linux-kernel - Re: [PATCH v5 1/1] hwmon: Add driver for Astera Labs PT5161L retimer

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Date: Mon, 5 Feb 2024 20:02:28 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Cosmo Chou <chou.cosmo@...il.com>
Cc: robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org,
 conor+dt@...nel.org, jdelvare@...e.com, corbet@....net, broonie@...nel.org,
 naresh.solanki@...ements.com, vincent@...emblay.dev,
 patrick.rudolph@...ements.com, luca.ceresoli@...tlin.com,
 bhelgaas@...gle.com, festevam@...x.de, alexander.stein@...tq-group.com,
 heiko@...ech.de, jernej.skrabec@...il.com, macromorgan@...mail.com,
 forbidden405@...mail.com, sre@...nel.org, linus.walleij@...aro.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-hwmon@...r.kernel.org, linux-doc@...r.kernel.org,
 cosmo.chou@...ntatw.com
Subject: Re: [PATCH v5 1/1] hwmon: Add driver for Astera Labs PT5161L retimer

On 2/5/24 19:53, Cosmo Chou wrote:
> On Tue, Feb 06, 2024 at 11:26 AM +0800, Guenter Roeck wrote:
>>
>> On 2/5/24 19:05, Cosmo Chou wrote:
>>> On Tue, Feb 06, 2024 at 3:43 AM +0800, Guenter Roeck wrote:
>>>>
>>>> On Mon, Feb 05, 2024 at 11:20:13PM +0800, Cosmo Chou wrote:
>>>>> This driver implements support for temperature monitoring of Astera Labs
>>>>> PT5161L series PCIe retimer chips.
>>>>>
>>>>> This driver implementation originates from the CSDK available at
>>>>> Link: https://github.com/facebook/openbmc/tree/helium/common/recipes-lib/retimer-v2.14
>>>>> The communication protocol utilized is based on the I2C/SMBus standard.
>>>>>
>>>>> Signed-off-by: Cosmo Chou <chou.cosmo@...il.com>
>>>>> ---
>>>> [ ... ]
>>>>
>>>>> +static ssize_t pt5161l_debugfs_read_fw_ver(struct file *file, char __user *buf,
>>>>> +                                        size_t count, loff_t *ppos)
>>>>> +{
>>>>> +     struct pt5161l_data *data = file->private_data;
>>>>> +     int ret;
>>>>> +     char ver[32];
>>>>> +
>>>>> +     mutex_lock(&data->lock);
>>>>> +     ret = pt5161l_fwsts_check(data);
>>>>> +     mutex_unlock(&data->lock);
>>>>> +     if (ret)
>>>>> +             return ret;
>>>>> +
>>>>> +     ret = snprintf(ver, sizeof(ver), "%u.%u.%u\n", data->fw_ver.major,
>>>>> +                    data->fw_ver.minor, data->fw_ver.build);
>>>>> +     if (ret < 0)
>>>>> +             return ret;
>>>>> +
>>>>
>>>> You almost got me here ;-). snprintf() never returns a negative error code,
>>>> so checking for it is not necessary.
>>>>
>>> Oh! You're right.
>>>
>>>>> +     return simple_read_from_buffer(buf, count, ppos, ver, ret + 1);
>>>>
>>>> Number of bytes written plus 1 ? Why ?
>>> It's just to include the string terminator '\0'.
>>>
>>
>> If that was needed, it would be risky. snprintf() truncates the output
>> if the buffer is not large enough. You might want to consider using
>> scnprintf() instead. But then I am not sure if that is needed in the first
>> place. Almost all code I checked doesn't do that, and it seems to be likely
>> that the few drivers who do that are simply wrong. Can you explain why the
>> string terminator needs to be added to the output ?
>>
>> Thanks,
>> Guenter
>>
> It's just in case someone reads and prints this, but with a dirty
> buffer and doesn't handle the terminator.

That needs a better reason. It is not conceivable that 99% of drivers
don't do this but this one would need it for some reason. I am not going
to accept this unless you can show that debugfs files are supposed to
include a terminating '\0' in the response. This is like claiming that
printf() should include a terminating '\0' in the output just in case
the output is read by a broken application which needs to see the
terminator.

Guenter