linux-kernel - Re: [PATCH v8 3/3] net: ax88796c: ASIX AX88796C SPI Ethernet Adapter Driver

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Date:   Thu, 17 Dec 2020 09:06:51 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Lukasz Stelmach <l.stelmach@...sung.com>
Cc:     Andrew Lunn <andrew@...n.ch>, jim.cromie@...il.com,
        Heiner Kallweit <hkallweit1@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Rob Herring <robh+dt@...nel.org>,
        Kukjin Kim <kgene@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Russell King <linux@...linux.org.uk>, netdev@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org,
        Bartłomiej Żolnierkiewicz 
        <b.zolnierkie@...sung.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH v8 3/3] net: ax88796c: ASIX AX88796C SPI Ethernet
 Adapter Driver

On Thu, 17 Dec 2020 12:46:57 +0100 Lukasz Stelmach wrote:
> > to the correct values so the stack pre-allocates the needed spaces,
> > when it can.  
> 
> Yes, I fonud these. However, I am not sure setting needed_tailroom has
> any effect. In many places where alloc_skb() is called needed_headrom
> and hard_header_len are refered to via the LL_RESERVED_SPACE macro. But
> the macro does not refer to needed_tailroom. Once (f5184d267c1a ("net:
> Allow netdevices to specify needed head/tailroom") there was
> LL_ALLOCATED_SPACE macro, but but it was removed in 56c978f1da1f ("net:
> Remove LL_ALLOCATED_SPACE"). And now only some protocols refer to
> needet_tailroom.

Yeah, tailroom is used a lot less often. Only really crappy HW requires
it.

> BTW. What is hard_header_len for? Is it the length of the link layer
> header? Considering "my" hardware requires some headers with each
> packet, I find hard_headr_len name a bit confusing.

Yup, L2 headers, not hardware. Not sure why "hard" was chosen, that must
have happened way back.