Linux操作系统的一大优势就是支持数以万计的芯片设备，大大小小的芯片厂商工程师都在积极地向Linux kernel提交设备驱动代码。能让这个目标得以实现，这背后隐藏着一个看不见的技术优势：Linux内核提供了一套易于扩展和维护的设备驱动框架。Linux内核本身提供一套设备驱动模型，此模型提供了Linux内核对设备的一般性抽象描述，包括设备的电源管理、对象生命周期管理、用户空间呈现等等。在设备模型的帮助下，设备驱动开发工程师从设备的一般性抽象中解脱出来。但是每个设备的具体功能实现还需要大量开发工作，如果每个设备都从头开发，那工作量无疑相当巨大。而这些设备可以按功能进行分类，每个设备类在业界或者标准组织中定义了硬件标准规范，所以针对每个设备类，如果有一个针对此设备类的功能性抽象框架，这将大大加快新设备的添加和开发效率。设备标准规范的存在，无疑对设备驱动框架的设计提供了有力的支撑。
但是坏消息也不少：
这些差异性需求下，一个设备类就像一颗树，其树干为设备标准规范（可能有多个，如下图），每个分支为厂商设备类或设备子类规范，而每片树叶就是每个具体的设备。设备框架的目的就是能帮助驱动工程师简洁优雅地添加一片树叶。这些差异性需求对框架设计是一个不小的挑战，如何很好地支持这些需求，是考验优秀设备驱动框架的试金石。
本文的目的就是总结一些内核设备驱动框架的优秀设计方案，以供大家参考。如有疏漏，也欢迎大家留言补充。
ATA驱动模块管理众多的SATA、PATA设备。以SATA设备为例，又分支持和不支持 port multiplier功能的，支持port multiplier的。而且SATA总线又存在多种标准规范定义的（ahci、fsl、 sil24 etc.），就算是用ahci 总线标准的，有些厂商总在某些地方做的跟标准不一致。
框架设计相关代码示例：
struct ata_port_operations {/** Command execution*/int (*qc_defer)(struct ata_queued_cmd *qc);int (*check_atapi_dma)(struct ata_queued_cmd *qc);void (*qc_prep)(struct ata_queued_cmd *qc);unsigned int (*qc_issue)(struct ata_queued_cmd *qc);bool (*qc_fill_rtf)(struct ata_queued_cmd *qc);/** Configuration and exception handling*/int (*cable_detect)(struct ata_port *ap);unsigned long (*mode_filter)(struct ata_device *dev, unsigned long xfer_mask);void (*set_piomode)(struct ata_port *ap, struct ata_device *dev);void (*set_dmamode)(struct ata_port *ap, struct ata_device *dev);int (*set_mode)(struct ata_link *link, struct ata_device **r_failed_dev);unsigned int (*read_id)(struct ata_device *dev, struct ata_taskfile *tf, u16 *id);void (*dev_config)(struct ata_device *dev);void (*freeze)(struct ata_port *ap);void (*thaw)(struct ata_port *ap);ata_prereset_fn_t prereset;ata_reset_fn_t softreset;ata_reset_fn_t hardreset;ata_postreset_fn_t postreset;ata_prereset_fn_t pmp_prereset;ata_reset_fn_t pmp_softreset;ata_reset_fn_t pmp_hardreset;ata_postreset_fn_t pmp_postreset;void (*error_handler)(struct ata_port *ap);void (*lost_interrupt)(struct ata_port *ap);void (*post_internal_cmd)(struct ata_queued_cmd *qc);/** Optional features*/int (*scr_read)(struct ata_link *link, unsigned int sc_reg, u32 *val);int (*scr_write)(struct ata_link *link, unsigned int sc_reg, u32 val);void (*pmp_attach)(struct ata_port *ap);void (*pmp_detach)(struct ata_port *ap);int (*enable_pm)(struct ata_port *ap, enum link_pm policy);void (*disable_pm)(struct ata_port *ap);/** Start, stop, suspend and resume*/int (*port_suspend)(struct ata_port *ap, pm_message_t mesg);int (*port_resume)(struct ata_port *ap);int (*port_start)(struct ata_port *ap);void (*port_stop)(struct ata_port *ap);void (*host_stop)(struct ata_host *host);#ifdef CONFIG_ATA_SFF/** SFF / taskfile oriented ops*/void (*sff_dev_select)(struct ata_port *ap, unsigned int device);u8 (*sff_check_status)(struct ata_port *ap);u8 (*sff_check_altstatus)(struct ata_port *ap);void (*sff_tf_load)(struct ata_port *ap, const struct ata_taskfile *tf);void (*sff_tf_read)(struct ata_port *ap, struct ata_taskfile *tf);void (*sff_exec_command)(struct ata_port *ap,const struct ata_taskfile *tf);unsigned int (*sff_data_xfer)(struct ata_device *dev,unsigned char *buf, unsigned int buflen, int