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1. 前言
device和device driver是Linux驱动开发的基本概念。Linux kernel的思路很简单:驱动开发,就是要开发指定的软件(driver)以驱动指定的设备(device),所以kernel就为设备和驱动它的driver定义了两个数据结构,分别是device和device_driver。因此本文将会围绕这两个数据结构,介绍Linux设备模型的核心逻辑,包括:
设备及设备驱动在kernel中的抽象、使用和维护;
设备及设备驱动的注册、加载、初始化原理;
设备模型在实际驱动开发过程中的使用方法。
注:在介绍device和device_driver的过程中,会遇到很多额外的知识点,如Class、Bus、DMA、电源管理等等,这些知识点都很复杂,任何一个都可以作为一个单独的专题区阐述,因此本文不会深入解析它们,而会在后续的文章中专门描述。
2. struct device和struct device_driver
在阅读Linux内核源代码时,通过核心数据结构,即可理解某个模块60%以上的逻辑,设备模型部分尤为明显。
在include/linux/device.h中,Linux内核定义了设备模型中最重要的两个数据结构,struct device和struct device_driver。
/**
* struct device - The basic device structure
* @parent: The device's "parent" device, the device to which it is attached.
* In most cases, a parent device is some sort of bus or host
* controller. If parent is NULL, the device, is a top-level device,
* which is not usually what you want.
* @p: Holds the private data of the driver core portions of the device.
* See the comment of the struct device_private for detail.
* @kobj: A top-level, abstract class from which other classes are derived.
* @init_name: Initial name of the device.
* @type: The type of device.
* This identifies the device type and carries type-specific
* information.
* @mutex: Mutex to synchronize calls to its driver.
* @bus: Type of bus device is on.
* @driver: Which driver has allocated this
* @platform_data: Platform data specific to the device.
* Example: For devices on custom boards, as typical of embedded
* and SOC based hardware, Linux often uses platform_data to point
* to board-specific structures describing devices and how they
* are wired. That can include what ports are available, chip
* variants, which GPIO pins act in what additional roles, and so
* on. This shrinks the "Board Support Packages" (BSPs) and
* minimizes board-specific #ifdefs in drivers.
* @driver_data: Private pointer for driver specific info.
* @power: For device power management.
* See Documentation/power/devices.txt for details.
* @pm_domain: Provide callbacks that are executed during system suspend,
* hibernation, system resume and during runtime PM transitions
* along with subsystem-level and driver-level callbacks.
* @pins: For device pin management.
* See Documentation/pinctrl.txt for details.
* @msi_list: Hosts MSI descriptors
* @msi_domain: The generic MSI domain this device is using.
* @numa_node: NUMA node this device is close to.
* @dma_mask: Dma mask (if dma'ble device).
* @coherent_dma_mask: Like dma_mask, but for alloc_coherent mapping as not all
* hardware supports 64-bit addresses for consistent allocations
* such descriptors.
* @dma_pfn_offset: offset of DMA memory range relatively of RAM
* @dma_parms: A low level driver may set these to teach IOMMU code about
* segment limitations.
* @dma_pools: Dma pools (if dma'ble device).
* @dma_mem: Internal for coherent mem override.
* @cma_area: Contiguous memory area for dma allocations
* @archdata: For arch-specific additions.
* @of_node: Associated device tree node.
* @fwnode: Associated device node supplied by platform firmware.
* @devt: For creating the sysfs "dev".
* @id: device instance
* @devres_lock: Spinlock to protect the resource of the device.
* @devres_head: The resources list of the device.
* @knode_class: The node used to add the device to the class list.
* @class: The class of the device.
* @groups: Optional attribute groups.
* @release: Callback to free the device after all references have
* gone away. This should be set by the allocator of the
* device (i.e. the bus driver that discovered the device).
* @iommu_group: IOMMU group the device belongs to.
* @iommu_fwspec: IOMMU-specific properties supplied by firmware.
*
* @offline_disabled: If set, the device is permanently online.
* @offline: Set after successful invocation of bus type's .offline().
*
* At the lowest level, every device in a Linux system is represented by an
* instance of struct device. The device structure contains the information
* that the device model core needs to model the system. Most subsystems,
* however, track additional information about the devices they host. As a
* result, it is rare for devices to be represented by bare device structures;
* instead, that structure, like kobject structures, is usually embedded within
* a higher-level representation of the device.
