When we do the fwrite to the file /sys/class/leds/LED1/brightness , how it turns on/off the LED.Can any one give me some brief details about this..i meant from the user space to Driver how the flow is happening,and what are the modules involved in this.

Thanks in advance!

/sys, like /proc, is a virtual filesystem. Actually, it is a very sophisticated API, since none of the "files" and "directories" in it actually exist.

Here's a good foundation paper on the subject: https://www.kernel.org/pub/linux/ker...005/mochel.pdf.

Of course, since it was written many years ago now, the implementation details will have changed. But it is an excellent overview of how this interface works, and how it was implemented.

Everything that hooks into this API emulates one or more files, directories, symlinks, and so on. (Refer to the PDF, especially starting in Section 6 and beyond.) In this regard, /sys is a bit more-ambitious than /proc, but the essential concepts and facilities are very similar.

At some point, there will be a software object that corresponds to that LED-control, and it will be expecting (probably ...) a sysctl() call. Linux will direct the call to this object when it comes in, and the code, executing in kernel-mode, must do the right thing.

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On my laptop, the brightness is adjustable via writing to /sys/class/backlight/radeon_bl0/brightness. Only Root is allowed to write, the rest can only read the value. 0 is off, 1 is minimum, 255 is maximum brightness.

So I wrote a little C program that takes one number as a parameter and writes the value to that file. The compiled binary needs to belong to root and have the suid bit set in order to allow everyone to change the backlight brightness.

There are some brightness symbols on the keyboard, so I used xev to find the keycodes. Then I wrote a shell script that reads the value from /sys/class/backlight/radeon_bl0/brightness and increases/decreases it according to the given parameter. Then the script writes that value out by using the program I mentioned before. And finally I added two keyboard shortcuts to the openbox rc.xml. So now I can use Fn+F6/Fn+F7 to adjust the brightness. (And I could use the program on it's own to set the brightness to a specific value during startup or when the power supply is connected/disconnected.)

Would you like to see the code or would you like to figure out the rest on your own?

... and, if you want to learn how such things work, you should grab a copy of the Kernel source-code and look at how this brightness-control pseudo-file was implemented.

All of the pseudo- file-systems work in basically the same way, and all of the source code is available for your amusement and amazement . . .

Remember that egrep is your best friend. For instance, look for \bbrightness\b ...