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Date: Fri, 16 Oct 2026 18:25:08 -0700
From: Jakub Kicinski <kuba@kernel.org>
To: Nicolai Buchwitz <nb@tipi-net.de>
Cc: Doug Berger <opendmb@gmail.com>, Florian Fainelli
 <florian.fainelli@broadcom.com>, Broadcom internal kernel review list
 <bcm-kernel-feedback-list@broadcom.com>, Andrew Lunn
 <andrew+netdev@lunn.ch>, "David S. Miller" <davem@davemloft.net>, Eric
 Dumazet <edumazet@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
 netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Dave Stevenson
 <dave.stevenson@raspberrypi.com>, Justin Chen <justin.chen@broadcom.com>,
 Pierre-Marin Leclercq <pierremarinleclercq88@gmail.com>, Florian Fainelli
 <f.fainelli@gmail.com>
Subject: Re: [PATCH net-next v5 6/8] net: bcmgenet: pad transmit frames out
 of the packet ready window
Message-ID: <20261016182508.1100f391@kernel.org>
In-Reply-To: <20261013-nb-genet-mtu-nn-v2-v5-6-076b6fc240be@tipi-net.de>
References: <20261013-nb-genet-mtu-nn-v2-v5-0-076b6fc240be@tipi-net.de>
	<20261013-nb-genet-mtu-nn-v2-v5-6-076b6fc240be@tipi-net.de>
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On Tue, 13 Oct 2026 16:20:35 +0200 Nicolai Buchwitz wrote:
> A frame that ends a few bytes past the transmit packet ready threshold
> stops the transmitter as soon as a shorter frame follows. Tx DMA then
> refuses to halt, so every later bcmgenet_init_dma() fails and the interface
> cannot be opened again. IP fragmentation generates that pattern on every
> datagram, full frames and a short tail.

This is really odd. Sounds like the IP can't really support high MTU
properly. Should the user be somehow informed that the driver will
append out-of-spec bytes to their frames or better still opt into
this hack?