On Fri, Aug 28, 2015 at 10:34 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> Hello,
>
> I am looking at this code in __ps2_command again:
>
> /*
> * The reset command takes a long time to execute.
> */
> timeout = msecs_to_jiffies(command == PS2_CMD_RESET_BAT ? 4000 : 500);
>
> timeout = wait_event_timeout(ps2dev->wait,
> !(READ_ONCE(ps2dev->flags) & PS2_FLAG_CMD1), timeout);
>
> if (smp_load_acquire(&ps2dev->cmdcnt) &&
> !(smp_load_acquire(&ps2dev->flags) & PS2_FLAG_CMD1)) {
> timeout = ps2_adjust_timeout(ps2dev, command, timeout);
> wait_event_timeout(ps2dev->wait,
> !(smp_load_acquire(&ps2dev->flags) &
> PS2_FLAG_CMD), timeout);
> }
>
> if (param)
> for (i = 0; i < receive; i++)
> param[i] = ps2dev->cmdbuf[(receive - 1) - i];
>
>
> Here are two moments I don't understand:
> 1. The last parameter of ps2_adjust_timeout is timeout in jiffies (it
> is compared against 100ms). However, timeout is assigned to result of
> wait_event_timeout, which returns 0 or 1. This does not make sense to
> me. What am I missing?
The fact that wait_event_timeout can return value greater than one:
* Returns:
* 0 if the @condition evaluated to %false after the @timeout elapsed,
* 1 if the @condition evaluated to %true after the @timeout elapsed,
* or the remaining jiffies (at least 1) if the @condition evaluated
^^^^^^^^^^^^^^^^^^^^^^^^^
> 2. This code pays great attention to timeouts, but in the end I don't
> see how it handles timeouts. That is, if a timeout is happened, we
> still copyout (garbage) from cmdbuf. What am I missing here?
Once upon a time wait_event() did not return positive value when
timeout expired and then condition satisfied. So we just examine the
final state (psmpouse->cmdcnt should be 0 if command actually
succeeded) and even if we copy in garbage nobody should care since
we'll return error in this case.
Thanks.
--
Dmitry
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