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From: Jonathan Cameron <Jonathan.Cameron@huawei.com>
To: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Eugen Hristev <eugen.hristev@microchip.com>,
	Jonathan Cameron <jic23@kernel.org>,
	<ludovic.desroches@microchip.com>,
	<alexandre.belloni@bootlin.com>,
	<linux-arm-kernel@lists.infradead.org>,
	<devicetree@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
	<linux-iio@vger.kernel.org>, <linux-input@vger.kernel.org>,
	<nicolas.ferre@microchip.com>, <robh@kernel.org>,
	<lars@metafoo.de>
Subject: Re: [PATCH v3 06/11] iio: inkern: add module put/get on iio dev module when requesting channels
Date: Wed, 18 Apr 2018 10:35:48 +0100	[thread overview]
Message-ID: <20180418103548.000078b7@huawei.com> (raw)
In-Reply-To: <20180417191906.GB67621@dtor-ws>

On Tue, 17 Apr 2018 12:19:06 -0700
Dmitry Torokhov <dmitry.torokhov@gmail.com> wrote:

> Hi Eugen,
> 
> On Tue, Apr 17, 2018 at 10:39:24AM +0300, Eugen Hristev wrote:
> > 
> > 
> > On 17.04.2018 02:58, Dmitry Torokhov wrote:  
> > > On Sun, Apr 15, 2018 at 08:33:21PM +0100, Jonathan Cameron wrote:  
> > > > On Tue, 10 Apr 2018 11:57:52 +0300
> > > > Eugen Hristev <eugen.hristev@microchip.com> wrote:
> > > >   
> > > > > When requesting channels for a particular consumer device,
> > > > > besides requesting the device (incrementing the reference counter), also
> > > > > do it for the driver module of the iio dev. This will avoid the situation
> > > > > where the producer IIO device can be removed and the consumer is still
> > > > > present in the kernel.
> > > > > 
> > > > > Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
> > > > > ---
> > > > >   drivers/iio/inkern.c | 8 +++++++-
> > > > >   1 file changed, 7 insertions(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/drivers/iio/inkern.c b/drivers/iio/inkern.c
> > > > > index ec98790..68d9b87 100644
> > > > > --- a/drivers/iio/inkern.c
> > > > > +++ b/drivers/iio/inkern.c
> > > > > @@ -11,6 +11,7 @@
> > > > >   #include <linux/slab.h>
> > > > >   #include <linux/mutex.h>
> > > > >   #include <linux/of.h>
> > > > > +#include <linux/module.h>
> > > > >   #include <linux/iio/iio.h>
> > > > >   #include "iio_core.h"
> > > > > @@ -152,6 +153,7 @@ static int __of_iio_channel_get(struct iio_channel *channel,
> > > > >   	if (index < 0)
> > > > >   		goto err_put;
> > > > >   	channel->channel = &indio_dev->channels[index];
> > > > > +	try_module_get(channel->indio_dev->driver_module);  
> > > > 
> > > > And if it fails? (the module we are trying to get is going away...)
> > > > 
> > > > We should try and handle it I think. Be it by just erroring out of here.  
> > > 
> > > Even more, this has nothing to do with modules. A device can go away for
> > > any number of reasons (we unbind it manually via sysfs, we pull the USB
> > > plug from the host in case it is USB-connected device, we unload I2C
> > > adapter for the bus device resides on, we kick underlying PCI device)
> > > and we should be able to handle this in some fashion. Handling errors
> > > from reads and ignoring garbage is one of methods.
> > > 
> > > FWIW this is a NACK from me.
> > > 
> > > Thanks.  
> > Hello,
> > 
> > This patch is actually a "best effort attempt" for the consumer driver
> > (touch driver) to get a reference to the producer of the data (the IIO
> > device), when it requests the specific channels.
> > As of this moment, there is no attempt whatsoever for the consumer to have a
> > reference on the producer driver. Thus, the producer can be removed at any
> > time, and the consumer will fail ungraciously.  
> 
> This is the root of the issue. The consumer should be prepared to handle
> errors from producer.
> 
> > I can change the perspective from "best effort" to "mandatory" to get a
> > reference to the producer, or you wish to stop trying to get any reference
> > at all (remove this patch completely) ?  
> 
> You should take reference to the device itself (if it is not taken
> already), so it does not disappear completely and you can continue using
> IIO API to access it, and IIO API should be prepared to deal with "dead"
> devices, but as I pointed in my other email, trying to pin the driver
> is quite pointless as there are myriad other ways of device stopping
> working besides module unloading.
> 
> In any case, I think this problem is outside of the scope of this
> patchset that adds a generic resistive touchscreen, so if you want to
> continue working on this I'd recommend moving it into a separate series.
> 
> Thanks.
> 
Agreed, this one has come up a number of times before.  Quite a lot of
work got done by (IIRC) Lars Peter Clausen to stabilize things in various
unexpected 'going away' events.  Of course there may be paths we have
added since that (it was years ago) that can cause trouble...

