The U-Boot Driver Model Project
===============================
MMC system analysis
===================
Marek Vasut <marek.vasut@gmail.com>
2012-02-25

I) Overview
-----------

The MMC subsystem is already quite dynamic in it's nature. It's only necessary
to flip the subsystem to properly defined API.

The probing process of MMC drivers start by calling "mmc_initialize()",
implemented by MMC framework, from the architecture initialization file. The
"mmc_initialize()" function in turn calls "board_mmc_init()" function and if
this doesn't succeed, "cpu_mmc_init()" function is called. It is important to
note that both of the "*_mmc_init()" functions have weak aliases to functions
which automatically fail.

Both of the "*_mmc_init()" functions though serve only one purpose. To call
driver specific probe function, which in turn actually registers the driver with
MMC subsystem. Each of the driver specific probe functions is currently done in
very ad-hoc manner.

The registration with the MMC subsystem is done by calling "mmc_register()",
whose argument is a runtime configured structure of information about the MMC
driver. Currently, the information structure is intermixed with driver's internal
data. The description of the structure follows:

struct mmc {
 /*
  * API: Allows this driver to be a member of the linked list of all MMC drivers
  *      registered with MMC subsystem
  */
  struct list_head link;

  /* DRIVER: Name of the registered driver */
  char name[32];

  /* DRIVER: Driver's private data */
  void *priv;

  /* DRIVER: Voltages the host bus can provide */
  uint voltages;

  /* API: Version of the card */
  uint version;

  /* API: Test if the driver was already initialized */
  uint has_init;

  /* DRIVER: Minimum frequency the host bus can provide */
  uint f_min;

  /* DRIVER: Maximum frequency the host bus can provide */
  uint f_max;

  /* API: Is the card SDHC */
  int high_capacity;

  /* API: Actual width of the bus used by the current card */
  uint bus_width;

  /*
   * DRIVER: Clock frequency to be configured on the host bus, this is read-only
   *         for the driver.
   */
  uint clock;

  /* API: Capabilities of the card */
  uint card_caps;

  /* DRIVER: MMC bus capabilities */
  uint host_caps;

  /* API: Configuration and ID data retrieved from the card */
  uint ocr;
  uint scr[2];
  uint csd[4];
  uint cid[4];
  ushort rca;

  /* API: Partition configuration */
  char part_config;

  /* API: Number of partitions */
  char part_num;

  /* API: Transmission speed */
  uint tran_speed;

  /* API: Read block length */
  uint read_bl_len;

  /* API: Write block length */
  uint write_bl_len;

  /* API: Erase group size */
  uint erase_grp_size;

  /* API: Capacity of the card */
  u64 capacity;

  /* API: Descriptor of this block device */
  block_dev_desc_t block_dev;

  /* DRIVER: Function used to submit command to the card */
  int (*send_cmd)(struct mmc *mmc,
                  struct mmc_cmd *cmd, struct mmc_data *data);

  /* DRIVER: Function used to configure the host */
  void (*set_ios)(struct mmc *mmc);

  /* DRIVER: Function used to initialize the host */
  int (*init)(struct mmc *mmc);

  /* DRIVER: Function used to report the status of Card Detect pin */
  int (*getcd)(struct mmc *mmc);

  /*
   * DRIVER: Maximum amount of blocks sent during multiblock xfer,
   *         set to 0 to autodetect.
   */
  uint b_max;
};

The API above is the new API used by most of the drivers. There're still drivers
in the tree that use old, legacy API though.

2) Approach
-----------

To convert the MMC subsystem to a proper driver model, the "struct mmc"
structure will have to be properly split in the first place. The result will
consist of multiple parts, first will be the structure defining operations
provided by the MMC driver:

struct mmc_driver_ops {
  /* Function used to submit command to the card */
  int  (*send_cmd)(struct mmc *mmc,
                  struct mmc_cmd *cmd, struct mmc_data *data);
  /* DRIVER: Function used to configure the host */
  void (*set_ios)(struct mmc *mmc);
  /* Function used to initialize the host */
  int  (*init)(struct mmc *mmc);
  /* Function used to report the status of Card Detect pin */
  int  (*getcd)(struct mmc *mmc);
}

The second part will define the parameters of the MMC driver:

struct mmc_driver_params {
  /* Voltages the host bus can provide */
  uint32_t voltages;
  /* Minimum frequency the host bus can provide */
  uint32_t f_min;
  /* Maximum frequency the host bus can provide */
  uint32_t f_max;
  /* MMC bus capabilities */
  uint32_t host_caps;
  /*
   * Maximum amount of blocks sent during multiblock xfer,
   * set to 0 to autodetect.
   */
  uint32_t b_max;
}

