Date: Wed, 1 Jul 2009 11:06:16 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: David =?ISO-8859-1?Q?H=E4rdeman?= <david@hardeman.nu>
Cc: linux-kernel@vger.kernel.org, linux-acpi@vger.kernel.org,
       linux-input@vger.kernel.org, jbarnes@virtuousgeek.org
Subject: Re: [PATCH 2/2] Add a driver for the Winbond WPCD376I IR 
 functionality.
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On Wed, 1 Jul 2009 09:47:46 +0200 (CEST)
David H__rdeman <david@hardeman.nu> wrote:

> >> +	struct wbcir_data *data = input_get_drvdata(dev);
> >> +
> >> +	if (scancode < 0 || scancode > 0xFFFFFFFF)
> >
> > Neither of the comparisons in this expression can ever be true.
> 
> Didn't know if I could be certain that int is always 32 bit on all
> platforms which use/might use the chip...can I?

Yep, int and unsigned int are always 32-bit.

It's not a big deal at all - the compiler will optimise the tests away
completely.  The tests may have some value as code commentary, and they
provide robustness should someone change the type of `scancode'.

> >> +		return -EINVAL;
> >> +
> >> +	*keycode = (int)wbcir_do_getkeycode(data, (u32)scancode);
> >
> > uneeded casts.
> >
> > Something has gone wrong with the types here.  Where does the fault lie
> > - this driver, or input core?
> 
> Two issues:
> 
> a) the input layer API confused me
> 
> include/linux/input.h provides:
> 
> struct input_event {
>          struct timeval time;
>          __u16 type;
>          __u16 code;
>          __s32 value;
> };
> 
> (keycode is an unsigned 16 bit integer)
> 
> int input_get_keycode(struct input_dev *dev, int scancode, int *keycode);
> int input_set_keycode(struct input_dev *dev, int scancode, int keycode);
> 
> (keycode is an int)
> 
> static inline void input_report_key(struct input_dev *dev,
>                                     unsigned int code,
>                                     int value)
> {
>         input_event(dev, EV_KEY, code, !!value);
> }
> 
> (keycode is an uint)
> 
> 
> b) 32 bit scancodes
> 
> I wanted to use 32 bit scancodes in my driver since the largest IR message
> supported by the driver is RC6 mode 6A which can potentially have a 1 + 15
> bits "customer" field + 8 bits address + 8 bits command = 32 bits.
> 
> Casting the int scancode passed to input_[get__set]_keycode to an uint and
> assuming it would be at least 32 bits on all platforms using the chip was
> the best solution I could come up with without changing the input API.

erp.  Hopefully this is all something which Dmitry can help us with.

> >> +{
> >> +        struct wbcir_data *data = (struct wbcir_data *)cookie;
> >> +        unsigned long flags;
> >> +
> >> +        /*
> >> +         * data->keyup_jiffies is used to prevent a race condition if a
> >> +         * hardware interrupt occurs at this point and the keyup timer
> >> +         * event is moved further into the future as a result.
> >> +         */
> >
> >hm.  I don't see what the race is, nor how the comparison fixes it.  If
> >I _did_ understand this, perhaps I could suggest alternative fixes.
> >But I don't so I can't.  Oh well.
> 
> When the interrupt service routine detects an IR command it reports a
> keydown event and sets a timer to report a keyup event in the future if no
> repeated ir messages are detected (in which case the timer-driven keyup
> should be pushed further into the future to allow the input core to do its
> auto-repeat-handling magic).
> 
> What I wanted to protect against was something like this:
> 
> Thread 1                        Thread 2
> --------                        --------
> ISR called, grabs
> wbcir_lock, reports
> keydown for key X,
> sets up keyup timer,
> releases lock, exits
> 
>                     (many ms later)
> 
>                                 keyup timer function called
>                                 and preempted before grabbing
>                                 wbcir_lock
> 
> ISR called, grabs wbcir_lock,
> notices a repeat event for
> key X, pushes the keyup timer
> further into the future using
> mod_timer (thus reenabling the
> timer), releases lock, exits
>                                 keyup timer function grabs
>                                 wbcir_lock, reports keyup,
>                                 exits.
>                     (many ms later)
> 
>                                 keyup timer function called *again*,
>                                 reports keyup, exits.
> 
> The result would be (if I understood the timer implementation correctly)
> that a keyup event is reported immediately after the second ISR even
> though the "first" timer function call should have been cancelled/pushed
> further into the future at that point.
> 
> Does this make any sense? :)

yes.  The timer will be rescheduledin the scenario which you describe.

Here's my problem: often when I ask a question about some code, what I
_really_ mean is "if I didn't understand this code today, others
probably won't understand it when reading it a year from now.  Hence it
perhaps needs additional commentary to prevent this".  But I tire of
complaining about code comments ;)

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