Anyhow, separate issue as Dmitry says, let's deal with it separately.

Jonathan

  reply	other threads:[~2018-04-18  9:37 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-10  8:57 [PATCH v3 00/11] Add support for SAMA5D2 touchscreen Eugen Hristev
2018-04-10  8:57 ` [PATCH v3 01/11] iio: adc: at91-sama5d2_adc: fix channel configuration for differential channels Eugen Hristev
2018-04-15 19:23   ` Jonathan Cameron
2018-04-10  8:57 ` [PATCH v3 02/11] MAINTAINERS: add generic resistive touchscreen adc Eugen Hristev
2018-04-10  8:57 ` [PATCH v3 03/11] iio: Add channel for Position Relative Eugen Hristev
2018-04-15 19:29   ` Jonathan Cameron
2018-04-17  7:30     ` Eugen Hristev
2018-04-18  9:32       ` Jonathan Cameron
2018-04-10  8:57 ` [PATCH v3 04/11] dt-bindings: input: touchscreen: add pressure threshold touchscreen property Eugen Hristev
2018-04-13 18:33   ` Rob Herring
2018-04-10  8:57 ` [PATCH v3 05/11] dt-bindings: input: touchscreen: resistive-adc-touch: create bindings Eugen Hristev
2018-04-13 18:34   ` Rob Herring
2018-04-10  8:57 ` [PATCH v3 06/11] iio: inkern: add module put/get on iio dev module when requesting channels Eugen Hristev
2018-04-15 19:33   ` Jonathan Cameron
2018-04-16 23:58     ` Dmitry Torokhov
2018-04-17  7:39       ` Eugen Hristev
2018-04-17 19:19         ` Dmitry Torokhov
2018-04-18  9:35           ` Jonathan Cameron [this message]
2018-04-18 12:37             ` Lars-Peter Clausen
2018-04-10  8:57 ` [PATCH v3 07/11] iio: adc: at91-sama5d2_adc: add support for position and pressure channels Eugen Hristev
2018-04-15 19:45   ` Jonathan Cameron
2018-04-10  8:57 ` [PATCH v3 08/11] input: touchscreen: resistive-adc-touch: add generic resistive ADC touchscreen Eugen Hristev
2018-04-23 23:22   ` Dmitry Torokhov
2018-04-23 23:24     ` Dmitry Torokhov
2018-04-10  8:57 ` [PATCH v3 09/11] dt-bindings: iio: adc: at91-sama5d2_adc: add channel specific consumer info Eugen Hristev
2018-04-10  8:57 ` [PATCH v3 10/11] ARM: dts: at91: sama5d2: add channel cells for ADC device Eugen Hristev
2018-04-10  8:57 ` [PATCH v3 11/11] ARM: dts: at91: sama5d2: Add resistive touch device Eugen Hristev

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