I am trying to build an array of type "struct scatterlist", from a buffer pointed by a virtual kernel address (I know the byte size of the buffer, but it may be large). Ideally I would like to have function like init_sg_array_from_buf:
void my_function(void *buffer, int buffer_length)
{
struct scatterlist *sg;
int sg_count;
sg_count = init_sg_array_from_buf(buffer, buffer_length, sg);
}
Which function in the scatterlist api, does something similar? Currently the only possibility I see, is to manually determine the amount of pages, spanned by the buffer. Windows has a kernel macro called "ADDRESS_AND_SIZE_TO_SPAN_PAGES", but I didn't even manage to find something like this in the linux kernel.
I wrote a platform driver for a peripheral we developed and would like to expose some configuration options to the sysfs. I have managed to create the appropriate files using attribute structs (see below) and sysfs_create_file in the probe function, but I can't figure out how to attach the show/store functions to the structs in a platform driver.
Most resources I found online used a device_attribute struct or something similar to create their files, is that also appropriate here? Is there another way to do this for a platform driver?
My attribute struct looks like this:
struct attribute subkey_attr = {
.name = "subkeys",
.mode = S_IWUGO | S_IRUGO,
};
And I register the file using this call:
riddler_kobject = &pdev->dev.kobj;
ret_val = sysfs_create_file(riddler_kobject, &subkey_attr);
It boils down to next:
reuse existing kobject from struct device (from your struct platform_device) for sysfs_create_group() (instead of creating your own kobject)
use DEVICE_ATTR() to declare struct device_attribute instead of regular __ATTR(), which creates struct kobj_attribute.
Here is how I created sysfs attributes for my platform driver.
Create structure you'll be using as private data in show() / store() operations for your sysfs attribute (file). For example:
struct mydrv {
struct device *dev;
long myparam;
};
Allocate this structure in your driver's probe():
static int mydrv_probe(struct platform_device *pdev)
{
struct mydrv *mydrv;
mydrv = devm_kzalloc(&pdev->dev, sizeof(*mydrv), GFP_KERNEL);
mydrv->dev = &pdev->dev;
platform_set_drvdata(pdev, mydrv);
...
}
Create show() / store() functions:
static ssize_t mydrv_myparam_show(struct device *dev,
struct device_attribute *attr, char *buf)
{
struct mydrv *mydrv = dev_get_drvdata(dev);
int len;
len = sprintf(buf, "%d\n", mydrv->myparam);
if (len <= 0)
dev_err(dev, "mydrv: Invalid sprintf len: %d\n", len);
return len;
}
static ssize_t mydrv_myparam_store(struct device *dev,
struct device_attribute *attr, const char *buf, size_t count)
{
struct mydrv *mydrv = dev_get_drvdata(dev);
kstrtol(buf, 10, &mydrv->myparam);
return count;
}
Create device attribute for those functions (right after those functions):
static DEVICE_ATTR(myparam, S_IRUGO | S_IWUSR, mydrv_myparam_show,
mydrv_myparam_store);
Declare attributes table (listing in fact sysfs files for you driver):
static struct attribute *mydrv_attrs[] = {
&dev_attr_myparam.attr,
NULL
};
Declare attribute group (specifying in fact sysfs directory for your driver):
static struct attribute_group mydrv_group = {
.name = "mydrv",
.attrs = mydrv_attrs,
};
static struct attribute_group *mydrv_groups[] = {
&mydrv_group,
NULL
}
which can be actually replaced with one line:
ATTRIBUTE_GROUPS(mydrv);
Create sysfs directory and files in your driver's probe() function:
static int mydrv_probe(struct platform_device *pdev)
{
int ret;
...
ret = sysfs_create_group(&pdev->dev.kobj, &mydrv_group);
if (ret) {
dev_err(&pdev->dev, "sysfs creation failed\n");
return ret;
}
...
}
Remove your sysfs files in your driver's remove() function:
static int mydrv_remove(struct platform_device *pdev)
{
sysfs_remove_group(&pdev->dev.kobj, &mydrv_group);
...
}
Race condition note
As #FranzForstmayr correctly pointed out, there may be race condition when adding sysfs files with sysfs_create_group() in mydrv_probe(). That's because user-space can be already notified that those files exist before mydrv_probe() called (where those files are actually being created by sysfs_create_group() function). This issue covered in details in "How to Create a sysfs File Correctly" article by Greg Kroah-Hartman.
