On 10/14/19 5:59 PM, Olivier MOYSAN wrote: > Hello Jonathan, > > Thanks for your comment. > > On 10/12/19 10:57 AM, Jonathan Cameron wrote: >> On Fri, 11 Oct 2019 17:13:14 +0200 >> Olivier Moysan <olivier.moysan@xxxxxx> wrote: >> >>> The aim of this patch is to correct a recursive locking warning, >>> detected when setting CONFIG_PROVE_LOCKING flag (as shown in message below). >>> This message was initially triggered by the following call sequence >>> in stm32-dfsdm-adc.c driver, when using IIO hardware consumer interface. >>> >>> in stm32_dfsdm_read_raw() >>> iio_device_claim_direct_mode >>> mutex_lock(&indio_dev->mlock); -> lock on dfsdm device >>> iio_hw_consumer_enable >>> iio_update_buffers >>> mutex_lock(&indio_dev->mlock); -> lock on hw consumer device >> Hmm. I'm not sure I follow the logic. That lock is >> for one thing and one thing only, preventing access >> to the iio device that are unsafe when it is running >> in a buffered mode. We shouldn't be in a position where >> we both say don't do this if we are in buffered mode, + enter >> buffered mode whilst doing this, or we need special functions >> for entering buffering mode if in this state. We are in >> some sense combining internal driver logic with overall >> IIO states. IIO shouldn't care that the device is using >> the same methods under the hood for buffered and non >> buffered operations. >> >> I can't really recall how this driver works. Is it actually >> possible to have multiple hw_consumers at the same time? >> >> So do we end up with multiple buffers registered and have >> to demux out to the read_raw + the actual buffered path? >> Given we have a bit of code saying grab one sample, I'm >> going to guess we don't... >> >> If so, the vast majority of the buffer setup code in IIO >> is irrelevant here and we just need to call a few of >> the callbacks from this driver directly... (I think >> though I haven't chased through every corner. >> >> I'd rather avoid introducing this nesting for a corner >> case that makes no 'semantic' sense in IIO as it leaves us >> in two separate states at the same time that the driver >> is trying to make mutually exclusive. We can't both >> not be in buffered mode, and in buffered mode. >> >> Thanks and good luck with this nasty corner! >> >> Jonathan >> > Here I consider the following use case: > A single conversion is performed. The dfsdm (filter) is chained with a > front-end, which can be an ADC or a sensor. So we have two IIO devices, > the dfsdm and its front-end handled through the hw consumer interface. > > You are right. There is something wrong here, in buffered/non-buffered > mode mixing. > iio_hw_consumer_enable() call is used to enable the front-end device. > But this interface is intended for buffered mode. > So this is not coherent with the expected single conversion mode, > indeed. Another interface is required to manage the front-end device. I > have a poor knowledge of iio framework, but it seems to me that there is > no interface to manage this. > > My understanding regarding mlock, is that it is used to protect the > state of the iio device. > I we want to do a conversion from the chained devices, I think we need > to activate the first device > and keep it performing conversion, as long as the second device has done > its conversion. > We need to protect both devices, and we should have to do it in a nested > way. > So, I guess that anyway, nested mutexes would be required in this case. > Others like regmap have solved this by having a lockclass per instance. Although that is not ideal either since it will slow down lockdep. See https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/regmap.h#n629
