On Fri, Aug 28, 2015 at 10:34 AM, Dmitry Vyukov [off-list ref] 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