And finally, the internal per-card data of the MMC subsystem core:

struct mmc_card_props {
  /* Version of the card */
  uint32_t version;
  /* Test if the driver was already initializes */
  bool     has_init;
  /* Is the card SDHC */
  bool     high_capacity;
  /* Actual width of the bus used by the current card */
  uint8_t  bus_width;
  /* Capabilities of the card */
  uint32_t card_caps;
  /* Configuration and ID data retrieved from the card */
  uint32_t ocr;
  uint32_t scr[2];
  uint32_t csd[4];
  uint32_t cid[4];
  uint16_t rca;
  /* Partition configuration */
  uint8_t  part_config;
  /* Number of partitions */
  uint8_t  part_num;
  /* Transmission speed */
  uint32_t tran_speed;
  /* Read block length */
  uint32_t read_bl_len;
  /* Write block length */
  uint32_t write_bl_len;
  /* Erase group size */
  uint32_t erase_grp_size;
  /* Capacity of the card */
  uint64_t capacity;
  /* Descriptor of this block device */
  block_dev_desc_t block_dev;
}

The probe() function will then register the MMC driver by calling:

  mmc_device_register(struct instance *i, struct mmc_driver_ops *o,
                                          struct mmc_driver_params *p);

The struct mmc_driver_params will have to be dynamic in some cases, but the
driver shouldn't modify it's contents elsewhere than in probe() call.

Next, since the MMC drivers will now be consistently registered into the driver
tree from board file, the functions "board_mmc_init()" and "cpu_mmc_init()" will
disappear altogether.

As for the legacy drivers, these will either be converted or removed altogether.

III) Analysis of in-tree drivers
--------------------------------

  1) arm_pl180_mmci.c
  -------------------
  Follows the new API and also has a good encapsulation of the whole driver. The
  conversion here will be simple.

  2) atmel_mci.c
  --------------
  This driver uses the legacy API and should be removed unless converted. It is
  probably possbible to replace this driver with gen_atmel_mci.c . No conversion
  will be done on this driver.

  3) bfin_sdh.c
  -------------
  Follows the new API and also has a good encapsulation of the whole driver. The
  conversion here will be simple.

  4) davinci_mmc.c
  ----------------
  Follows the new API and also has a good encapsulation of the whole driver. The
  conversion here will be simple.

  5) fsl_esdhc.c
  --------------
  Follows the new API and also has a good encapsulation of the whole driver. The
  conversion here will be simple, unless some problem appears due to the FDT
  component of the driver.

  6) ftsdc010_esdhc.c
  -------------------
  Follows the new API and also has a good encapsulation of the whole driver. The
  conversion here will be simple.

  7) gen_atmel_mci.c
  ------------------
  Follows the new API and also has a good encapsulation of the whole driver. The
  conversion here will be simple.

  8) mmc_spi.c
  ------------
  Follows the new API and also has a good encapsulation of the whole driver. The
  conversion here will be simple.

  9) mv_sdhci.c
  -------------
  This is a component of the SDHCI support, allowing it to run on Marvell
  Kirkwood chip. It is probable the SDHCI support will have to be modified to
  allow calling functions from this file based on information passed via
  platform_data.

  10) mxcmmc.c
  ------------
  Follows the new API and also has a good encapsulation of the whole driver. The
  conversion here will be simple.

  11) mxsmmc.c
  ------------
  Follows the new API and also has a good encapsulation of the whole driver. The
  conversion here will be simple.

  12) omap_hsmmc.c
  ----------------
  Follows the new API and also has a good encapsulation of the whole driver. The
  conversion here will be simple.

  13) pxa_mmc.c
  -------------
  This driver uses the legacy API and is written in a severely ad-hoc manner.
  This driver will be removed in favor of pxa_mmc_gen.c, which is proved to work
  better and is already well tested. No conversion will be done on this driver
  anymore.

  14) pxa_mmc_gen.c
  -----------------
  Follows the new API and also has a good encapsulation of the whole driver. The
  conversion here will be simple.

  15) s5p_mmc.c
  -------------
  Follows the new API and also has a good encapsulation of the whole driver. The
  conversion here will be simple.

  16) sdhci.c
  -----------
  Follows the new API and also has a good encapsulation of the whole driver. The
  conversion here will be simple, though it'd be necessary to modify this driver
  to also support the Kirkwood series and probably also Tegra series of CPUs.
  See the respective parts of this section for details.

  17) sh_mmcif.c
  --------------
  Follows the new API and also has a good encapsulation of the whole driver. The
  conversion here will be simple.

  18) tegra2_mmc.c
  ----------------
  Follows the new API and also has a good encapsulation of the whole driver. The
  conversion here will be simple.