rw);u8 (*sff_irq_on)(struct ata_port *);void (*sff_irq_clear)(struct ata_port *);void (*bmdma_setup)(struct ata_queued_cmd *qc);void (*bmdma_start)(struct ata_queued_cmd *qc);void (*bmdma_stop)(struct ata_queued_cmd *qc);u8 (*bmdma_status)(struct ata_port *ap);void (*drain_fifo)(struct ata_queued_cmd *qc);#endif /* CONFIG_ATA_SFF */ssize_t (*em_show)(struct ata_port *ap, char *buf);ssize_t (*em_store)(struct ata_port *ap, const char *message,size_t size);ssize_t (*sw_activity_show)(struct ata_device *dev, char *buf);ssize_t (*sw_activity_store)(struct ata_device *dev,enum sw_activity val);/** Obsolete*/void (*phy_reset)(struct ata_port *ap);void (*eng_timeout)(struct ata_port *ap);/** ->inherits must be the last field and all the preceding* fields must be pointers.*/const struct ata_port_operations *inherits;[在对象ata_port_operations 最后一个字段定义一个指向ata_port_operations 的指针。ata_port_operations 类似于 C++ 中的 vtable， 这里模仿 C++ 中继承基类vtable的子类vtable内存布局。]};const struct ata_port_operations ata_base_port_ops = {.prereset = ata_std_prereset,.postreset = ata_std_postreset,.error_handler = ata_std_error_handler,};[基类vtable]const struct ata_port_operations sata_port_ops = {.inherits = &ata_base_port_ops,.qc_defer = ata_std_qc_defer,.hardreset = sata_std_hardreset,};[继承ata_base_port_ops的子类vtable]const struct ata_port_operations sata_pmp_port_ops = {.inherits = &sata_port_ops,.pmp_prereset = ata_std_prereset,.pmp_hardreset = sata_std_hardreset,.pmp_postreset = ata_std_postreset,.error_handler = sata_pmp_error_handler,};static struct ata_port_operations ahci_ops = {.inherits = &sata_pmp_port_ops,.qc_defer = sata_pmp_qc_defer_cmd_switch,.qc_prep = ahci_qc_prep,.qc_issue = ahci_qc_issue,.qc_fill_rtf = ahci_qc_fill_rtf,.freeze = ahci_freeze,.thaw = ahci_thaw,.softreset = ahci_softreset,.hardreset = ahci_hardreset,.postreset = ahci_postreset,.pmp_softreset = ahci_softreset,.error_handler = ahci_error_handler,.post_internal_cmd = ahci_post_internal_cmd,.dev_config = ahci_dev_config,.scr_read = ahci_scr_read,.scr_write = ahci_scr_write,.pmp_attach = ahci_pmp_attach,.pmp_detach = ahci_pmp_detach,.enable_pm = ahci_enable_alpm,.disable_pm = ahci_disable_alpm,.em_show = ahci_led_show,.em_store = ahci_led_store,.sw_activity_show = ahci_activity_show,.sw_activity_store = ahci_activity_store,#ifdef CONFIG_PM.port_suspend = ahci_port_suspend,.port_resume = ahci_port_resume,#endif.port_start = ahci_port_start,.port_stop = ahci_port_stop,};static struct ata_port_operations ahci_vt8251_ops = {.inherits = &ahci_ops,.hardreset = ahci_vt8251_hardreset,};[继承 ahci_ops的子类 vtable]static const struct ata_port_info ahci_port_info[] = {[board_ahci] ={.flags = AHCI_FLAG_COMMON,.pio_mask = ATA_PIO4,.udma_mask = ATA_UDMA6,.port_ops = &ahci_ops,},[board_ahci_vt8251] ={AHCI_HFLAGS (AHCI_HFLAG_NO_NCQ | AHCI_HFLAG_NO_PMP),.flags = AHCI_FLAG_COMMON,.pio_mask = ATA_PIO4,.udma_mask = ATA_UDMA6,.port_ops = &ahci_vt8251_ops,[初始化 对象ahci_port_info[board_ahci_vt8251] 的vtable入口port_ops 为ahci_vt8251_ops]},}static void ata_finalize_port_ops(struct ata_port_operations *ops){static DEFINE_SPINLOCK(lock);const struct ata_port_operations *cur;void **begin = (void **)ops;void **end = (void **)&ops->inherits;void **pp;if (!ops || !ops->inherits)return;spin_lock(&lock);for (cur = ops->inherits; cur; cur = cur->inherits) {void **inherit = (void **)cur;for (pp = begin; pp < end; pp++, inherit++)if (!