linux-kernel - Re: [PATCH 1/3] of: base: add support to get machine compatible string

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Date:   Wed, 23 Nov 2016 17:17:43 +0530
From:   Sekhar Nori <nsekhar@...com>
To:     Sudeep Holla <sudeep.holla@....com>
CC:     Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Kevin Hilman <khilman@...libre.com>,
        Michael Turquette <mturquette@...libre.com>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Ujfalusi <peter.ujfalusi@...com>,
        Russell King <linux@...linux.org.uk>,
        LKML <linux-kernel@...r.kernel.org>,
        arm-soc <linux-arm-kernel@...ts.infradead.org>,
        linux-drm <dri-devel@...ts.freedesktop.org>,
        linux-devicetree <devicetree@...r.kernel.org>,
        Jyri Sarha <jsarha@...com>,
        Tomi Valkeinen <tomi.valkeinen@...com>,
        David Airlie <airlied@...ux.ie>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH 1/3] of: base: add support to get machine compatible
 string

On Wednesday 23 November 2016 03:35 PM, Sudeep Holla wrote:
> 
> 
> On 23/11/16 07:49, Sekhar Nori wrote:
>> On Tuesday 22 November 2016 09:16 PM, Sudeep Holla wrote:
>>> Hi Sekhar,
>>>
>>> On 22/11/16 15:06, Sekhar Nori wrote:
>>>> Hi Sudeep,
>>>>
>>>> On Tuesday 22 November 2016 04:23 PM, Sudeep Holla wrote:
>>>>>
>>>>>
>>>>> On 22/11/16 10:41, Bartosz Golaszewski wrote:
>>>>>> Add a function allowing to retrieve the compatible string of the root
>>>>>> node of the device tree.
>>>>>>
>>>>>
>>>>> Rob has queued [1] and it's in -next today. You can reuse that if you
>>>>> are planning to target this for v4.11 or just use open coding in your
>>>>> driver for v4.10 and target this move for v4.11 to avoid cross tree
>>>>> dependencies as I already mentioned in your previous thread.
>>>>
>>>> I dont have your original patch in my mailbox, but I wonder if
>>>> returning a pointer to property string for a node whose reference has
>>>> already been released is safe to do? Probably not an issue for the root
>>>> node, but still feels counter-intuitive.
>>>>
>>>
>>> I am not sure if I understand the issue here. Are you referring a case
>>> where of_root is freed ?
>>
>> Yes, right, thats what I was hinting at. Since you are giving up the
>> reference to the device node before the function returns, the user can
>> be left with a dangling reference.
>>
> 
> Yes I agree.

So, the if(!of_node_get()) is just an expensive NULL pointer check. I think 
it is better to be explicit about it by not using of_node_get/put() at all. 
How about:

+int of_machine_get_model_name(const char **model)
+{
+       int error;
+
+       if (!of_root)
+               return -EINVAL;
+
+       error = of_property_read_string(of_root, "model", model);
+       if (error)
+               error = of_property_read_string_index(of_root, "compatible",
+                                                     0, model);
+       return error;
+}
+EXPORT_SYMBOL(of_machine_get_model_name);

I know the patch is already in -next so I guess it depends on how strongly 
Rob feels about this.

Thanks,
Sekhar