*/
struct device {
struct device *parent;
struct device_private *p;
struct kobject kobj;
const char *init_name; /* initial name of the device */
const struct device_type *type;
struct mutex mutex; /* mutex to synchronize calls to
* its driver.
*/
struct bus_type *bus; /* type of bus device is on */
struct device_driver *driver; /* which driver has allocated this
device */
void *platform_data; /* Platform specific data, device
core doesn't touch it */
void *driver_data; /* Driver data, set and get with
dev_set/get_drvdata */
struct dev_pm_info power;
struct dev_pm_domain *pm_domain;
#ifdef CONFIG_GENERIC_MSI_IRQ_DOMAIN
struct irq_domain *msi_domain;
#endif
#ifdef CONFIG_PINCTRL
struct dev_pin_info *pins;
#endif
#ifdef CONFIG_GENERIC_MSI_IRQ
struct list_head msi_list;
#endif
#ifdef CONFIG_NUMA
int numa_node; /* NUMA node this device is close to */
#endif
u64 *dma_mask; /* dma mask (if dma'able device) */
u64 coherent_dma_mask;/* Like dma_mask, but for
alloc_coherent mappings as
not all hardware supports
64 bit addresses for consistent
allocations such descriptors. */
unsigned long dma_pfn_offset;
struct device_dma_parameters *dma_parms;
struct list_head dma_pools; /* dma pools (if dma'ble) */
struct dma_coherent_mem *dma_mem; /* internal for coherent mem
override */
#ifdef CONFIG_DMA_CMA
struct cma *cma_area; /* contiguous memory area for dma
allocations */
#endif
/* arch specific additions */
struct dev_archdata archdata;
struct device_node *of_node; /* associated device tree node */
struct fwnode_handle *fwnode; /* firmware device node */
dev_t devt; /* dev_t, creates the sysfs "dev" */
u32 id; /* device instance */
spinlock_t devres_lock;
struct list_head devres_head;
struct klist_node knode_class;
struct class *class;
const struct attribute_group **groups; /* optional groups */
void (*release)(struct device *dev);
struct iommu_group *iommu_group;
struct iommu_fwspec *iommu_fwspec;
bool offline_disabled:1;
bool offline:1;
};
device结构很复杂(不过linux内核的开发人员素质是很高的,该接口的注释写的非常详细,感兴趣的同学可以参考内核源代码),这里将会选一些对理解设备模型非常关键的字段进行说明。
parent,该设备的父设备,一般是该设备所从属的bus、controller等设备。
p,一个用于struct device的私有数据结构指针,该指针中会保存子设备链表、用于添加到bus/driver/prent等设备中的链表头等等,具体可查看源代码。
kobj,该数据结构对应的struct kobject。
init_name,该设备的名称。
注1:在设备模型中,名称是一个非常重要的变量,任何注册到内核中的设备,都必须有一个合法的名称,可以在初始化时给出,也可以由内核根据“bus name + device ID”的方式创造。
type,struct device_type结构是新版本内核新引入的一个结构,它和struct device关系,非常类似stuct kobj_type和struct kobject之间的关系,后续会再详细说明。
bus,该device属于哪个总线(后续会详细描述)。
driver,该device对应的device driver。
platform_data,一个指针,用于保存具体的平台相关的数据。具体的driver模块,可以将一些私有的数据,暂存在这里,需要使用的时候,再拿出来,因此设备模型并不关心该指针得实际含义。
power、pm_domain,电源管理相关的逻辑,后续会由电源管理专题讲解。
pins,"PINCTRL”功能,暂不描述。
numa_node,"NUMA”功能,暂不描述。
dma_mask~archdata,DMA相关的功能,暂不描述。
devt,dev_t是一个32位的整数,它由两个部分(Major和Minor)组成,在需要以设备节点的形式(字符设备和块设备)向用户空间提供接口的设备中,当作设备号使用。在这里,该变量主要用于在sys文件系统中,为每个具有设备号的device,创建/sys/dev/* 下的对应目录,如下:
1|root@android:/storage/sdcard0 #ls /sys/dev/char/1\:
1:1/ 1:11/ 1:13/ 1:14/ 1:2/ 1:3/ 1:5/ 1:7/ 1:8/ 1:9/
1|root@android:/storage/sdcard0 #ls /sys/dev/char/1:1
1:1/ 1:11/ 1:13/ 1:14/
1|root@android:/storage/sdcard0 # ls /sys/dev/char/1\:1
/sys/dev/char/1:1class,该设备属于哪个class。
groups,该设备的默认attribute集合。将会在设备注册时自动在sysfs中创建对应的文件。
iommu_fwspec,固件提供的IOMMU特定属性
offline_disabled,如果设置,则设备永久在线
offline,成功调用总线类型的.offline()后设置
在最低级别,Linux系统中的每个设备都由一个结构设备的实例。 设备结构包含信息 设备模型核心需要为系统建模。 ‘
大多数子系统, 但是,请跟踪有关其托管设备的其他信息。 作为一个 结果,设备很少用裸设备结构表示;相反,该
结构,如kobject结构,通常嵌入其中 设备的更高级别表示。
- struct device_driver
/**
* struct device_driver - The basic device driver structure
* @name: Name of the device driver.
