引子

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：

1. 标准规范往往落后于产品，特别对新兴设备来说。
2. 一个设备类可能是一套标准规范定义，也有可能是多套标准规范定义。
3. 芯片厂商之间，为了达到提升自己的技术优势、市场壁垒等目的，在标准规范之内或之外，做了很多vendor specific的实现。

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设备驱动框架设计

现状描述

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。

设计要点

• 所有功能点抽象成接口，再抽象成数据结构struct ata_port_operations，实现对象的多态；
• 通过struct ata_port_operations的inherits字段实现对象的继承；
• 新的设备驱动添加，就是添加新的struct ata_port_operations对象，而此对象可以从已有的相似对象节点上，通过inherits字段继承大部分的功能。

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设备驱动框架设计

现状描述

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。

设计要点

• 通过struct pmbus_data对pmbus设备进行抽象性描述，此对象聚合对象pmbus_driver_info和pmbus_sensor。通过struct pmbus_driver_info对pmbus设备标准规范进行抽象性描述，struct pmbus_sensor对pmbus传感器进行抽象性描述。
• 可选功能集通过pmbus_driver_info的u32 func[PMBUS_PAGES]字段描述。
• 非标准寄存器通过虚拟寄存器（Virtual registers）统一到pmbus驱动框架中。虚拟寄存器到设备自定义寄存器的映射通过pmbus_driver_info的4个接口：read_byte_data/read_word_data/write_word_data/write_byte来完成。　
• 所以当添加一个新设备时，其差异性需求都封装在pmbus_driver_info中，这样pmbus_data和pmbus_sensor做为公共功能则无需修改，
• 通过构造新的pmbus_driver_info对象即可完成新的设备驱动的添加。

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块设备驱动框架设计

现状描述

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：

• 设备描述符中subclass、protocol字段为空或者不正确；
• 应答sense长度非标准；
• INQUIRY设备请求非标准；
• bulk传输协议中的 tag 不匹配；
• 类似问题近20个

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。

设计要点

• 通过struct scsi_host_template对scsi块设备进行抽象性描述，USB块设备通过创建一个scsi_host_template对象，集成到SCSI块设备驱动框架中；
• 通过struct us_data对USB块设备进行抽象性描述，通过us_data的fflags字段和unusual_dev字段对USB块设备中的非标准行为进行抽象性描述；
• 通过struct usb_device_id对USB块设备vendor-specific属性进行抽象描述，这样通过添加一个新的usb_device_id实例，即可完成对一个新的USB块设备驱动的添加。

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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