Message-ID: <20151021123740.GE25824@odux.rfo.atmel.com>
Date:	Wed, 21 Oct 2015 14:37:40 +0200
From:	Ludovic Desroches <ludovic.desroches@...el.com>
To:	Peter Rosin <peda@...ator.liu.se>
CC:	Cyrille Pitchen <cyrille.pitchen@...el.com>, <wsa@...-dreams.de>,
	<linux-i2c@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <nicolas.ferre@...el.com>
Subject: Re: [PATCH] i2c: at91: fix write transfers by clearing pending
 interrupt first

On Wed, Oct 21, 2015 at 09:42:40AM +0200, Peter Rosin wrote:
> On 2015-10-21 09:21, Peter Rosin wrote:
> > On 2015-10-20 15:27, Ludovic Desroches wrote:
> >> On Mon, Oct 19, 2015 at 12:49:03PM +0200, Peter Rosin wrote:
> >>> On 2015-10-19 10:51, Ludovic Desroches wrote:
> >>>> Hi Peter,
> >>>>
> >>>> On Fri, Oct 16, 2015 at 11:08:42AM +0200, Peter Rosin wrote:
> >>>>> On 2015-10-16 01:47, Peter Rosin wrote:
> >>>>>> On 2015-10-14 07:43, Ludovic Desroches wrote:
> >>>>>>> On Tue, Oct 13, 2015 at 08:01:34PM +0200, Peter Rosin wrote:
> >>>>>>>> On 2015-10-13 18:47, Cyrille Pitchen wrote:
> >>>>>>>>> Le 13/10/2015 17:19, Peter Rosin a écrit :
> >>>>>>>>>> On 2015-10-13 16:21, Ludovic Desroches wrote:
> >>>>
> >>>> [...]
> >>>>
> >>>>>> I have started to get this when I run with this patch:
> >>>>>>
> >>>>>> [   73.310000] at91_i2c f0014000.i2c: RXRDY still set!
> >>>>>> [  198.200000] at91_i2c f0014000.i2c: RXRDY still set!
> >>>>>> [  509.880000] at91_i2c f0014000.i2c: RXRDY still set!
> >>>>>> [  615.750000] at91_i2c f0014000.i2c: RXRDY still set!
> >>>>>> [  617.750000] at91_i2c f0014000.i2c: RXRDY still set!
> >>>>>> [ 1766.640000] at91_i2c f0014000.i2c: RXRDY still set!
> >>>>>> [ 2035.380000] at91_i2c f0014000.i2c: RXRDY still set!
> >>>>>> [ 2227.190000] at91_i2c f0014000.i2c: RXRDY still set!
> >>>>>> [ 2313.100000] at91_i2c f0014000.i2c: RXRDY still set!
> >>>>>>
> >>>>>> My USB serial dongle was hung which was why I didn't notice until just now.
> >>>>>>
> >>>>>> This is probably not when communication with the eeprom though, and certainly not
> >>>>>> writing to it, but perhpaps when polling the temperature (using the jc42 driver).
> >>>>>> I'll investigate further in the morning to see if I can pinpoint it.
> >>>>>
> >>>>> Yep, it's the jc42 accesses that triggers this (to the same chip as the
> >>>>> eeprom, but a different block of transistors I suppose).
> >>>>>
> >>>>> Looking at the i2c bus, the accesses normally go like this:
> >>>>>
> >>>>> [0.000000] S 0x18 W 0x05 S 0x18 R 0xc1 0xbe NACK P
> >>>>> [0.000521] S 0x18 W 0x04 S 0x18 R 0x00 0x00 NACK P
> >>>>> [0.001024] S 0x18 W 0x03 S 0x18 R 0x00 0x00 NACK P
> >>>>> [0.001524] S 0x18 W 0x02 S 0x18 R 0x00 0x00 NACK P
> >>>>> [0.196991] S 0x18 W 0x05 S 0x18 R 0xc1 0xbe NACK P
> >>>>> [0.197514] S 0x18 W 0x04 S 0x18 R 0x00 0x00 NACK P
> >>>>> [0.198019] S 0x18 W 0x03 S 0x18 R 0x00 0x00 NACK P
> >>>>> [0.198520] S 0x18 W 0x02 S 0x18 R 0x00 0x00 NACK P
> >>>>>
> >>>>> I.e. chunks of four transfers every ~200 ms (I removed the 1Hz rate
> >>>>> limiter in the jc42 driver to get more frequent incidents).
> >>>>>
> >>>>> But when the diagnostic (RXRDY still set!) is output it continues
> >>>>> with this:
> >>>>>
> >>>>> [0.399755] S 0x18 W 0x05 S 0x18 R 0xc1 0xbe NACK P
> >>>>> [0.404998] S 0x18 W 0x04 S 0x18 R 0x00 0x00 NACK P
> >>>>> [0.405508] S 0x18 W 0x03 S 0x18 R 0x00 0x00 NACK P
> >>>>> [0.406008] S 0x18 W 0x02 S 0x18 R 0x00 0x00 NACK P
> >>>>>
> >>>>> Notice the ~5 ms delay (about the time it should take to output
