linux-kernel - RE: [EXT] [PATCH] mwifiex: set needed_headroom, not hard_header

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Message-ID: <DB7PR04MB524240B38FF6603D89D694538FE20@DB7PR04MB5242.eurprd04.prod.outlook.com>
Date:   Thu, 5 Mar 2020 04:00:42 +0000
From:   Ganapathi Bhat <ganapathi.bhat@....com>
To:     Brian Norris <briannorris@...omium.org>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nishant Sarmukadam <nishants@...vell.com>,
        Amitkumar Karwar <amitkarwar@...il.com>,
        Xinming Hu <huxinming820@...il.com>,
        Arend Van Spriel <arend@...adcom.com>
Subject: RE: [EXT] [PATCH] mwifiex: set needed_headroom, not hard_header_len

Hi Brian,
 
> hard_header_len provides limitations for things like AF_PACKET, such that
> we don't allow transmitting packets smaller than this.


OK; However, are we not supposed to mention hard_header_len also?

> 
> needed_headroom provides a suggested minimum headroom for SKBs, so
> that we can trivally add our headers to the front.
> 
> The latter is the correct field to use in this case, while the former mostly just
> prevents sending small AF_PACKET frames.
> 
> In any case, mwifiex already does its own bounce buffering [1] if we don't
> have enough headroom, so hints (not hard limits) are all that are needed.
> 
> This is the essentially the same bug (and fix) that brcmfmac had, fixed in
> commit cb39288fd6bb ("brcmfmac: use ndev->needed_headroom to reserve
> additional header space").

OK; I read this commit:

"... According to definition of LL_RESERVED_SPACE() and hard_header_len, we should use hard_header_len to reserve for L2 header, like ethernet header(ETH_HLEN) in our case and use needed_headroom for the additional headroom needed by hardware..."

So, does it mean, hard_header_len is already considered by upper layer?


Regards,
Ganapathi