* @bus: The bus which the device of this driver belongs to.
* @owner: The module owner.
* @mod_name: Used for built-in modules.
* @suppress_bind_attrs: Disables bind/unbind via sysfs.
* @probe_type: Type of the probe (synchronous or asynchronous) to use.
* @of_match_table: The open firmware table.
* @acpi_match_table: The ACPI match table.
* @probe: Called to query the existence of a specific device,
* whether this driver can work with it, and bind the driver
* to a specific device.
* @remove: Called when the device is removed from the system to
* unbind a device from this driver.
* @shutdown: Called at shut-down time to quiesce the device.
* @suspend: Called to put the device to sleep mode. Usually to a
* low power state.
* @resume: Called to bring a device from sleep mode.
* @groups: Default attributes that get created by the driver core
* automatically.
* @pm: Power management operations of the device which matched
* this driver.
* @p: Driver core's private data, no one other than the driver
* core can touch this.
*
* The device driver-model tracks all of the drivers known to the system.
* The main reason for this tracking is to enable the driver core to match
* up drivers with new devices. Once drivers are known objects within the
* system, however, a number of other things become possible. Device drivers
* can export information and configuration variables that are independent
* of any specific device.
*/
struct device_driver {
const char *name;
struct bus_type *bus;
struct module *owner;
const char *mod_name; /* used for built-in modules */
bool suppress_bind_attrs; /* disables bind/unbind via sysfs */
enum probe_type probe_type;
const struct of_device_id *of_match_table;
const struct acpi_device_id *acpi_match_table;
int (*probe) (struct device *dev);
int (*remove) (struct device *dev);
void (*shutdown) (struct device *dev);
int (*suspend) (struct device *dev, pm_message_t state);
int (*resume) (struct device *dev);
const struct attribute_group **groups;
const struct dev_pm_ops *pm;
struct driver_private *p;
};
device_driver就简单多了(在早期的内核版本中driver的数据结构为"struct driver”,不知道从哪个版本开始,就改成device_driver了):
name,该driver的名称。和device结构一样,该名称非常重要,后面会再详细说明。
bus,该driver所驱动设备的总线设备。为什么driver需要记录总线设备的指针呢?因为内核要保证在driver运行前,设备所依赖的总线能够正确初始化。
owner、mod_name,內核module相关的变量,暂不描述。
suppress_bind_attrs,是不在sysfs中启用bind和unbind attribute,如下:root@android:/storage/sdcard0 # ls /sys/bus/platform/drivers/switch-gpio/
bind uevent unbind
在kernel中,bind/unbind是从用户空间手动的为driver绑定/解绑定指定的设备的机制。这种机制是在bus.c中完成的,后面会详细解释。probe、remove,这两个接口函数用于实现driver逻辑的开始和结束。Driver是一段软件code,因此会有开始和结束两个代码逻辑,就像PC程序,会有一个main函数,main函数的开始就是开始,return的地方就是结束。而内核driver却有其特殊性:在设备模型的结构下,只有driver和device同时存在时,才需要开始执行driver的代码逻辑。这也是probe和remove两个接口名称的由来:检测到了设备和移除了设备(就是为热拔插起的!)。
shutdown、suspend、resume、pm,电源管理相关的内容,会在电源管理专题中详细说明。
groups,和struct device结构中的同名变量类似,driver也可以定义一些默认attribute,这样在将driver注册到内核中时,内核设备模型部分的代码(driver/base/driver.c)会自动将这些attribute添加到sysfs中。
p,driver core的私有数据指针,其它模块不能访问。
3. 设备模型框架下驱动开发的基本步骤
