linux-kernel - Re: [PATCH v4 1/3] mfd: tps6586x: add version detection

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Date:	Thu, 05 Dec 2013 10:40:21 -0700
From:	Stephen Warren <swarren@...dotorg.org>
To:	Stefan Agner <stefan@...er.ch>
CC:	thierry.reding@...il.com, dev@...xeye.de, lee.jones@...aro.org,
	lgirdwood@...il.com, broonie@...nel.org,
	kai.poggensee@...onic-design.de, sameo@...ux.intel.com,
	linux-tegra@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/3] mfd: tps6586x: add version detection

On 12/05/2013 10:43 AM, Stefan Agner wrote:
> Am 2013-12-05 18:06, schrieb Stephen Warren:
> <snip>
>>> @@ -493,13 +527,12 @@ static int tps6586x_i2c_probe(struct i2c_client *client,
>>>  		return -EIO;
>>>  	}
>>>
>>> -	dev_info(&client->dev, "VERSIONCRC is %02x\n", ret);
>>> -
>>>  	tps6586x = devm_kzalloc(&client->dev, sizeof(*tps6586x), GFP_KERNEL);
>>> -	if (tps6586x == NULL) {
>>> -		dev_err(&client->dev, "memory for tps6586x alloc failed\n");
>>> +	if (!tps6586x)
>>>  		return -ENOMEM;
>>> -	}
>>> +
>>> +	tps6586x->version = ret;
>>
>> I have to say, I dislike this version of the patch. Separating the
>> reading of the version register from the assignment to tps6586x->version
>> doesn't make any sense, especially given that the version value is
>> stored in a variable named "ret"; that name isn't remotely related to
>> what's stored there. What if someone comes along later and adds more
>> code that assigns to ret between where it's repurposed for the version
>> value and where it's assigned to tps6586x->version? It'd be extremely
>> difficult for a patch reviewer to spot that given the limited context in
>> a diff, and quite non-obvious to the person changing the code too..
> 
> The value comes from the return value of i2c_smbus_read_byte_data. If
> the value is below zero its an EIO error. 
> 
> I could add a variable "version", but for me it felt strange because we
> check if version is below zero. This feels like its a wrong version
> rather than a transmit error. So I would prefer ret over version. But I
> agree, when one just reads the patch, its not obvious what exactly
> happens.

In my opinion, using a variable named "version" here would be
preferable. Testing that against <0 is just the way the I2C API works,
so the same argument could be applied to any I2C access.
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