Messages in this thread | | | From | Adam Thomson <> | Subject | RE: [PATCH 1/2] mfd: da9062: enable being a system-power-controller | Date | Fri, 24 Jan 2020 10:17:19 +0000 |
| |
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
| |