netdev - Re: [PATCH net-next 1/7] net: dsa: hide dsa_uses_tagged

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Message-Id: <20170530.110923.2297838959849946238.davem@davemloft.net>
Date:   Tue, 30 May 2017 11:09:23 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     andrew@...n.ch
Cc:     vivien.didelot@...oirfairelinux.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel@...oirfairelinux.com,
        f.fainelli@...il.com
Subject: Re: [PATCH net-next 1/7] net: dsa: hide dsa_uses_tagged_protocol
 code

From: Andrew Lunn <andrew@...n.ch>
Date: Tue, 30 May 2017 17:01:44 +0200

> On Tue, May 30, 2017 at 10:21:25AM -0400, Vivien Didelot wrote:
>> Hide the implementation of dsa_uses_tagged_protocol in dsa.c since this
>> helper will be extended to access the opaque dsa_device_ops structure.
>> 
>> At the same time, fix the checkpatch comparison check:
>> 
>>     CHECK: Comparison to NULL could be written "dst->rcv"
>>     #41: FILE: net/dsa/dsa.c:32:
>>     +	return dst->rcv != NULL;
>> 
>> Signed-off-by: Vivien Didelot <vivien.didelot@...oirfairelinux.com>
>> ---
>>  include/net/dsa.h | 5 +----
>>  net/dsa/dsa.c     | 5 +++++
>>  2 files changed, 6 insertions(+), 4 deletions(-)
>> 
>> diff --git a/include/net/dsa.h b/include/net/dsa.h
>> index c0e567c0c824..cb5d668b265d 100644
>> --- a/include/net/dsa.h
>> +++ b/include/net/dsa.h
>> @@ -464,10 +464,7 @@ struct mii_bus *dsa_host_dev_to_mii_bus(struct device *dev);
>>  
>>  struct net_device *dsa_dev_to_net_device(struct device *dev);
>>  
>> -static inline bool dsa_uses_tagged_protocol(struct dsa_switch_tree *dst)
>> -{
>> -	return dst->rcv != NULL;
>> -}
>> +bool dsa_uses_tagged_protocol(struct dsa_switch_tree *dst);
>>  
>>  static inline bool netdev_uses_dsa(struct net_device *dev)
>>  {
>> diff --git a/net/dsa/dsa.c b/net/dsa/dsa.c
>> index 3288a80d4d6c..7a8a0358299b 100644
>> --- a/net/dsa/dsa.c
>> +++ b/net/dsa/dsa.c
>> @@ -27,6 +27,11 @@
>>  
>>  #include "dsa_priv.h"
>>  
>> +bool dsa_uses_tagged_protocol(struct dsa_switch_tree *dst)
>> +{
>> +	return !!dst->rcv;
>> +}
>> +
> 
> Hi Vivien
> 
> You need to be careful here. This is in the hot path. Every frame
> received uses this code. And think about a distro kernel, which might
> have DSA enabled by default, yet is unlikely to have any switches. You
> are adding a function call which can be called millions of times per
> second....

Yeah, we really can't make this change.

This isn't glibc where we're trying to hide the implementation of "FILE *"
behind accessor functions that caller can't see.  We inline things when
performance dictates, and it does here.