netdev - Re: [PATCH 03/12] net: ll_temac: Fix support for 64-bit platforms

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Date:   Fri, 26 Apr 2019 22:59:12 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     Esben Haabendal <esben@...nix.com>, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Michal Simek <michal.simek@...inx.com>,
        Yang Wei <yang.wei9@....com.cn>,
        YueHaibing <yuehaibing@...wei.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/12] net: ll_temac: Fix support for 64-bit platforms

On Fri, Apr 26, 2019 at 11:40:13AM -0700, Jakub Kicinski wrote:
> On Fri, 26 Apr 2019 09:32:22 +0200, Esben Haabendal wrote:
> > The use of buffer descriptor APP4 field (32-bit) for storing skb pointer
> > obviously does not work on 64-bit platforms.
> > As APP3 is also unused, we can use that to store the other half of 64-bit
> > pointer values.
> > 
> > Contrary to what is hinted at in commit message of commit 15bfe05c8d63
> > ("net: ethernet: xilinx: Mark XILINX_LL_TEMAC broken on 64-bit")
> > there are no other pointers stored in cdmac_bd.
> > 
> > Signed-off-by: Esben Haabendal <esben@...nix.com>
> 
> This is a bit strange, the driver stores the host's virtual address into
> the HW descriptor?

Hi Jukub

This is reasonably common. You need some sort of cookie which links
the hardware descriptor to the skbuf it points to. The hardware makes
no use of it, it is just a cookie.

    Andrew