entry point is as usual supports_device(). This method is called only for devices which may be SDHCI controllers, thanks to filtering done in the device manager to probe only the relevant devices. The probing is done on-demand, currently when the system is enumerating /dev/disk in the devfs. Later on, when we have SDIO support, probing will also be triggered in other cases.

The function identifies the device by checking the class and subclass, as well as a limited set of hardcoded PCI device and vendor IDs for devices that do not use the assigned subclass.

Once a compatible device is found, register_child_devices() is used to publish device nodes for each slot to be controlled by the mmc bus manager. The registers for each device are mapped into virtual memory, using the information from the PCI bar registers. struct registers is defined so that it matches the register layout, and provide a little abstraction to raw register access.

An SdhciBus object is created to manage each of these busses at the SDHCI level. It will be responsible for executing SD commands on that bus, and dealing with the resulting interrupts.

The Bus Manager

The MMC bus manager manages the MMC bus (duh). Its tasks are:

  • enumerating SD cards on the bus

  • assigning RCAs to the cards for identifying them when sending commands

  • setting the bus clock speed according to what the cards can handle

  • remember which SD card is currently active (CMD7)

  • manage cards state

  • publish device nodes for each card

Disk Driver

The disk driver is attached to devices implementing SDSC or SDHC/SDXC commands. There will be other drivers for non-storage (SDIO) cards.

To help with this, the MMC bus manager provides the device with the information it gathered while initializing the device. According to the commands recognized by the card during the initialization sequence, it’s possible to know if it’s SDSC, SDHC/SDXC, or something else (SDIO, legacy MMC, etc).

The disk driver publishes devfs entries in /dev/disk/mmc and implements the usual interface for disk devices. From this point on, the device can be used just like any other mass storage device.

Getting everything loaded

The device manager is not completely implemented yet. As a result, some decisions about which drivers to load are hardcoded in device_manager.cpp.

It has been adjusted to handover SDHCI devices to the MMC bus. Whenever a “disk” device is requested, the MMC busses are searched, which results in loading the SDHCI driver and probing for SD cards. When we get support for other types of SDIO devices, we will need to adjust the device manager to probe the SDHCI bus when these type of devices are requested, too.

Tasks to be completed

The SDHCI driver is able to send and receive commands. However it does not handle card insertion and removal interrupts yet, so the card must be already inserted when the driver is loaded.

The mmc_disk driver is complete and working, but was not tested for MMC and eMMC devices. Some changes may be needed.

There is also work to be done for better performance: making sure we switch to the high-speed clock when an SD card supports it, and use the 4-bit data transfer mode instead of the default 1-bit if possible.

Drivers for SDIO devices should also be added. The mmc_bus and SDHCI drivers have been tested only with one card on the bus at a time (for lack of hardware allowing more complex setups).

If you find it difficult to understand the driver development and it’s functioning and role, please refer docs/develop/kernel/device_manager_introduction.html