接口设计

以下接口皆为 MMC 子系统所需要的标准接口，通过 mmc_host_ops 注册到 MMC 子系统。

static const struct mmc_host_ops artinchip_mmc_ops = {
    .request        = artinchip_mmc_request,
    .pre_req        = artinchip_mmc_pre_req,
    .post_req       = artinchip_mmc_post_req,
    .set_ios        = artinchip_mmc_set_ios,
    .get_ro         = artinchip_mmc_get_ro,
    .get_cd         = artinchip_mmc_get_cd,
    .hw_reset               = artinchip_mmc_hw_reset,
    .enable_sdio_irq    = artinchip_mmc_enable_sdio_irq,
    .ack_sdio_irq       = artinchip_mmc_ack_sdio_irq,
    .execute_tuning     = artinchip_mmc_execute_tuning,
    .card_busy      = artinchip_mmc_card_busy,
    .start_signal_voltage_switch = artinchip_mmc_switch_voltage,
    .init_card      = artinchip_mmc_init_card,
};