*pp)*pp = *inherit;}for (pp = begin; pp < end; pp++)if (IS_ERR(*pp))*pp = NULL;ops->inherits = NULL;[扫描多重继承的虚函数接口]spin_unlock(&lock);}int ata_host_start(struct ata_host *host){int have_stop = 0;void *start_dr = NULL;int i, rc;if (host->flags & ATA_HOST_STARTED)return 0;ata_finalize_port_ops(host->ops);[host对象初始化时，调用ata_finalize_port_ops初始化对象vtable指针host->ops]…}
PMBus有一套标准规范，其中有些是基本功能，有些是可选功能。基本功能是必须要实现的，而且寄存器接口也进行标准化。而可选功能由各设备厂商自由决定，而且这些可选功能的寄存器接口也无统一规范，支持PMBus设备厂商的自定义寄存器。
框架设计相关代码示例：
struct pmbus_data {
struct device *dev;
struct device *hwmon_dev;
u32 flags; /* from platform data */
int exponent; /* linear mode: exponent for output voltages */
const struct pmbus_driver_info *info;
int max_attributes;
int num_attributes;
struct attribute_group group;
struct pmbus_sensor *sensors;
struct mutex update_lock;
bool valid;
unsigned long last_updated; /* in jiffies */
/*
* A single status register covers multiple attributes,
* so we keep them all together.
*/
u8 status[PB_NUM_STATUS_REG];
u8 status_register;
u8 currpage;
};
struct pmbus_driver_info {
int pages; /* Total number of pages */
enum pmbus_data_format format[PSC_NUM_CLASSES];
/*
* Support one set of coefficients for each sensor type
* Used for chips providing data in direct mode.
*/
int m[PSC_NUM_CLASSES]; /* mantissa for direct data format */
int b[PSC_NUM_CLASSES]; /* offset */
int R[PSC_NUM_CLASSES]; /* exponent */
u32 func[PMBUS_PAGES]; /* Functionality, per page */
/*
* The following functions map manufacturing specific register values
* to PMBus standard register values. Specify only if mapping is
* necessary.
* Functions return the register value (read) or zero (write) if
* successful. A return value of -ENODATA indicates that there is no
* manufacturer specific register, but that a standard PMBus register
* may exist. Any other negative return value indicates that the
* register does not exist, and that no attempt should be made to read
* the standard register.
*/
int (*read_byte_data)(struct i2c_client *client, int page, int reg);
int (*read_word_data)(struct i2c_client *client, int page, int reg);
int (*write_word_data)(struct i2c_client *client, int page, int reg,
u16 word);
int (*write_byte)(struct i2c_client *client, int page, u8 value);
/*
* The identify function determines supported PMBus functionality.
* This function is only necessary if a chip driver supports multiple
* chips, and the chip functionality is not pre-determined.
*/
int (*identify)(struct i2c_client *client,
struct pmbus_driver_info *info);
};
struct pmbus_sensor {
struct pmbus_sensor *next;
char name[PMBUS_NAME_SIZE]; /* sysfs sensor name */
struct device_attribute attribute;
u8 page; /* page number */
u16 reg; /* register */
enum pmbus_sensor_classes class; /* sensor class */
bool update; /* runtime sensor update needed */
int data; /* Sensor data.
