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

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Message-ID: <b2a1db4167d319d355dd2a3a844553bc@agner.ch>
Date:	Thu, 05 Dec 2013 18:43:38 +0100
From:	Stefan Agner <stefan@...er.ch>
To:	Stephen Warren <swarren@...dotorg.org>
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

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 v2, I moved the i2c_smbus_read_byte_data function call after the
allocation, so it was more obvious for the reader. But then, as Thierry
Reding pointed out, not moving it is an optimization: In case reading
fails, we don't allocate memory first.
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