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Subject: Re: [PATCH][RFC] iio: core: add a class hierarchy on iio device lock
To:     Olivier MOYSAN <olivier.moysan@st.com>,
        Jonathan Cameron <jic23@kernel.org>
Cc:     "knaack.h@gmx.de" <knaack.h@gmx.de>,
        "pmeerw@pmeerw.net" <pmeerw@pmeerw.net>,
        "mcoquelin.stm32@gmail.com" <mcoquelin.stm32@gmail.com>,
        Alexandre TORGUE <alexandre.torgue@st.com>,
        Fabrice GASNIER <fabrice.gasnier@st.com>,
        "linux-iio@vger.kernel.org" <linux-iio@vger.kernel.org>,
        "linux-stm32@st-md-mailman.stormreply.com" 
        <linux-stm32@st-md-mailman.stormreply.com>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Benjamin GAIGNARD <benjamin.gaignard@st.com>
References: <20191011151314.5365-1-olivier.moysan@st.com>
 <20191012095747.3acd95e6@archlinux>
 <db362ddf-390e-0847-1269-f3cd0c757d2a@st.com>
From:   Lars-Peter Clausen <lars@metafoo.de>
Message-ID: <9ddc41c4-3d84-cc94-5494-a5ef06697ce8@metafoo.de>
Date:   Tue, 15 Oct 2019 23:11:43 +0200
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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@st.com> 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