linux-kernel - Re: [PATCH 2/5] input: Use driver hint to compute the evdev buffer size (rev3)

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Message-ID: <4C223F60.6080904@euromail.se>
Date:	Wed, 23 Jun 2010 19:07:44 +0200
From:	Henrik Rydberg <rydberg@...omail.se>
To:	Ping Cheng <pinglinux@...il.com>
CC:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jiri Kosina <jkosina@...e.cz>, Ping Cheng <pingc@...om.com>,
	Benjamin Tissoires <tissoire@...a.fr>,
	Chase Douglas <chase.douglas@...onical.com>,
	Rafi Rubin <rafi@...s.upenn.edu>
Subject: Re: [PATCH 2/5] input: Use driver hint to compute the evdev buffer
 	size (rev3)

Hi Ping,
[...]
>> @@ -51,7 +52,9 @@ static DEFINE_MUTEX(evdev_table_mutex);
>>
>>  static int evdev_compute_buffer_size(struct input_dev *dev)
>>  {
>> -       return EVDEV_MIN_BUFFER_SIZE;
>> +       int nev = dev->hint_events_per_packet * EVDEV_BUF_PACKETS;
>> +       nev = max(nev, EVDEV_MIN_BUFFER_SIZE);
>> +       return roundup_pow_of_two(nev);
> 
> I think we have a backward compatibility issue here. This routine will
> return 7 when nev falls to the default value
> (EVDEV_MIN_BUFFER_SIZE/64).  This could happen to those drivers that
> don't report MT events or forget/don't feel the need to set
> hint_events_per_packet since the old BUFFER_SIZE worked perfectly for
> them.  We need to keep the return value for those drivers as 64 so we
> could allocate the same space as it was in [PATCH 1/5].

Are you perhaps confusing EVDEV_BUF_PACKETS and EVDEV_MIN_BUFFER_SIZE? The last
line ensures that the value returned is a power of two (hence not 7). The
second-to-last line ensures the value is at least equal to 64 (hence not 7). The
default hint value for a driver that does not do anything is zero, which leads
to a return value of 64, just as it is today.

Thanks,
Henrik

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