linux-kernel - Re: [PATCH v2 04/23] counter: Provide a wrapper to access device private data

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Message-ID: <cb583f3f-2cf9-2de5-8beb-da7c11ff1481@metafoo.de>
Date:   Mon, 27 Dec 2021 12:34:28 +0100
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc:     William Breathitt Gray <vilhelm.gray@...il.com>,
        kernel@...gutronix.de,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 04/23] counter: Provide a wrapper to access device
 private data

On 12/27/21 12:02 PM, Greg Kroah-Hartman wrote:
> On Mon, Dec 27, 2021 at 10:45:07AM +0100, Uwe Kleine-König wrote:
>> For now this just wraps accessing struct counter_device::priv. However
>> this is about to change and converting drivers to this helper
>> individually makes fixing device lifetime issues result in easier to
>> review patches.
>>
>> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
>> ---
>>   drivers/counter/counter-core.c | 12 ++++++++++++
>>   include/linux/counter.h        |  2 ++
>>   2 files changed, 14 insertions(+)
>>
>> diff --git a/drivers/counter/counter-core.c b/drivers/counter/counter-core.c
>> index f053a43c6c04..00c41f28c101 100644
>> --- a/drivers/counter/counter-core.c
>> +++ b/drivers/counter/counter-core.c
>> @@ -45,6 +45,18 @@ static struct bus_type counter_bus_type = {
>>   
>>   static dev_t counter_devt;
>>   
>> +/**
>> + * counter_priv - access counter device private data
>> + * @counter: counter device
>> + *
>> + * Get the counter device private data
>> + */
>> +void *counter_priv(const struct counter_device *const counter)
>> +{
>> +	return counter->priv;
>> +}
>> +EXPORT_SYMBOL_GPL(counter_priv);
> Shouldn't this be usin gdev_get_drvdata() and using the private data
> pointer that is already on the struct device structure itself?  The void
> *priv; should be dropped from struct counter_device entirely.
>
> Oh ick, I just now looked at 'struct counter_device', there are other
> reference counting issues in there (hint, struct cdev has a reference
> count...)  But that's independent of this patch series...
This is not a problem. The struct cdev holds a reference to the struct 
dev. This allows them to use the same allocation. As long as there is a 
reference to the cdev there will be a reference to the dev and the 
memory will be kept alive.