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Easiest way to read from device inside another kernel driver

Posted on 2008-06-24
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Last Modified: 2010-04-21
I have some data available in /dev/mtd/0ro. I have no problem to get this data from userspace by opening this device and reading from it. But I need to get this data also within some kernel driver.

What is the easiest way I can do this?
What should I watch for?
Any hints/examples are welcomed.

Thank you.
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Question by:ineya
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kolrabi earned 500 total points
ID: 21878149
I never tried it, but this should work:

1. Get a file pointer for the device you want to open using filp_open.
2. Use the vfs_* functions to operate on the file. (See your linux/fs.h for syntax)
3. Close the file using filp_close.

Something like the snippet (which lacks any error checking).

HTH :)
struct file *f = filp_open("/dev/mtd/0ro", O_RDONLY, 0444);
 
/* seek to the beginning, not sure if this is really needed. */
vfs_llseek(f, 0, 0);
 
/* read some data */
vfs_read(f, buffer, buffer_size, &f->f_pos);
 
/* do something with the data ... */
 
/* close the file pointed to by f. */
filp_close(f, 0);

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Author Comment

by:ineya
ID: 21882558
Another solution seems to be to rely on EXPORT_SYMBOL of mtd driver:
http://linux.derkeiler.com/Mailing-Lists/Kernel/2003-10/3671.html
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Author Comment

by:ineya
ID: 21908926
@kolrabi: Do you know if there is some documentation for that filp_open, vfs_read? I'm failed trying to google it.
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Assisted Solution

by:kolrabi
kolrabi earned 500 total points
ID: 21914665
As far as I can tell from the header files, the syntax for filp_open() is the same as for open() but returning a "struct file" pointer instead of a file descriptor.

The syntax for the vfs_* functions seems to be the same as the syntax for the callbacks specified in the f_op member (struct file_operations) of the file pointer.

The file struct and the file_operations struct are explained in a little bit more detail here:

http://www.moses.uklinux.net/patches/lki-3.html#ss3.4

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Author Closing Comment

by:ineya
ID: 31470094
I didn't get to implement it, but it looks promising, thanks for the effort.
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