Re: [PATCH v2 0/2] i2c: Add new driver for Renesas RZ/V2M controller

[Philipp Zabel <p.zabel@pengutronix.de>]

Hi Phil,

On Do, 2022-06-30 at 13:43 +0000, Phil Edworthy wrote:
> Hi Philipp, Geert,
> 
> On 29 June 2022 18:18 Geert Uytterhoeven wrote:
> > On Wed, Jun 29, 2022 at 6:21 PM Philipp Zabel wrote:
> > > On Di, 2022-06-28 at 20:45 +0100, Phil Edworthy wrote:
> > > > The Renesas RZ/V2M SoC (r9a09g011) has a new i2c controller. This
> > series
> > > > add the driver. One annoying problem is that the SoC uses a single
> > reset
> > > > line for two i2c controllers, and unfortunately one of the controllers
> > > > is managed by some firmware, not by Linux. Therefore, the driver just
> > > > deasserts the reset.
> > > 
> > > This sounds scary. If the driver is never loaded, and the reset is
> > > never deasserted, what happens to the firmware trying to access the
> > > other i2c controller? Does it hang? Or write to the reset controller
> > > registers to deassert the reset? If so, is there any protection against
> > > concurrent access from firmware and reset controller driver?
> Where a common reset is used by Linux and some firmware, I think we have to
> ensure/assume that both only ever de-assert it.

We also have to make sure that no read-modify-write cycles are required
to deassert the resets if we can't lock between firmware and kernel.
Otherwise concurrent access could cause a deassert to be reverted.

> In this particular SoC, the register used to assert/de-assert the reset
> has write enable bits in the upper half of the reg. There shouldn't be any
> issues with both trying to de-assert the reset at the same time.

Which reset driver is handling the reset for this i2c module?

> > In response to v1, I wrote
> > 
> > > That is actually an integration issue, not an i2c controller issue.
> > > 
> > > Perhaps we need a RESET_IS_CRITICAL flag, cfr. CLK_IS_CRITICAL,
> > > to be set by the reset provider?
> 
> From what I understand, there are two main use cases for resets:
> 1. Often reset lines may be asserted at power on and so a driver needs to
>    de-assert them so that the module can be used.

There are resets that are not initially asserted (among them the self-
deasserting resets) that are required to be asserted some time during
boot, to put some hardware into a well defined state.
I don't think those should be shared, but they sometimes are.

> 2. A driver may need to reset the module for some reason. I have only
>    seen this with watchdog timers with no way out.

Grep for device_reset() or reset_control_reset() for some examples.
Also there are quite a few assert/udelay/deassert calls in drivers.

Also many drivers assert the reset again during remove(). Whether that
is always necessary or useful, I can't say.

It's sometimes nice during development, to be able to reload a kernel
module or rebind a driver to reset some locked up hardware.

> So if a driver does not need to reset the module, shouldn't the driver
> only ever be de-asserting the reset line?

I'm not sure the driver can always know this if it is used on different
platforms.

> If so, it also doesn’t matter whether the reset is shared with other
> modules or not.
> If a driver needs to reset the module, then the reset cannot be shared
> with other modules used by firmware or Linux, or we cannot use any
> other modules that share the reset line.

It can be shared for the special case of multiple modules requiring a
shared reset line to be asserted once, at some time before the modules
are used. The reset controller API supports this for the
reset_control_reset() call.

regards
Philipp