Date:	Tue, 23 Jun 2015 14:02:03 -0300
From:	Lucas De Marchi <lucas.de.marchi@...il.com>
To:	christian.ruppert@...tech.com
Cc:	Christian Ruppert <christian.ruppert@...lis.com>,
	Fabio Mello <fabio.mello@...el.com>,
	Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
	linux-i2c@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>,
	Lucas De Marchi <lucas.demarchi@...el.com>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Wolfram Sang <wsa@...-dreams.de>
Subject: Re: [PATCH] i2c: designware: use enable on resume instead initialization

On Tue, Jun 23, 2015 at 1:45 PM,  <christian.ruppert@...tech.com> wrote:
> Hello,
>
> Christian Ruppert/ALi_GVA/ALi wrote on 10.06.2015 17:05:16:
>> Mika Westerberg <mika.westerberg@...ux.intel.com> wrote on 10.06.
>> 2015 09:07:22:
>> > On Tue, Jun 09, 2015 at 03:29:01PM -0300, Lucas De Marchi wrote:
>> > > Hi Mika,
>> > >
>> > > On Tue, Jun 9, 2015 at 5:51 AM, Mika Westerberg
>> > > <mika.westerberg@...ux.intel.com> wrote:
>> > > > On Mon, Jun 08, 2015 at 02:50:28PM -0300,
>> lucas.de.marchi@...il.com wrote:
>> > > >> From: Fabio Mello <fabio.mello@...el.com>
>> > > >>
>> > > >> According to documentation and tests, initialization is not
>> > > >> necessary on module resume, since the controller keeps its state
>> > > >> between disable/enable. Change the target address is also
> allowed.
>> > > >>
>> > > >> So, this patch replaces the initialization on module resume with
> a
>> > > >> simple enable, and removes the (non required anymore) enables and
>> > > >> disables.
>> > > >>
>> > > >> Signed-off-by: Fabio Mello <fabio.mello@...el.com>
>> > > >> Signed-off-by: Lucas De Marchi <lucas.demarchi@...el.com>
>> > > >> ---
>> > > >>
>> > > >> These pictures explain a little more the consequence of letting
> the
>> > > >> enable+disable in the code:
>> > > >>
>> > > >>       http://pub.politreco.com/paste/TEK0011-before.jpg
>> > > >>       http://pub.politreco.com/paste/TEK0007-after.jpg
>> > > >>
>> > > >> The yellow line is a GPIO toggle in userspace to mark when we
>> > start and finish
>> > > >> the i2c transactions.  The blue line is the SCL in that i2c
>> > bus. Take a look on
>> > > >> the huge pauses we have between any 2 transactions.  These
>> > pauses are removed
>> > > >> with this patch and we are able to read our sensor's values in
>> > 950usec rather
>> > > >> than 5.24msec we had before.  We are testing this using a
>> > Minnowboard Max that
>> > > >> has a designware i2c controller.
>> > > >
>> > > > Did you test this on any other platform than Intel Baytrail?
>> > >
>> > > No. The only soc we have here with this controller is the Baytrail.
>> >
>> > My concern is that this patch might break some non-Intel platform. It
>> > would be nice if someone (Christian?) could try this out.
>>
>> Ouch, this one brings back painful memories. Take a look at patch
>> 38d7fadef4973bb94e36897fcb6bb6a12fdd10c9 (https://git.kernel.org/
>> cgit/linux/kernel/git/torvalds/linux.git/commit/?
>> id=38d7fadef4973bb94e36897fcb6bb6a12fdd10c9) for the context.
>>
>> In brief:
>> - Before patch 38d7fade, the driver disabled the hardware after
>> successful transfers. I do not know the reason for this and I cannot
>> judge whether this is necessary or not. I thus decided not to modify
>> this behaviour in patch 38d7fade.
>>
>> - After patch 38d7fade, the driver disabled the dw controller after
>> all transfers, in particular in the case of unsuccessful transfers.
>> This modification was necessary because of a race condition
>> triggered by an i2c slave device which interrupted transfers in the
>> middle. In this case, the dw controller (at least our version) seems
>> to send spurious interrupts later if it is not disabled. The
>> interrupt handler is not designed to be called on already aborted
>> transfers, however, which leads to undesirable behaviour if the
>> interrupt occurs at the wrong moment (system hangs in irq loop).
>>
>> I will try to dig out the test setup we used to validate the patch
>> at the time but given the fact that this was two years ago this
>> might take a little while. In the meantime, do you have any means to
>> stress test the case of unexpected events on the bus (client aborts
>> transfer, timeout etc.)? An alternative might be to only disable the
>> controller in the case of errors and leave it active after
>> successful transfers. We should understand why the controller was
>> disabled after successful transfers in the first place, however.
>> Maybe some quirk with older versions of the hardware? Mika, do you
>> have any memories about this?
>
> As promised I tried to dig out the test setup we used to validate patch
> 38d7fade at the time but without success. (I half expected that after such
> a long time...)
>
> So I said to myself, let's give the patch a try nevertheless and see if it
> works in our system at least in the nominal case (no i2c bus errors).
>
> The result is not very encouraging: Out of five (identical) designware i2c
> controllers we have on my test SOC, the first one initialises properly but
> the second one gets stuck in the famous irq loop right away when the
> module is enabled in i2c_dw_init. The system never gets around to try

Are you using the pci or platform driver?  I noticed yesterday the pci
version is failing here with a NULL pointer dereference.

> initialising the remaining three controllers due to the irq loop. I didn't
> have the time to investigate the details yet but I suspect this is
> triggered by some nastily behaved device on the bus. For example, some of
> our external devices are notorious for keeping i2c  lines tied to zero
> before being properly powered on/reset/initialised by their respective
> drivers (which in turn depend on the i2c master to be initialised first,
> obviously). I'll grab an oscilloscope and dump the waves to confirm this
> suspicion on the occasion.

Yeah, it'd be great to have it.

thanks for testing it with your setup.

-- 
Lucas De Marchi
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