linux-kernel - Re: [PATCH v2 net-next 1/2] net: dsa: lan9303: Add port_fast_age and port_fdb

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Date:   Thu, 19 Oct 2017 17:42:26 +0200
From:   Egil Hjelmeland <privat@...l-hjelmeland.no>
To:     David Laight <David.Laight@...LAB.COM>,
        'Andrew Lunn' <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...oirfairelinux.com>
Cc:     "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 net-next 1/2] net: dsa: lan9303: Add port_fast_age and
 port_fdb_dump methods

On 19. okt. 2017 17:15, David Laight wrote:
> From: Andrew Lunn
>> Sent: 19 October 2017 15:15
>>>> +/* Clear learned (non-static) entry on given port */
>>>> +static void alr_loop_cb_del_port_learned(struct lan9303 *chip, u32 dat0,
>>>> +					 u32 dat1, int portmap, void *ctx)
>>>> +{
>>>> +	int *port = ctx;
>>>
>>> You can get the value directly to make the line below more readable:
>>>
>>>      int port = *(int *)ctx;
>>
>> You have to be a bit careful with this. You often see people
>> submitting patches taking away casts for void * pointers.
>> If they do that here, it should at least not compile...
>>
>> So maybe do it in two steps?
>>
>>     int * pport = ctx;
>>     int port = *pport;
> 
> IMHO it is best to define a struct for the 'ctx and then do:
> 	..., void *v_ctx)
> {
> 	foo_ctx *ctx = v_ctx;
> 	int port = ctx->port;
> 
> That stops anyone having to double-check that the *(int *)
> is operating on a pointer to an integer of the correct size.
> 

Does casting to a struct pointer require less manual double-check than
to a int-pointer? In neither cases the compiler can protect us, IFAIK.
But on the other hand, a the text "foo_ctx" can searched in the editor.
So in that respect it can somewhat aid to the double-checking.

So I can do that.


> One of the syntax checkers probably ought to generate a warning
> for *(integer_type *)foo since it is often a bug.
> 
> 	David
> 
> 

Egil