Message-ID: <AM6PR10MB22636902FCF576272AC8F373800E0@AM6PR10MB2263.EURPRD10.PROD.OUTLOOK.COM>
Date:   Fri, 24 Jan 2020 10:17:19 +0000
From:   Adam Thomson <Adam.Thomson.Opensource@...semi.com>
To:     Helmut Grohne <helmut.grohne@...enta.de>,
        Lee Jones <lee.jones@...aro.org>,
        Adam Thomson <Adam.Thomson.Opensource@...semi.com>
CC:     Support Opensource <Support.Opensource@...semi.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/2] mfd: da9062: enable being a system-power-controller

On 24 January 2020 08:53, Helmut Grohne wrote:

> Hi,
> 
> Thank you for reviewing the code.
> 
> On Thu, Jan 23, 2020 at 04:51:37PM +0100, Adam Thomson wrote:
> > I have concerns about using regmap/I2C within the pm_power_off() callback
> > function although I am aware there are other examples of this in the kernel. At
> > the point that is called I believe IRQs are disabled so it would require a
> > platform to have an atomic version of the I2C bus's xfer function. Don't know
> > if there's a check to see if the bus supports this, but if not then maybe it's
> > something worth adding? That way we can then only support the
> pm_power_off()
> > approach on systems which can actually do it.
> 
> On arm, machine_power_off calls the pm_power_off callback after issuing
> local_irq_disable() and smp_send_stop(). So I think your intuition is
> correct that we are running with only one CPU left with IRQs disabled.
> 
> I have tested this code on a board with an i2c-cadence bus. This driver
> seems to use IRQs for completion tracking with no fallback to polling.
> I'm now puzzled as to why this works at all. Given that I'm using
> regmap_update_bits on a volatile register, it would have to complete the
> read before performing the relevant write. Nevertheless, it reliably
> turns off here. So I'm starting to wonder whether there is a flaw in the
> analysis.
> 
> I also looked into whether linux/i2c.h would tell us about the
> availability of an atomic xfer function. Indeed, the i2c_algorithm
> structure has a master_xfer_atomic specifically for this purpose. The
> i2c core will automatically use this function when irqs are disabled.
> Unfortunately, very few buses implement this function. In particular,
> i2c-cadence lacks it.
> 
> So I could check for i2c_dev->adapter->algo->master_xfer_atomic != NULL
> indeed. Possibly this could be wrapped in a central inline function.

Yes, I'd be tempted to make this a nice wrapper function to hide the
particulars, were someone to implement this.

> 
> I concur that quite a few other drivers use a regmap/i2c from their
> pm_power_off hook. Examples include:
>  * arch/powerpc/platforms/83xx/mcu_mpc8349emitx.c (i2c without regmap)
>  * drivers/mfd/axp20x.c (regmap without i2c)
>  * drivers/mfd/dm355evm_msp.c (i2c without regmap)
>  * drivers/mfd/max77620.c (regmap and i2c)
>  * drivers/mfd/max8907.c (regmap and i2c)
>  * drivers/mfd/palmas.c (regmap and i2c)
>  * drivers/mfd/retu-mfd.c (regmap and i2c)
>  * drivers/mfd/rn5t618.c (regmap and i2c)
>  * drivers/mfd/tps6586x.c (regmap and i2c)
>  * drivers/mfd/tps65910.c (regmap and i2c)
>  * drivers/mfd/tps80031.c (regmap and i2c)
>  * drivers/mfd/twl4030-power.c (i2c without regmap)
>  * drivers/regulator/act8865-regulator.c (regmap and i2c)
> 
> For this reason, I think the practice of using regmap/i2c within
> pm_power_off is well-established and should not be questioned for an
> individual device. In addition, the relevant functionality must be
> explicitly requested by modifying a board-specific device-tree. It can
> be assumed that an integrator would test whether the mfd actually works
> as a power controller when adding the relevant property. Given that we
> turned off other CPUs and IRQs, the behaviour should be fairly reliable.

I never like assumptions and they tend to catch people out. A lot of the time
driver developers will use another as a template/example and so the same
possible mistakes can be duplicated. I don't know for certain these are mistakes
but the code seems to indicate that could be the case, and there's a good
reason that atomic I2C transfer code has been added into the kernel. I also
prefer code that helps people identify where a problem might lie so having a
check for I2C atomic support would be useful to indicate if there is a problem.

Lee, do you have any further insight into any of this? Am I barking up the
wrong tree here?

> 
> I think that requiring atomic transfers for pm_power_off would be
> relatively easy to implement (for all mfds). However, I also think that
> it would break a fair number of boards, because so few buses implement
> atomic transfers. As such, I don't think we can actually require it
> before requiring all buses to implement atomic transfers. At that point,
> the check becomes useless, because the i2c core will automatically use
> atomic transfers during pm_power_off.
> 
> Given these reasons (consistency with other drivers, testing and "don't
> break"), I think that including the functionality without an additional
> check is a reasonable thing to do.
> 
> Helmut

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