Re: POLL in char drivers

[Chetan Nanda <chetannanda@xxxxxxxxx>]

On Sun, Aug 16, 2009 at 8:23 PM, shankarGanesh <shankarganeshk@xxxxxxxxx> wrote:

Chetan Nanda wrote:

On Thu, Aug 13, 2009 at 6:30 PM, Shankar Ganesh <shankarganeshk@xxxxxxxxx <mailto:shankarganeshk@xxxxxxxxx>> wrote:



   On Thu, Aug 13, 2009 at 5:23 PM, Daniel Baluta

   <daniel.baluta@xxxxxxxxx <mailto:daniel.baluta@xxxxxxxxx>> wrote:

       On Thu, Aug 13, 2009 at 2:17 PM, Shankar
       Ganesh<shankarganeshk@xxxxxxxxx

       <mailto:shankarganeshk@xxxxxxxxx>> wrote:
       > Hi ,
       >
       > I have a doubt regarding poll function in char driver . Let
       us consider
       > following code
       >
       >
       --------------------------------------------------------------------------------------
       > static unsigned int random_poll(struct file *file,
       poll_table * wait)
       > {
       >      unsigned int mask;
       >    poll_wait(file, &random_read_wait, wait);
       >    poll_wait(file, &random_write_wait, wait);
       >    mask = 0;
       > ........
       >      return mask;
       > }
       >
       >
       > read()  / write()
       > {
       >        wait_event_interuptable()
       > }
       >
       --------------------------------------------------------------------------------------
       >
       > Does the above code (poll function) is in sleep state until both
       > random_read_wait & random_write_wait are awakened ? . If i
       have use case If
       > i want my user space application to poll only for read ,
       then can i remove
       > one of the above  ?
       t beautiful thing we can experience is the mysterious. It is the
       > source of all true art and science."
       >
       AFAIK your actual random_poll function nevers sleeps. The
       calls for
       poll_wait just inform the VFS layer
       about the existence of random_read_wait and random_write_wait wait
       queues. An upper layer will use the return mask
       to actual put your current process to sleep if there are no data
       available for reading/writing.


   Thanks. How do VFS know which of the queue is woke-up (read -WQ or
   write - WQ ) ? . As any ISR or relevant function wil wake-up the
   wait-queue.

It is decided by value return by drivers poll function. e.g. if driver detects that device is ready for reading then poll function should return 'POLLRDNORM' to VFS layer. Similarly 'POLLWRNORM' for writable case.

Thanks Chetan .
Return Value  - POLLRDNORM & POLLWRNORM describes the operations  that could be immediately performed without blocking. I would like to know the case where there is blocking

Hi Shankar,

Driver need to return a bitmask from its poll function, if value of this bitmask is zero then kernel VFS layer automatically put calling process into sleep state on waitqueue passed in 'poll_wait'.
Rest of the handiling is done inside the kernel and driver writers just need to detect the conditions and to return a proper values from their polll function.

For better understanding have a look at section 6.3 of LDD3.

Thanks,
Chetan Nanda

       I think you should not care about what type of operation a
       user space
       application is watching for.

       Daniel.




   --    With Regards,
   ShankarGanesh K .


Thanks,
Chetan Nanda






   "The most beautiful thing we can experience is the mysterious. It
   is the source of all true art and science."