So in our case of platform_device, instead of calling sysfs_create_group() (and its counterpart sysfs_remove_group()), you can use default attribute group. To do so, you need to assign corresponding .groups field of your struct device to your attribute groups variable:
static int mydrv_probe(struct platform_device *pdev)
{
...
pdev->dev.groups = mydrv_groups;
...
}
DISCLAIMER: I didn't test this code, though it should work, because of this code.
See [1,2,3] links for more insights on mentioned race condition.
For more examples, run next command in kernel source directory:
$ git grep -l --all-match -e platform_device -e attribute -e '\.groups =' -- drivers/
Also you can search by "default attribute" in commit messages:
$ git log --no-merges --oneline --grep="default attribute" -- drivers/
Some commits I found this way: [4,5,6,7].
References
[1] My attributes are way too racy, what should I do?
[2] PATCH: sysfs: add devm_sysfs_create_group() and friends
[3] [GIT PATCH] Driver core patches for 3.11-rc2
[4] commit 1
[5] commit 2
[6] commit 3
[7] commit 4
Not enough reputation to post a comment, but I just want to comment on the default attribute group note from the accepted answer.
My understanding is that this should not be added in the probe function, as given in the example, but instead should be set in the device struct, (or device_driver, class, or bus depending on your driver) usually defined at the end of your file.
For example:
static struct device iio_evgen_dev = {
.bus = &iio_bus_type,
.groups = iio_evgen_groups,
.release = &iio_evgen_release,
};
from this example
Strangely, according to this it doesn't work correctly when using DEVICE_INT_ATTR to create the attribute, so not sure what that's all about.
Also, I'm not 100% sure, but I think that this is invoked when the driver is loaded, not when the device is probed.
This is an addition to Sam Protsenko's and Anthony's answers
If you create device attributes via the DEVICE_ATTR macros then you have to register the attribute groups (mydrv_groups) in the .dev_groups instead of the .groups field.
static struct device iio_evgen_dev = {
.bus = &iio_bus_type,
.dev_groups = iio_evgen_groups, // .dev_groups for DEVICE_ATTR
.groups = another_attr_group, // .groups for DRIVER_ATTR
.release = &iio_evgen_release,
};
Then the attributes are automatically registered correctly without setting up anything in the probe/remove functions, as described in Greg Kroah-Hartman's article.
Assume that the module has been loaded into the kernel and the driver is registered in
/sys/bus/platform/drivers/mydrv
every device instances will be a subdirectory of the driver's folder like
/sys/bus/platform/drivers/mydrv/mydrv1
/sys/bus/platform/drivers/mydrv/mydrv2
Registering attributes in the .groups field creates the attributes in the driver folder.
Registering attributes in the .dev_groups field creates the attributes in the device's instance folder.
mydrv
├── driver_attr1
├── driver_attr2
└── mydrv1
├── device_attr1
└── device_attr2
The show/store functions of the attributes in the .groups field do not have access to the driver data set via platform_set_drvdata(pdev, mydrv).
At least not by accessing it via dev_get_drvdata(dev).
Accessing the driver data via dev_get_drvdata(dev) returns NULL and dereferencing it will result in a kernel oops.
In turn, he show/store functions of the attributes in the .dev_groups field have access to the driver data via
struct mydrv *mydrv = dev_get_drvdata(dev);
Can anyone tell me how a Char Driver is bind to the corresponding physical device?
Also, I would like to know where inside a char driver we are specifying the physical device related information, which can be used by kernel to do the binding.
Thanks !!
A global array — bdev_map for block and cdev_map for character devices — is used to implement a hash table, which employs the device major number as hash key.
while registering for char driver following calls get in invoked to get major and minor numbers.
int register_chrdev_region(dev_t from, unsigned count, const char *name)
int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count,
const char *name);
After a device number range has been obtained, the device needs to be activated by adding it to the character device database.
void cdev_init(struct cdev *cdev, const struct file_operations *fops);
int cdev_add(struct cdev *p, dev_t dev, unsigned count);
Here on cdev structure initialize with file operation and respected character device.