Negative if there was a read error */
};
static struct pmbus_driver_info tps53667_info = {
.pages = 1,
.format[PSC_VOLTAGE_IN] = linear,
.format[PSC_VOLTAGE_OUT] = vid,
.format[PSC_TEMPERATURE] = linear,
.format[PSC_CURRENT_IN] = linear,
.format[PSC_CURRENT_OUT] = linear,
.format[PSC_POWER] = linear,
.read_word_data = tps53667_read_word_data,
.write_word_data = tps53667_write_word_data,
.func[0] = PMBUS_HAVE_VIN |
PMBUS_HAVE_VOUT |
PMBUS_HAVE_IIN |
PMBUS_HAVE_IOUT |
PMBUS_HAVE_PIN |
PMBUS_HAVE_POUT |
PMBUS_HAVE_TEMP |
PMBUS_HAVE_STATUS_VOUT |
PMBUS_HAVE_STATUS_IOUT |
PMBUS_HAVE_STATUS_INPUT |
PMBUS_HAVE_STATUS_TEMP,
};
static int tps53667_probe(struct i2c_client *client,
const struct i2c_device_id *id)
{
return pmbus_do_probe(client, id, &tps53667_info);
}
int pmbus_do_probe(struct i2c_client *client, const struct i2c_device_id *id,
struct pmbus_driver_info *info)
{
struct device *dev = &client->dev;
const struct pmbus_platform_data *pdata = dev->platform_data;
struct pmbus_data *data;
int ret;
if (!info)
return -ENODEV;
if (!i2c_check_functionality(client->adapter, I2C_FUNC_SMBUS_WRITE_BYTE
| I2C_FUNC_SMBUS_BYTE_DATA
| I2C_FUNC_SMBUS_WORD_DATA))
return -ENODEV;
data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
if (!data)
return -ENOMEM;
i2c_set_clientdata(client, data);
mutex_init(&data->update_lock);
data->dev = dev;
if (pdata)
data->flags = pdata->flags;
data->info = info;
ret = pmbus_init_common(client, data, info);
if (ret < 0)
return ret;
ret = pmbus_find_attributes(client, data);
if (ret)
goto out_kfree;
/*
* If there are no attributes, something is wrong.
* Bail out instead of trying to register nothing.
*/
if (!data->num_attributes) {
dev_err(dev, "No attributes foundn");
ret = -ENODEV;
goto out_kfree;
}
/* Register sysfs hooks */
ret = sysfs_create_group(&dev->kobj, &data->group);
if (ret) {
dev_err(dev, "Failed to create sysfs entriesn");
goto out_kfree;
}
data->hwmon_dev = hwmon_device_register(dev);
if (IS_ERR(data->hwmon_dev)) {
ret = PTR_ERR(data->hwmon_dev);
dev_err(dev, "Failed to register hwmon devicen");
goto out_hwmon_device_register;
}
ret = device_create_file(dev, &dev_attr_clear_fault);
if (ret)
goto out_hwmon_device_register;
return 0;
out_hwmon_device_register:
sysfs_remove_group(&dev->kobj, &data->group);
out_kfree:
kfree(data->group.attrs);
return ret;
}
USB块设备有一套标准规范定义，但USB设备类型众多（如U盘、MP3播放器、手机、GPS设备等等），各厂商水平参差不齐，实现混乱。而且因为USB规范相对较新，一直在演进变化中，这也加剧了这一混乱。比如：
另外，Linux内核已有一套通用的SCSI块设备驱动框架，如果USB块设备能复用这个框架，那是一个最好的选择。
框架设计相关代码示例：
/* Driver for USB Mass Storage compliant devices
* SCSI layer glue code
#define US_DO_ALL_FLAGS
US_FLAG(SINGLE_LUN, 0x00000001)
/* allow access to only LUN 0 */
US_FLAG(NEED_OVERRIDE, 0x00000002)
/* unusual_devs entry is necessary */
US_FLAG(SCM_MULT_TARG, 0x00000004)
/* supports multiple targets */
US_FLAG(FIX_INQUIRY, 0x00000008)
/* INQUIRY response needs faking */
US_FLAG(FIX_CAPACITY, 0x00000010)
/* READ CAPACITY response too big */
US_FLAG(IGNORE_RESIDUE, 0x00000020)
/* reported residue is wrong */
US_FLAG(BULK32, 0x00000040)
/* Uses 32-byte CBW length */
US_FLAG(NOT_LOCKABLE, 0x00000080)
/* PREVENT/ALLOW not supported */
US_FLAG(GO_SLOW, 0x00000100)
/* Need delay after Command phase */
