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[68.111.84.250]) by smtp.gmail.com with ESMTPSA id e11sm4138039pfm.160.2020.10.16.19.44.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 16 Oct 2020 19:44:39 -0700 (PDT) To: Christian Eggers , Andrew Lunn , Vivien Didelot , Vladimir Oltean , Jakub Kicinski , Kurt Kanzenbach Cc: "David S . Miller" , Woojung Huh , Microchip Linux Driver Support , netdev@vger.kernel.org, linux-kernel@vger.kernel.org References: <20201016200226.23994-1-ceggers@arri.de> From: Florian Fainelli Subject: Re: [PATCH 0/3] net: dsa: move skb reallocation to dsa_slave_xmit Message-ID: <9795d523-c296-cc18-4188-9214fb9cc25d@gmail.com> Date: Fri, 16 Oct 2020 19:44:37 -0700 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Firefox/78.0 Thunderbird/78.3.2 MIME-Version: 1.0 In-Reply-To: <20201016200226.23994-1-ceggers@arri.de> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 10/16/2020 1:02 PM, Christian Eggers wrote: [snip] > On Friday, 16 October 2020, 20:03:11 CEST, Jakub Kicinski wrote: >> FWIW if you want to avoid the reallocs you may want to set >> needed_tailroom on the netdev. > I haven't looked for this yet. If this can really solve the tagging AND > padding problem, I would like to do this in a follow up patch. The comment in netdevice.h says: * @needed_headroom: Extra headroom the hardware may need, but not in all * cases can this be guaranteed * @needed_tailroom: Extra tailroom the hardware may need, but not in all * cases can this be guaranteed. Some cases also use * LL_MAX_HEADER instead to allocate the skb and while I have never seen a reallocation occur while pushing a descriptor status block in front of a frame on transmit after setting the correct needed_headroom, it was not exercised in a very complicated way either, just TCP or UDP over IPv4 or IPv6. This makes me think that the comment is cautionary about more complicated transmit scenarios with stacked devices, tunneling etc. > > Wishing a nice weekend for netdev. Likewise! -- Florian