linux-kernel - Re: [PATCH v2] drivers/cdrom: improved ioctl for media change detection

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Date:   Wed, 8 Sep 2021 17:42:30 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Phillip Potter <phil@...lpotter.co.uk>, hch@...radead.org
Cc:     axboe@...nel.dk, linux-kernel@...r.kernel.org, lumip@...ip.de
Subject: Re: [PATCH v2] drivers/cdrom: improved ioctl for media change
 detection

On 9/8/21 5:17 PM, Phillip Potter wrote:
>>> +static int cdrom_ioctl_timed_media_change(struct cdrom_device_info *cdi,
>>> +		unsigned long arg)
>>> +{
>>> +	int ret;
>>> +	struct cdrom_timed_media_change_info __user *info;
>>> +	struct cdrom_timed_media_change_info tmp_info;
>>> +
>>> +	if (!CDROM_CAN(CDC_MEDIA_CHANGED))
>>> +		return -ENOSYS;
>>> +
>>> +	info = (struct cdrom_timed_media_change_info __user *)arg;
>>> +	cd_dbg(CD_DO_IOCTL, "entering CDROM_TIMED_MEDIA_CHANGE\n");
>>> +
>>> +	ret = cdrom_ioctl_media_changed(cdi, CDSL_CURRENT);
>>> +	if (ret < 0)
>>> +		return ret;
>>> +
>>> +	if (copy_from_user(&tmp_info, info, sizeof(tmp_info)) != 0)
>>> +		return -EFAULT;
>>> +
>>> +	tmp_info.has_changed = ((tmp_info.last_media_change - cdi->last_media_change_ms) < 0);
>>
> Dear Christoph,
> 
> Sorry didn't see this e-mail before I replied to Lukas, so I hope you
> don't think I was ignoring you :-)
> 
>> Overly long line here, but more importantly this is much cleaner with
>> a good old if:
>>
>>
>> 	if (tmp_info.last_media_change - cdi->last_media_change_ms) < 0)
>> 		tmp_info.has_changed = 1;
>>
> 
> Whilst I don't disagree this is technically cleaner, the existing style
> certainly read well to me. In terms of line length, checkpatch doesn't
> complain about it, so I guess you mean purely from a visual perspective?

Documentation/process/coding-style.rst says:

   The preferred limit on the length of a single line is 80 columns.

checkpatch only checks lines > 100 columns since that is OK in a few
cases, like a long quoted string.

So try to limit line lengths to 80 columns unless there is some
other reason not to do that.

>>> +{
>>> +	__s64	last_media_change;	/* Timestamp of the last detected media
>>> +					 * change in ms. May be set by caller, updated
>>> +					 * upon successful return of ioctl.
>>> +					 */
>>> +	__u64	has_changed;		/* Set to 1 by ioctl if last detected media
>>
>> More overly long lines.  Also why is has_changed a u64 if it is used as
>> a boolean flag?

Might as well be explicit about it and also make it obvious that there
is some space available for other fields.

thanks.
-- 
~Randy