在设备模型框架下,设备驱动的开发是一件很简单的事情,主要包括2个步骤:
步骤1:分配一个struct device类型的变量,填充必要的信息后,把它注册到内核中。
步骤2:分配一个struct device_driver类型的变量,填充必要的信息后,把它注册到内核中。
这两步完成后,内核会在合适的时机(后面会讲),调用struct device_driver变量中的probe、remove、suspend、resume等回调函数,从而触发或者终结设备驱动的执行。而所有的驱动程序逻辑,都会由这些回调函数实现,此时,驱动开发者眼中便不再有“设备模型”,转而只关心驱动本身的实现。
以上两个步骤的补充说明:
1. 一般情况下,Linux驱动开发很少直接使用device和device_driver,因为内核在它们之上又封装了一层,如soc device、platform device等等,而这些层次提供的接口更为简单、易用(也正是因为这个原因,本文并不会过多涉及device、device_driver等模块的实现细节)。
2. 内核提供很多struct device结构的操作接口(具体可以参考include/linux/device.h和drivers/base/core.c的代码),主要包括初始化(device_initialize)、注册到内核(device_register)、分配存储空间+初始化+注册到内核(device_create)等等,可以根据需要使用。
3. device和device_driver必须具备相同的名称,内核才能完成匹配操作,进而调用device_driver中的相应接口。这里的同名,作用范围是同一个bus下的所有device和device_driver。
4. device和device_driver必须挂载在一个bus之下,该bus可以是实际存在的,也可以是虚拟的。
5. driver开发者可以在struct device变量中,保存描述设备特征的信息,如寻址空间、依赖的GPIOs等,因为device指针会在执行probe等接口时传入,这时driver就可以根据这些信息,执行相应的逻辑操作了。
device部分的实现细节:
https://blog.csdn/qq_16777851/article/details/81437352
device_driver部分的实现细节
https://mp.csdn/postedit/81459931
4. 设备驱动probe的时机
所谓的"probe”,是指在Linux内核中,如果存在相同名称的device和device_driver(注:还存在其它方式,我们先不关注了),内核就会执行device_driver中的probe回调函数,而该函数就是所有driver的入口,可以执行诸如硬件设备初始化、字符设备注册、设备文件操作ops注册等动作("remove”是它的反操作,发生在device或者device_driver任何一方从内核注销时,其原理类似,就不再单独说明了)。
设备驱动prove的时机有如下几种(分为自动触发和手动触发):
- 将struct device类型的变量注册到内核中时自动触发(device_register,device_add,device_create_vargs,device_create)
- 将struct device_driver类型的变量注册到内核中时自动触发(driver_register)
- 手动查找同一bus下的所有device_driver,如果有和指定device同名的driver,执行probe操作(device_attach)
- 手动查找同一bus下的所有device,如果有和指定driver同名的device,执行probe操作(driver_attach)
- 自行调用driver的probe接口,并在该接口中将该driver绑定到某个device结构中----即设置dev->driver(device_bind_driver)
注2:probe动作实际是由bus模块(会在下一篇文章讲解)实现的,这不难理解:device和device_driver都是挂载在bus这根线上,因此只有bus最清楚应该为哪些device、哪些driver配对。
注3:每个bus都有一个drivers_autoprobe变量,用于控制是否在device或者driver注册时,自动probe。该变量默认为1(即自动probe),bus模块将它开放到sysfs中了,因而可在用户空间修改,进而控制probe行为。
注4:上面的手动触发和自动触发以及probe机制都在设备模型六和七有分析,要看细节请看那两篇博客。
5. 其它杂项
5.1 device_attribute和driver_attribute
在Linux设备模型四(attribure)中,我们有讲到,大多数时候,attribute文件的读写数据流为:vfs---->sysfs---->kobject---->attibute---->kobj_type---->sysfs_ops---->xxx_attribute,其中kobj_type、sysfs_ops和xxx_attribute都是由包含kobject的上层数据结构实现。
Linux内核中关于该内容的例证到处都是,device也不无例外的提供了这种例子,如下
#define to_dev_attr(_attr) container_of(_attr, struct device_attribute, attr)
static ssize_t dev_attr_show(struct kobject *kobj, struct attribute *attr,
char *buf)
{
struct device_attribute *dev_attr = to_dev_attr(attr);
struct device *dev = kobj_to_dev(kobj);
ssize_t ret = -EIO;
if (dev_attr->show)
ret = dev_attr->show(dev, dev_attr, buf);
if (ret >= (ssize_t)PAGE_SIZE) {
print_symbol("dev_attr_show: %s returned bad count\n",
(unsigned long)dev_attr->show);
}
return ret;
}
static ssize_t dev_attr_store(struct kobject *kobj, struct attribute *attr,
const char *buf, size_t count)
{
struct device_attribute *dev_attr = to_dev_attr(attr);
struct device *dev = kobj_to_dev(kobj);
ssize_t ret = -EIO;
if (dev_attr->store)
