Re: Discovering select(2) parameters from driver's poll method

["linux-os \(Dick Johnson\)" <linux-os@analogic.com>]

On Fri, 9 Jun 2006, Ozan Eren Bilgen wrote:

> *** Please CC me your responses ***
>
> Hi,
>
> I am writing a device driver and I have problem with poll method.  For
> some reason, I need learn the timeout and descriptor sets of select(2)
> call.  Other words to say, if the user space process calls:
>
> 	select(n, &readfds, NULL, &exceptionfds, &tv);
>
> With the help of my poll implementation in device driver, I want to
> learn that only the write fds is empty.  I am also interested in the
> value of timeout parameter.  Please let me know if this is possible.
>

If you want to know, your driver is HORRIBLY BROKEN beyond any
repair.

The kernel handles poll in a POSIX correct manner. There is a
link into your device driver which is __not__ directly attached
to the user's poll call, even though its name is similar. Your
driver must properly signal the kernel, with the proper wake-up
call, after the proper bits are put into the poll variable, any
time anything has changed.

If or when user action (read, write, etc.) changes those bits,
then they must also be updated -- and the kernel again signaled
that those bits have changed.

> By the way, I checked out some Linux device drivers, which are
> implemented poll method, and related books like LDD.  Everywhere,
> poll_wait is called for both read and write queues, without taking the
> select(2) call's parameters into account.  For example it still waits
> for the read queue although the select call was looking only for write
> fds.  My second question is, why a poll method queries all the queues,
> instead of querying only the necessary one?

Wrong. The kernel calls the poll function in the driver. The user code
does not call this directly. The kernel knows what fds are active
for whatever. Your driver **MUST NOT**!

>
> Thank you in advance,
>
> Ozan Eren Bilgen

Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.4 on an i686 machine (5592.88 BogoMips).
New book: http://www.AbominableFirebug.com/
_


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