linux-kernel - Re: [PATCH] i2c: tegra: proper handling of error cases

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Message-ID: <20160418073118.GC13078@ulmo.ba.sec>
Date:	Mon, 18 Apr 2016 09:31:18 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Shardar Mohammed <smohammed@...dia.com>
Cc:	Laxman Dewangan <ldewangan@...dia.com>,
	"swarren@...dotorg.org" <swarren@...dotorg.org>,
	"linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"wsa@...-dreams.de" <wsa@...-dreams.de>,
	"gnurou@...il.com" <gnurou@...il.com>
Subject: Re: [PATCH] i2c: tegra: proper handling of error cases

On Sat, Apr 16, 2016 at 08:08:50AM +0000, Shardar Mohammed wrote:
> Thanks for the review, updated with comments inline with Shardar.
> 
> > On Fri, Apr 15, 2016 at 06:51:47PM +0530, Shardar Shariff Md wrote:
> > > From: Shardar Shariff Md <smohammed@...dia.com>
> > >
> > > To summarize the issue observed in error cases:
> > >
> > > In SW: For i2c message transfer, packet header and data payload is
> > > posted and then required error/packet completion interrupts are
> > > enabled later.
> > >
> > > In HW flow: HW process the packet just after packet header is posted,
> > > if ARB lost/NACK error occurs (SW will not handle immediately when
> > > error happens as error interrupts are not enabled at this point). HW
> > > assumes error is acknowledged and clears current data in FIFO, But SW
> > > here posts the remaining data payload which still stays in FIFO as
> > > stale data (data without packet header).
> > >
> > > Now once the interrupts are enabled, SW handles ARB lost/NACK error by
> > > clearing the ARB lost/NACK interrupt. Now HW assumes that SW attended
> > > the error and will parse/process stale data (data without packet
> > > header) present in FIFO which causes invalid NACK errors.
> > >
> > > Fix: Enable the error interrupts before posting the packet into FIFO
> > > which make sure HW to not clear the fifo. Also disable the packet mode
> > > before acknowledging errors (ARB lost/NACK error) to not process any
> > > stale data.
> > >
> > > As error interrupts are enabled before posting the packet header use
> > > spinlock to avoid preempting.
> > 
> > Please try to use up the full 78 characters per line in the commit message
> > (with the exception being the subject line that should be shorter).
> 
> [Shardar] Will take care of this.

I just realized my comment was wrong. The rule is to use 72 characters
per line for the commit message. Sorry about that.

Thierry

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