ret = dev_attr->store(dev, dev_attr, buf, count);
return ret;
}
static const struct sysfs_ops dev_sysfs_ops = {
.show = dev_attr_show,
.store = dev_attr_store,
};
static struct kobj_type device_ktype = {
.release = device_release,
.sysfs_ops = &dev_sysfs_ops,
.namespace = device_namespace,
};
struct sysfs_ops {
ssize_t (*show)(struct kobject *, struct attribute *,char *);
ssize_t (*store)(struct kobject *,struct attribute *,const char *, size_t);
};
/* interface for exporting device attributes */
struct device_attribute {
struct attribute attr;
ssize_t (*show)(struct device *dev, struct device_attribute *attr,
char *buf);
ssize_t (*store)(struct device *dev, struct device_attribute *attr,
const char *buf, size_t count);
};
至于driver的attribute,则要简单的多,其数据流为:vfs---->sysfs---->kobject---->attribute---->driver_attribute,如下:
/* sysfs interface for exporting driver attributes */
struct driver_attribute {
struct attribute attr;
ssize_t (*show)(struct device_driver *driver, char *buf);
ssize_t (*store)(struct device_driver *driver, const char *buf,
size_t count);
};
#define DRIVER_ATTR(_name, _mode, _show, _store) \
struct driver_attribute driver_attr_##_name = __ATTR(_name, _mode, _show, _store)
5.2 device_type
device_type是内嵌在struct device结构中的一个数据结构,用于指明设备的类型,并提供一些额外的辅助功能。它的的形式如下:
/*
* The type of device, "struct device" is embedded in. A class
* or bus can contain devices of different types
* like "partitions" and "disks", "mouse" and "event".
* This identifies the device type and carries type-specific
* information, equivalent to the kobj_type of a kobject.
* If "name" is specified, the uevent will contain it in
* the DEVTYPE variable.
*/
struct device_type {
const char *name;
const struct attribute_group **groups;
int (*uevent)(struct device *dev, struct kobj_uevent_env *env);
char *(*devnode)(struct device *dev, umode_t *mode,
kuid_t *uid, kgid_t *gid);
void (*release)(struct device *dev);
const struct dev_pm_ops *pm;
};
device_type的功能包括:
- name表示该类型的名称,当该类型的设备添加到内核时,内核会发出"DEVTYPE=‘name’”类型的uevent,告知用户空间某个类型的设备available了
- groups,该类型设备的公共attribute集合。设备注册时,会同时注册这些attribute。这就是面向对象中“继承”的概念
- uevent,同理,所有相同类型的设备,会有一些共有的uevent需要发送,由该接口实现
- devnode,devtmpfs有关的内容,暂不说明
- release,如果device结构没有提供release接口,就要查询它所属的type是否提供。用于释放device变量所占的空间
5.3 root device
在sysfs中有这样一个目录:/sys/devices,系统中所有的设备,都归集在该目录下。有些设备,是通过device_register注册到Kernel并体现在/sys/devices/xxx/下。但有时候我们仅仅需要在/sys/devices/下注册一个目录,该目录不代表任何的实体设备,这时可以使用下面的接口:
/*
* Root device objects for grouping under /sys/devices
*/
extern struct device *__root_device_register(const char *name,
struct module *owner);
/* This is a macro to avoid include problems with THIS_MODULE */
#define root_device_register(name) \
__root_device_register(name, THIS_MODULE)
extern void root_device_unregister(struct device *root);
该接口会调用device_register函数,向内核中注册一个设备,但是(你也想到了),没必要注册与之对应的driver(顺便提一下,内核中有很多不需要driver的设备,这是之一)。
参考文章:
http://www.wowotech/device_model/device_and_driver.html
本文标签: 模型设备Linuxdevicedevicedriver
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