Waiting / Blocking in Linux Driver Part – 4

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In the last article, we discussed the usage of wait queues in Linux kernel. We saw the variants of wait_event(). Just for completeness, we will discuss how the wait queues are implemented internally. First step is the creation and initialization of wait queue entry. This is done as below:

DEFINE_WAIT(wait_entry);

Next step is to add the wait queue entry to the queue and set the process state. Both of these things are being done by a single function declared below:

prepare_to_wait(wait_queue_head_t *wq_head, wait_queue_t *wq, int state);

After this, we schedule out the process by invoking the schedule() API. Once we are done with waiting, next step is to clean up with the API below:

finish_wait(wait_queue_head_t *wq_head, wait_queue_t *wq);

All the above are available from:

#include <linux/wait.h>

One more point worth noting is that the condition has to be tested manually as was done in our earlier article.

Pradeep D Tewani (12 Posts)

The author works at Intel, Bangalore. The author is a Linux enthusiast and is interested in Linux porting, Linux Kernel Internal & Linux device drivers. He shares his learnings on Linux & embedded systems through his weekend workshops. Learn more about his experiments at http://sysplay.in.


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3 thoughts on “Waiting / Blocking in Linux Driver Part – 4

  1. Pingback: Waiting / Blocking in Linux Driver Part – 3 | Playing with Systems

  2. Viswanath

    Nice article. My question is, shall we use this concept in character drivers(i2c/spi/uart).
    Like say, an i2c driver reading data from device — shall i use this concept either wait/blocking
    Not like wait in read function call and wake-up in the write function call.

    Or
    existing character drivers(i2c/spi/uart) are already blocking/wait device drivers.
    Could you please help in understanding this concept.

    Reply
    1. admin

      Frankly speaking, this is a generic concept. It can be used anywhere, where it suits the best. Moreover, if I understand, the one shown above is just the framework, which is used internally by many others. The already blocking drivers may be ultimately using this internally.

      Reply

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