Re: [PATCH] i2c: mv64xxx: Fix random system lock caused by runtime PM

["Marek Behún" <kabel@kernel.org>]

On Wed, 14 Apr 2021 15:28:18 +0200
Gregory CLEMENT <gregory.clement@bootlin.com> wrote:

> > On Thu, Apr 08, 2021 at 04:00:00AM +0200, Marek Behún wrote:  
> >> I noticed a weird bug with this driver on Marvell CN9130 Customer
> >> Reference Board.
> >> 
> >> Sometime after boot, the system locks with the following message:
> >>  [104.071363] i2c i2c-0: mv64xxx: I2C bus locked, block: 1, time_left: 0
> >> 
> >> The system does not respond afterwards, only warns about RCU stalls.
> >> 
> >> This first appeared with commit e5c02cf54154 ("i2c: mv64xxx: Add runtime
> >> PM support").
> >> 
> >> With further experimentation I discovered that adding a delay into
> >> mv64xxx_i2c_hw_init() fixes this issue. This function is called before
> >> every xfer, due to how runtime PM works in this driver. It seems that in
> >> order to work correctly, a delay is needed after the bus is reset in
> >> this function.  
> 
> Marek,
> 
> As you mentioned it was related to reset and the issue occurred with the
> support of runtime PM. Did you try to add the delay only in the function
> mv64xxx_i2c_runtime_resume(), just after the mv64xxx_i2c_hw_init() call ?
> 

I did indeed discover this when I added this delay into the resume
function. In fact I discovered this when I added printf()s into suspend
and resume when debugging. The problem disappeared with these printf()s
(UART is slow so printf() counted as the necessary delay it seems).

I then moved the delay into the hw_init() function because that is what
made sense to me, that the delay is necessary after the reset, not only
when resuming, but always. We just did not notice because a xfer was
never done immediately after reset before the PM patch. (But maybe I am
wrong, maybe it is not needed in the reset. It just makes the most
sense to me...)

Marek