linux-kernel - Re: [PATCH RESEND 2/2] i2c: pnx: Fix read transactions of >= 2 bytes

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Message-ID: <5030A7A0.3080308@antcom.de>
Date:	Sun, 19 Aug 2012 10:45:20 +0200
From:	Roland Stigge <stigge@...com.de>
To:	Wolfram Sang <w.sang@...gutronix.de>
CC:	vitalywool@...il.com, khali@...ux-fr.org, ben-linux@...ff.org,
	grant.likely@...retlab.ca, linux-i2c@...r.kernel.org,
	linux-kernel@...r.kernel.org, kevin.wells@....com,
	srinivas.bakki@....com, aletes.xgr@...il.com, jonsmirl@...il.com
Subject: Re: [PATCH RESEND 2/2] i2c: pnx: Fix read transactions of >= 2 bytes

Hi,

On 18/08/12 11:52, Wolfram Sang wrote:
> On Wed, Aug 08, 2012 at 09:42:32AM +0200, Roland Stigge wrote:
>> On transactions with n>=2 bytes, the controller actually wrongly
>> clocks in n+1 bytes. This is caused by the (wrong) assumption
>> that RFE in the Status Register is 1 iff there is no byte already
>> ordered (via a dummy TX byte). This lead to the implementation of
>> synchronized byte ordering, e.g.:
>> 
>> Dummy-TX - RX - Dummy-TX - RX - ...
>> 
>> But since RFE actually stays high after some Dummy-TX, it rather
>> looks like:
>> 
>> Dummy-TX - Dummy-TX - RX - Dummy-TX - RX - (RX)
>> 
>> The last RX byte is clocked in by the bus controller, but ignored
>> by the kernel when filling the userspace buffer.
>> 
>> This patch fixes the issue by asking for RX via Dummy-TX
>> asynchronously. Introducing a separate counter for TX bytes.
>> 
>> Signed-off-by: Roland Stigge <stigge@...com.de>
>> 
>> --- Applies to v3.6-rc1
>> 
>> This patch for i2c-pnx affects PNX4008 and LPC32xx (and LPC31xx,
>> not yet in mainline). Can you please test?
>> 
>> Thanks in advance!
> 
> I assume you checked this on LPC32xx?

Yes.

The bug surfaced when we debugged unexpected behaviour with I2C
clients. A colleague noticed that sometimes, there were more bytes
transferred than expected, confusing some I2C clients.

The patch fixes exactly that.

Thanks,

Roland
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