Re: [PATCH] platform/chromeos: cros_ec: Use per-device lockdep key

[Tzung-Bi Shih <tzungbi@kernel.org>]

On Mon, Jan 09, 2023 at 02:19:38PM +0800, Chen-Yu Tsai wrote:
> On Mon, Jan 9, 2023 at 1:46 PM Tzung-Bi Shih <tzungbi@kernel.org> wrote:
> >
> > On Sat, Jan 07, 2023 at 01:43:57PM +0800, Chen-Yu Tsai wrote:
> > > On Fri, Jan 6, 2023 at 5:08 PM Tzung-Bi Shih <tzungbi@kernel.org> wrote:
> > > >
> > > > On Fri, Jan 06, 2023 at 12:55:37PM +0800, Chen-Yu Tsai wrote:
> > > > > Lockdep reports a bogus possible deadlock on MT8192 Chromebooks due to
> > > > > the following lock sequences:
> > > > >
> > > > > 1. lock(i2c_register_adapter) [1]; lock(&ec_dev->lock)
> > > > > 2. lock(&ec_dev->lock); lock(prepare_lock);
> > > > >
> > > > > The actual dependency chains are much longer. The shortened version
> > > > > looks somewhat like:
> > > > >
> > > > > 1. cros-ec-rpmsg on mtk-scp
> > > > >    ec_dev->lock -> prepare_lock
> > > > > 2. In rt5682_i2c_probe() on native I2C bus:
> > > > >    prepare_lock -> regmap->lock -> (possibly) i2c_adapter->bus_lock
> > > > > 3. In rt5682_i2c_probe() on native I2C bus:
> > > > >    regmap->lock -> i2c_adapter->bus_lock
> > > > > 4. In sbs_probe() on cros-ec-i2c (passthrough) I2C bus on cros-ec
> > > > >    i2c_adapter->bus_lock -> ec_dev->lock
> > > > >
> > > > > While lockdep is correct that the shared lockdep classes have a circular
> > > > > dependency, it is bogus because
> > > > >
> > > > >   a) 2+3 happen on a native I2C bus
> > > > >   b) 4 happens on the actual EC on ChromeOS devices
> > > > >   c) 1 happens on the SCP coprocessor on MediaTek Chromebooks that just
> > > > >      happen to expose a cros-ec interface, but do not have a passthrough
> > > > >      I2C bus
> > > > >
> > > > > In short, the "dependencies" are actually on different devices.
> > > >
> > > > Path of 4 looks weird to me.
> > > >
> > > > Could you point out where sbs_probe() gets to acquire ec_dev->lock?
> > >
> > > sbs_probe() calls sbs_get_battery_presence_and_health(), which
> > >
> > >   -> does an I2C transfer. This SBS instance is connected on the I2C bus
> > >      on the EC, so the I2C transfer
> > >
> > >      -> acquires i2c_adapter->bus_lock, and
> >
> > I see.
> >
> > Another question: the i2c_adapter here should be different from the native
> > I2C bus in 2 and 3.  Did they really form the circular dependencies?
> 
> That's why it's a false positive. lockdep normally doesn't track individual
> instances, only classes of locks. The class is declared as part of the
> mutex_init() macro.

Is the following understanding correct:
It has 2 ways to break the "fake" circular dependencies: separate lockdep key
for i2c_adapter vs. ec_dev.  The patch adopts the latter one because it has
limited impact for other I2C-related drivers.