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From: Arend van Spriel <arend.vanspriel@broadcom.com>
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Subject: Re: [Bridge] [PATCH] brcmfmac: drop Inter-Access Point Protocol
 packets by default
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Date: Wed, 14 Mar 2018 20:55:28 -0000
To: Kalle Valo <kvalo@codeaurora.org>, =?UTF-8?B?UmFmYcWCIE1pxYJlY2tp?= <rafal@milecki.pl>
Cc: James Hughes <james.hughes@raspberrypi.org>, brcm80211-dev-list.pdl@broadcom.com, bridge@lists.linux-foundation.org, netdev@vger.kernel.org, Chi-Hsien Lin <chi-hsien.lin@cypress.com>, =?UTF-8?B?UmFmYcWCIE1pxYJlY2tp?= <zajec5@gmail.com>, linux-wireless@vger.kernel.org, Hante Meuleman <hante.meuleman@broadcom.com>, Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>, Wright Feng <wright.feng@cypress.com>, Felix Fietkau <nbd@nbd.name>, brcm80211-dev-list@cypress.com, Franky Lin <franky.lin@broadcom.com>

On 3/14/2018 5:10 PM, Kalle Valo wrote:
> Rafał Miłecki <rafal@milecki.pl> writes:
>
>>>> +	unsigned char *eth_data = skb_mac_header(skb) + ETH_HLEN;
>>>> +#if !defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
>>>
>>> #ifndef?
>>
>> I followed what is used in the include/linux/etherdevice.h. Is that a
>> good exceuse? Could it be there any some good reason for #if defined()?
>
> Don't know, maybe just a matter of taste? But it would be nice to know
> the background behind #ifdef vs #if defined(), never figured it out why
> two different forms.

Well. In this case you could use either one, but if you have more 
conditions #if defined() is bit more efficient:

#ifdef A
#ifdef B
#endif
#endif

vs.

#if defined(A) && defined(B)

Regards,
Arend