Whenever a device file is opened, the various filesystem implementations invoke the init_special_inode function to create the inode for a block or character device file.
void init_special_inode(struct inode *inode, umode_t mode, dev_t rdev)
{
inode->i_mode = mode;
if (S_ISCHR(mode)) {
inode->i_fop = &def_chr_fops;
inode->i_rdev = rdev;
} else if (S_ISBLK(mode)) {
inode->i_fop = &def_blk_fops;
inode->i_rdev = rdev;
}
else
printk(KERN_DEBUG "init_special_inode: bogus i_mode (%o)\n",
mode);
}
now the default_chr_fpos chrdev_open() method will get invoked. which will look up for the inode->rdev device in cdev_map array and will get a instance of cdev structure. with the reference to cdev it will bind the file->f_op to cdev file operation and invoke the open method for character driver.
In a character driver like I2C client driver, We specify the slave address in the client structure's "addr" field and then call i2c_master_send() or i2c_master_receive() on this client . This calls will ultimately go to the main adapter controlling that line and the adapter then communicates with the device specified by the slave address.
And the binding of drivers operations is done mainly with cdev_init() and cdev_add() functions.
Also driver may choose to provide probe() function and let kernel find and bind all the devices which this driver is capable of supporting.
I have been studying I2C driver (client) code for a while.
I have seen this function "i2c_get_clientdata" and "i2c_set_clientdata" every where.
I have seen the this question here .
Use of pointer to structure instead of creating static local copy
Some times i think like it is like "container_of" macro to get a pointer to the structure.
But still i didn't understood properly why to use it and when to use it.
Below i am posting a sample code where I see its usage.
If any one could help me understand why it is used there and when we shall use it when we write our own drivers.
struct max6875_data {
struct i2c_client *fake_client;
struct mutex update_lock;
u32 valid;
u8 data[USER_EEPROM_SIZE];
unsigned long last_updated[USER_EEPROM_SLICES];
};
static ssize_t max6875_read(struct file *filp, struct kobject *kobj,
struct bin_attribute *bin_attr,
char *buf, loff_t off, size_t count)
{
struct i2c_client *client = kobj_to_i2c_client(kobj);
struct max6875_data *data = i2c_get_clientdata(client);
int slice, max_slice;
if (off > USER_EEPROM_SIZE)
return 0;
if (off + count > USER_EEPROM_SIZE)
count = USER_EEPROM_SIZE - off;
/* refresh slices which contain requested bytes */
max_slice = (off + count - 1) >> SLICE_BITS;
for (slice = (off >> SLICE_BITS); slice <= max_slice; slice++)
max6875_update_slice(client, slice);
memcpy(buf, &data->data[off], count);
return count;
}
Those functions are used to get/set the void *driver_data pointer that is part of the struct device, itself part of struct i2c_client.
This is a void pointer that is for the driver to use. One would use this pointer mainly to pass driver related data around.
That is what is happening in your example. The max6875_read is a callback getting a structu kobject. That kobject is an i2c_client which is enough to communicate with the underlying device using the driver_data pointer here allows to get back the driver related data (instead of using global variables for example).
i'd need to obtain a pointer to a particular device registered in linux. Briefly, this device represents a mii_bus object. The problem is that this device seems doesn't belong to a bus (its dev->bus is NULL) so i can't use for example the function bus_for_each_dev. The device is however registered by the Open Firmware layer and i can see the relative of_device (which is the parent of the device i'm interested in) in /sys/bus/of_platform. My device is also registered in a class so i can find it in /sys/class/mdio_bus. Now the questions:
It's possible to obtain the pointer using the pointer to the of_device that is the parent of the device we want?
How can i get a pointer to an already instantiated class by using only the name?If it was possible i could iterate over the devices of that class.
Any other advice would be very useful! Thank you all.
I found the way. I explain it briefly, maybe it could be useful. The method we could use is device_find_child. The method takes as third parameter a pointer to a function that implements the comparison logic. If the function returns not zero when called with a particular device as first parameter, device_find_child will return that pointer.
#include <linux/device.h>
#include <linux/of_platform.h>
static int custom_match_dev(struct device *dev, void *data)
{
/* this function implements the comaparison logic. Return not zero if device
pointed by dev is the device you are searching for.
*/
}
static struct device *find_dev()
{
struct device *ofdev = bus_find_device_by_name(&of_platform_bus_type,
NULL, "OF_device_name");
if (ofdev)
{
/* of device is the parent of device we are interested in */
struct device *real_dev = device_find_child(ofdev,
NULL, /* passed in the second param to custom_match_dev */
custom_match_dev);
if (real_dev)
return real_dev;
}
return NULL;
}