linux-kernel - Re: [PATCH] Add a quirk for the Dell XPS 13 (2015) when in PS/2 mode.

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Date:	Sat, 11 Apr 2015 00:39:40 +0200
From:	Pali Rohár <pali.rohar@...il.com>
To:	Mario Limonciello <mario_limonciello@...l.com>
Cc:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
	Rob <robr.bensson@...il.com>
Subject: Re: [PATCH] Add a quirk for the Dell XPS 13 (2015) when in PS/2 mode.

On Wednesday 25 February 2015 21:45:22 Pali Rohár wrote:
> On Wednesday 25 February 2015 19:48:55 Mario Limonciello wrote:
> > On 02/20/2015 02:41 PM, Pali Rohár wrote:
> > > On Friday 20 February 2015 20:56:23 Mario Limonciello
> > > wrote:
> > > 
> > > resetafter=0 means to never reset (even if driver receive
> > > e.g thousand invalid packets). I think this is very
> > > dangerous if there will be other bugs either in linux
> > > driver or some other HW problems.
> > > 
> > > For ALPS issue I added resetafter = pktsize * 2 (Allow 2
> > > invalid packets without resetting device). Cannot you find
> > > something similar for synaptics touchpads on XPS? (pktsize
> > > for ALPS is 6, no idea how big are synaptics packets).
> > 
> > Pali,
> > 
> > I've done some experimentation with increasing the size to
> > resetafter to up to pktsize * 4.  It will decrease the
> > number of occurrences of this problem, but the problem
> > still occurs eventually.  pktsize for synaptics is 6 as
> > well.  Would you recommend to continue to go higher than
> > that?  Since out_of_sync_cnt is reset when a full packet
> > gets received, some arbitrarily high number should likely
> > fix it to.
> > 
> > That being said, if you try to more closely follow what
> > Windows does for the mouse, it's not issuing a reconnect no
> > matter how much bad data is received.
> 
> I believe problem is similar to one as with ALPS devices.
> Driver always receive 6 bytes packet of data (no new byte is
> inserted and no byte is never lost), just one byte in packet
> is incorrect (does not match specification).
> 
> Setting resetafter to > 0 prevent problems when driver enters
> into undefined state (either by bug in driver of other SW/HW
> problem). So I think setting resetafter to 0 is not good idea.
> 
> But if we know that setting resetafter to 4*pktsize is not
> enough (e.g. with experimenting you saw that driver received
> more then 4 invalid packets consecutively), set it to higher
> value.
> 
> I think it is still good idea to ignore maximally as many
> packets which can be received in time which is equal to
> resetting device.
> 
> E.g. when period of time in which we are dropping all packets
> is higher then time needed to reset touchpad, we should stop
> dropping packets and immediately reset touchpad. In this case
> we could hit maybe problem in driver (there can be bugs) or
> touchpad is in some bad state and out-of-sync...
> 
> So if your tests show that there are never invalid 10 packets
> consecutively, then set resetafter to 10 packets. Value 10 is
> still not high and if it fix problem with touchpad I think it
> is acceptable. But rather ask Dmitry what he thinks about it.
> This is just my opinion.

Hello Mario,
have you patched synaptics driver with some resetafter parameter? 
And have some team in dell found reason for invalid packets?

-- 
Pali Rohár
pali.rohar@...il.com

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