US_FLAG(NO_WP_DETECT, 0x00000200)
/* Don't check for write-protect */
US_FLAG(MAX_SECTORS_64, 0x00000400)
/* Sets max_sectors to 64 */
US_FLAG(IGNORE_DEVICE, 0x00000800)
/* Don't claim device */
US_FLAG(CAPACITY_HEURISTICS, 0x00001000)
/* sometimes sizes is too big */
US_FLAG(MAX_SECTORS_MIN,0x00002000)
/* Sets max_sectors to arch min */
US_FLAG(BULK_IGNORE_TAG,0x00004000)
/* Ignore tag mismatch in bulk operations */
US_FLAG(SANE_SENSE, 0x00008000)
/* Sane Sense (> 18 bytes) */
US_FLAG(CAPACITY_OK, 0x00010000)
/* READ CAPACITY response is correct */
US_FLAG(BAD_SENSE, 0x00020000)
/* Bad Sense (never more than 18 bytes) */
US_FLAG(NO_READ_DISC_INFO, 0x00040000)
/* cannot handle READ_DISC_INFO */
US_FLAG(NO_READ_CAPACITY_16, 0x00080000)
/* cannot handle READ_CAPACITY_16 */
US_FLAG(INITIAL_READ10, 0x00100000)
/* Initial READ(10) (and others) must be retried */
US_FLAG(WRITE_CACHE, 0x00200000)
/* Write Cache status is not available */
#define US_FLAG(name, value) US_FL_##name = value ,
enum { US_DO_ALL_FLAGS };
#undef US_FLAG
struct us_data {
/* The device we're working with
* It's important to note:
* (o) you must hold dev_mutex to change pusb_dev
*/
struct mutex dev_mutex; /* protect pusb_dev */
struct usb_device *pusb_dev; /* this usb_device */
struct usb_interface *pusb_intf; /* this interface */
struct us_unusual_dev *unusual_dev; /* device-filter entry */
unsigned long fflags; /* fixed flags from filter */
unsigned long dflags; /* dynamic atomic bitflags */
unsigned int send_bulk_pipe; /* cached pipe values */
unsigned int recv_bulk_pipe;
unsigned int send_ctrl_pipe;
unsigned int recv_ctrl_pipe;
unsigned int recv_intr_pipe;
/* information about the device */
char *transport_name;
char *protocol_name;
__le32 bcs_signature;
u8 subclass;
u8 protocol;
u8 max_lun;
u8 ifnum; /* interface number */
u8 ep_bInterval; /* interrupt interval */
/* function pointers for this device */
trans_cmnd transport; /* transport function */
trans_reset transport_reset; /* transport device reset */
proto_cmnd proto_handler; /* protocol handler */
/* SCSI interfaces */
struct scsi_cmnd *srb; /* current srb */
unsigned int tag; /* current dCBWTag */
char scsi_name[32]; /* scsi_host name */
/* control and bulk communications data */
struct urb *current_urb; /* USB requests */
struct usb_ctrlrequest *cr; /* control requests */
struct usb_sg_request current_sg; /* scatter-gather req. */
unsigned char *iobuf; /* I/O buffer */
dma_addr_t iobuf_dma; /* buffer DMA addresses */
struct task_struct *ctl_thread; /* the control thread */
/* mutual exclusion and synchronization structures */
struct completion cmnd_ready; /* to sleep thread on */
struct completion notify; /* thread begin/end */
wait_queue_head_t delay_wait; /* wait during reset */
struct delayed_work scan_dwork; /* for async scanning */
/* subdriver information */
void *extra; /* Any extra data */
extra_data_destructor extra_destructor;/* extra data destructor */
#ifdef CONFIG_PM
pm_hook suspend_resume_hook;
#endif
/* hacks for READ CAPACITY bug handling */
int use_last_sector_hacks;
int last_sector_retries;
};
struct usb_device_id {
/* which fields to match against? */
