linux-kernel - Re: [RFC PATCH] Add bridge driver to connect sensors to CIO2 device via software nodes on ACPI platforms

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Message-ID: <20200918081609.GR18329@kadam>
Date:   Fri, 18 Sep 2020 11:16:09 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:     devel@...verdev.osuosl.org, robh@...nel.org, mchehab@...nel.org,
        jorhand@...ux.microsoft.com, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org, kieran.bingham@...asonboard.com,
        Daniel Scally <djrscally@...il.com>, kitakar@...il.com,
        yong.zhi@...el.com, bingbu.cao@...el.com,
        andriy.shevchenko@...ux.intel.com, davem@...emloft.net,
        tian.shu.qiu@...el.com, linux-media@...r.kernel.org
Subject: Re: [RFC PATCH] Add bridge driver to connect sensors to CIO2 device
 via software nodes on ACPI platforms

On Fri, Sep 18, 2020 at 09:40:43AM +0300, Sakari Ailus wrote:
> Hi Dan,
> 
> On Thu, Sep 17, 2020 at 01:49:41PM +0300, Dan Carpenter wrote:
> > On Thu, Sep 17, 2020 at 01:33:43PM +0300, Sakari Ailus wrote:
> > > > +static int connect_supported_devices(void)
> > > > +{
> > > > +	struct acpi_device *adev;
> > > > +	struct device *dev;
> > > > +	struct sensor_bios_data ssdb;
> > > > +	struct sensor *sensor;
> > > > +	struct property_entry *sensor_props;
> > > > +	struct property_entry *cio2_props;
> > > > +	struct fwnode_handle *fwnode;
> > > > +	struct software_node *nodes;
> > > > +	struct v4l2_subdev *sd;
> > > > +	int i, ret;
> > > 
> > > unsigned int i
> > > 
> > 
> > Why?
> > 
> > For list iterators then "int i;" is best...  For sizes then unsigned is
> > sometimes best.  Or if it's part of the hardware spec or network spec
> > unsigned is best.  Otherwise unsigned variables cause a ton of bugs.
> > They're not as intuitive as signed variables.  Imagine if there is an
> > error in this loop and you want to unwind.  With a signed variable you
> > can do:
> > 
> > 	while (--i >= 0)
> > 		cleanup(&bridge.sensors[i]);
> > 
> > There are very few times where raising the type maximum from 2 billion
> > to 4 billion fixes anything.
> 
> There's simply no need for the negative integers here. Sizes (as it's a
> size here) are unsigned, too, so you'd be comparing signed and unsigned
> numbers later in the function.

I'm not trying to be rude, I'm honestly puzzled by this...

The "i" variable is not a size, it's an iterator...  Comparing signed
and unsigned isn't necessarily a problem, but the only comparison in
this case is here:

   253          struct property_entry *cio2_props;
   254          struct fwnode_handle *fwnode;
   255          struct software_node *nodes;
   256          struct v4l2_subdev *sd;
   257          int i, ret;
   258  
   259          for (i = 0; i < ARRAY_SIZE(supported_devices); i++) {
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
That's obviously fine.  The compiler knows at compile time the value of
ARRAY_SIZE().  I feel like there must be a static checker which
complains about this?  ARRAY_SIZE() is size_t.

   260                  adev = acpi_dev_get_first_match_dev(supported_devices[i],
   261                                                      NULL, -1);
   262  
   263                  if (!adev)
   264                          continue;
   265  

regards,
dan carpenter