Re: [PATCH v3 06/11] iio: inkern: add module put/get on iio dev module when requesting channels

[Eugen Hristev <eugen.hristev@microchip.com>]

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.
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) ?

Thanks,
Eugen

>