> >>>>> the diagnostic at 115200 baud) after the access to register 0x05
> >>>>> at 0.399755.
> >>>>>
> >>>>> This is the only thing that I can observe on the bus, so it appears
> >>>>> to be harmless.
> >>>>>
> >>>>> It appears that the i2c access at 0.399755 finds the TWI
> >>>>> registers in an odd state, but nothing from the access at
> >>>>> 0.198520 appears to have gone wrong. Is this a race? Anyway,
> >>>>> the diagnostic is pretty frequent and annoying. printk_once?
> >>>>
> >>>> I'll try to reproduce it on my side. The only issue you have is having
> >>>> the message about RXRDY? I mean no issue with i2c transfers?
> >>>
> >>> Exactly, the only issue is the message, the bus looks good and transfers
> >>> work as they should AFAICT.
> >>>> It is not the possible bug we had in mind, this bug will prevent the
> >>>> master device to release the i2c bus. It will stop the transfer but
> >>>> without sending a stop on the bus.
> >>>
> >>> Agreed, this is not about the extra frame caused by the spurious write
> >>> to the THR register. This is something else.
> >>>
> >>> One suspicion is that the driver gets an unexpected irq from its own
> >>> NACK (the one that it puts out to end the read) and races with the
> >>> expected interrupt at TXCOMP?
> >>>
> >>
> >> I have discussed with the designer of this IP. So the NACK flag is only
> >> used for 'real' nack not protocol ones.
> >>
> >> Concerning the read issue, it have attached a patch (apply it on top of
> >> Cyrille's patch). Could you have a try? I have reproduced your issue only one
> >> time so it's hard for me to tell if it works well or not.
> > 
> > Since you removed the dev_err call, I don't really know if the underlying
> > issue is solved or not (if there ever was any).
> > 
> > Reading RHR in the if at the top of at91_twi_read_next_byte() never happens
> > for me (I added a witness and left it running overnight), so that
> > seems like an unrelated change.
> 
> Ahhh, now I see. Why is it that I have to hit send before I start to realize
> things? You have moved the RHR read from at91_do_twi_transfer()...
> 
> > Checking RXRDY before TXCOMP|NACK seems about equal, but what do I know?
> 
> ...and there is no else before the TXCOMP|NACK, so I added a witness inside the
> if (...RXRDY) that triggers if TXCOM|NACK is also set. That witness fires at
> about the same rate as the "RXRDY still set" message did before. For thrills I
> readded the if (sr & AT91_TWI_RXRDY) check in at91_do_twi_transfer() but that
> test is no longer firing.
> 
> So, yes, this is a good fix...
> 

Great thanks for your feedback, I will send it with Cyrille's one.

> > But the "problem" is gone of course, no flood of messages, but I can't tell
> > if this is just papering over or a fix.
> 
> ...and you can scratch that comment.

The twi device has a weird behavior (solved on SAMA5D2) when it concerns
reception. It sounds as when I have almost received a byte, I will do
internal stuff to prepare the reception of a new byte. If I have
received the order to send a stop command, this internal stuff should
not be done and everything goes well. But if the order is received too
late, the twi device may think it's still receiving data and set the
RXRDY flag. For that reason we don't set the stop order on the last data
but on the second last data. Unfortunately, it seems we still have bad
timing in some cases.

Regards

Ludovic
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