__u16 match_flags;
/* Used for product specific matches; range is inclusive */
__u16 idVendor;
__u16 idProduct;
__u16 bcdDevice_lo;
__u16 bcdDevice_hi;
/* Used for device class matches */
__u8 bDeviceClass;
__u8 bDeviceSubClass;
__u8 bDeviceProtocol;
/* Used for interface class matches */
__u8 bInterfaceClass;
__u8 bInterfaceSubClass;
__u8 bInterfaceProtocol;
/* Used for vendor-specific interface matches */
__u8 bInterfaceNumber;
/* not matched against */
kernel_ulong_t driver_info
__attribute__((aligned(sizeof(kernel_ulong_t))));
};
/*
* The table of devices
*/
#define UNUSUAL_DEV(id_vendor, id_product, bcdDeviceMin, bcdDeviceMax,
vendorName, productName, useProtocol, useTransport,
initFunction, flags)
{ USB_DEVICE_VER(id_vendor, id_product, bcdDeviceMin, bcdDeviceMax),
.driver_info = (flags) }
struct usb_device_id usb_storage_usb_ids[] = {
# include "unusual_devs.h"
{ } /* Terminating entry */
};
UNUSUAL_DEV( 0x22b8, 0x6426, 0x0101, 0x0101,
"Motorola",
"MSnc.",
USB_SC_DEVICE, USB_PR_DEVICE, NULL,
US_FL_FIX_INQUIRY | US_FL_FIX_CAPACITY | US_FL_BULK_IGNORE_TAG),
static int slave_configure(struct scsi_device *sdev)
{
...
if (us->fflags & US_FL_FIX_CAPACITY)
sdev->fix_capacity = 1;
...
}
/*
* read disk capacity
*/
static void
sd_read_capacity(struct scsi_disk *sdkp, unsigned char *buffer)
{
...
/* Some devices are known to return the total number of blocks,
* not the highest block number. Some devices have versions
* which do this and others which do not. Some devices we might
* suspect of doing this but we don't know for certain.
*
* If we know the reported capacity is wrong, decrement it. If
* we can only guess, then assume the number of blocks is even
* (usually true but not always) and err on the side of lowering
* the capacity.
*/
if (sdp->fix_capacity ||
(sdp->guess_capacity && (sdkp->capacity & 0x01))) {
sd_printk(KERN_INFO, sdkp, "Adjusting the sector count "
"from its reported value: %llun",
(unsigned long long) sdkp->capacity);
--sdkp->capacity;
}
...
}
struct scsi_host_template usb_stor_host_template = {
/* basic userland interface stuff */
.name = "usb-storage",
.proc_name = "usb-storage",
.show_info = show_info,
.write_info = write_info,
.info = host_info,
/* command interface -- queued only */
.queuecommand = queuecommand,
/* error and abort handlers */
.eh_abort_handler = command_abort,
.eh_device_reset_handler = device_reset,
.eh_bus_reset_handler = bus_reset,
/* queue commands only, only one command per LUN */
.can_queue = 1,
.cmd_per_lun = 1,
/* unknown initiator id */
.this_id = -1,
.slave_alloc = slave_alloc,
.slave_configure = slave_configure,
.target_alloc = target_alloc,
/* lots of sg segments can be handled */
.sg_tablesize = SCSI_MAX_SG_CHAIN_SEGMENTS,
/* limit the total size of a transfer to 120 KB */
.max_sectors = 240,
/* merge commands... this seems to help performance, but
* periodically someone should test to see which setting is more
* optimal.
*/
.use_clustering = 1,
/* emulated HBA */
.emulated = 1,
/* we do our own delay after a device or bus reset */
.skip_settle_delay = 1,
/* sysfs device attributes */
.sdev_attrs = sysfs_device_attr_list,
/* module management */
.module = THIS_MODULE
};
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