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

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Message-ID: <CA+ASDXNvjJLzH+_0g1OUNckmRoLyNJVwiAg9=OPwd7LGk-E2Cg@mail.gmail.com>
Date:   Wed, 4 Mar 2020 20:37:36 -0800
From:   Brian Norris <briannorris@...omium.org>
To:     Ganapathi Bhat <ganapathi.bhat@....com>
Cc:     "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        "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 Ganapathi,

On Wed, Mar 4, 2020 at 8:00 PM Ganapathi Bhat <ganapathi.bhat@....com> wrote:
> > 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?

No, my understanding is that we do not need to bother with
hard_header_len ourselves -- ether_setup() establishes the appropriate
L2 header parameters. I think that's covered a little better below.

> > 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..."

Yeah, that's probably a little more verbose and accurate description,
which is partly why I referred to that commit :)

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

Right, it's set by ether_setup().

Brian