From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 28C00C9830B for ; Wed, 23 Sep 2026 18:07:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=+vDPcquJM7GahtjddOoqJJfhDpqWnXxKLvU+vehlwfQ=; b=s1g3wEqqUzq/IEphrvDR9jGc2k qUFIQ4l2FGXs/ef80UTNCTiSs8HMzf5TLrADo4R4E/i+HP8+Wd9+vcmd4eISSoazZFxxrKdUIXqap hdhdi77pZUFm4vO70Bbd1UVPOsqsRofob89cca/mdgUMVK3g45BrhkXG6gh8ICv3xnInLGdIVTMOf Z1PlCnvLbnfde3P56fbwINiwaMBc/CpvZinrz+wliSSWUqsccikwlxL7bZ3ipIdSyVlnNtjGDQUvb 75a+ZUl2oKjuKRQc+3NO5fyYhFjCh4bdD6SddmqXwXt0vjOyw4VBd6+5jYWTlLzftlPNm8LwsXmDN Zv+mP+VQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1x9RNa-000000098b9-11ER; Wed, 23 Sep 2026 18:07:38 +0000 Received: from sea.source.kernel.org ([172.234.252.31]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1x9RNZ-000000098b2-38sv for linux-arm-kernel@lists.infradead.org; Wed, 23 Sep 2026 18:07:37 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 3507041B1D; Wed, 23 Sep 2026 18:07:37 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id F0BA91F000FF; Wed, 23 Sep 2026 18:07:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790186857; bh=+vDPcquJM7GahtjddOoqJJfhDpqWnXxKLvU+vehlwfQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=kZJpmvu7LipMdS7s6+mj9xDLvZhIbIEu5iavgnSnpKXpDsyvWxAwjkhQfEWK5EvB3 puajcWtvMf/x7BQSh8XixkLDEZbx1IP0iI8dXlEFD83TJlneF5YQjtadOY6PFulIF6 L0G2y+7enOt+6AmbfvtSPxtYFRJzsw6kPgFlIF6GL0488YwRZ2KqW28I0ftI63c6Jn Jh8OoFaA4UUyN8HygG5Q4MSkdgZkPKCj3SN2TP2BpTdxH/M1bgouZwMEDeqCyM3OFC swXiKbV5JxoXCdQRQsVCdO5HqcoPaSEEausVGKdlcYl+OQ9i14JSTEpnH+23iJUVkG 82Dr4cmCduoCA== Date: Wed, 23 Sep 2026 19:07:30 +0100 From: Simon Horman To: Daniel Machon Cc: netdev-bot+sashiko@kernel.org, andrew+netdev@lunn.ch, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horatiu.vultur@microchip.com, steen.hegelund@microchip.com, UNGLinuxDriver@microchip.com, ast@kernel.org, daniel@iogearbox.net, hawk@kernel.org, john.fastabend@gmail.com, sdf@fomichev.me, herve.codina@bootlin.com, arnd@arndb.de, gregkh@linuxfoundation.org, mohsin.bashr@gmail.com, richardcochran@gmail.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH net-next v7 10/14] net: lan966x: add PCIe FDMA support Message-ID: <20260923180730.GV13925@horms.kernel.org> References: <20260918-lan966x-pci-fdma-v7-10-0ecc179c8a2c@microchip.com> <179008205148.2160803.12818321368571406375@kernel.org> <20260923084538.wgkw2nt5ergoxcot@DEN-DL-M70577> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260923084538.wgkw2nt5ergoxcot@DEN-DL-M70577> X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Wed, Sep 23, 2026 at 10:45:38AM +0200, Daniel Machon wrote: ... > > [Severity: Medium] > > Is this path expected to work on a big-endian host? > > > > Until now the descriptor producer and the FDMA engine were both on the > > little-endian SoC, so the native-endian descriptor layout in > > drivers/net/ethernet/microchip/fdma/fdma_api.h was fine: > > > > struct fdma_db { > > u64 dataptr; > > u64 status; > > }; > > > > struct fdma_dcb { > > u64 nextptr; > > u64 info; > > struct fdma_db db[FDMA_DB_MAX]; > > }; > > > > With PCIe FDMA the descriptors live in host memory and are fetched by the > > endpoint, so the host and the engine can disagree on byte order. The > > translated addresses written here, and the status/info words written by > > __fdma_db_add()/__fdma_dcb_add(), all go out in host byte order, and the > > HW-written DONE/BLOCKL fields are read back the same way in > > lan966x_fdma_pci_get_next_dcb() and lan966x_fdma_pci_rx_check_frame(). > > > > CONFIG_MCHP_LAN966X_PCI in drivers/misc/Kconfig only has > > > > depends on PCI > > depends on OF_OVERLAY > > > > so a big-endian host (e.g. powerpc) can select it. On such a host, would > > the engine see byte-swapped nextptr/dataptr and length fields, i.e. would > > RX/TX not work at all? Should the descriptor fields become explicit > > little-endian types with accessors at the hardware boundary, or should the > > Kconfig entry gain a little-endian dependency? > > Hmm. Never tested on PowerPC, not this driver nor the DT overlay mechanism it > relies on for PCI deployment, which has only been exercised on ARM and x86 so > far (AFAICT) [1]. > > [1] https://lore.kernel.org/all/20240430183301.46568e35@bootlin.com/ I don't think that the baseline expectation is that drivers are tested on big-endian systems, but rather that reasonable effort is made to ensure that byte-order usage is correct. In this case I suggest that will involve using __le64 instead of u64, coupled with calling le64_to_cpu() and cpu_to_le64() as appropriate. Sparse can aid with getting this right. ... > > [Severity: Medium] > > Is rx_bytes meant to count the frame as it arrived on the wire? > > > > By the time this runs, skb->len has already been reduced twice: the > > skb_trim() above drops ETH_FCS_LEN, and eth_type_trans() pulls the > > MAC header out of the linear region, so skb->len is short by at least > > ETH_HLEN. If lan966x_hw_offload() ends up untagging a VLAN header, > > that is another four bytes gone. So every packet delivered through > > lan966x_fdma_pci_rx_get_frame() undercounts rx_bytes by 14 bytes or > > more, which is visible to userspace via ip -s link. > > > > The frame length is available before any of that surgery happens -- > > data_len from FDMA_DCB_STATUS_BLOCKL(db->status), or skb->len right > > after the skb_pull(skb, IFH_LEN_BYTES) -- so accounting could be done > > there instead. > > > > I realise this mirrors what the existing register/page path in > > lan966x_fdma.c does, so if the intent is to keep the two backends > > byte-for-byte consistent, please say so; otherwise it would be good > > not to copy the miscount into the new file. > > Not only lan966x, but sparx5 and lan969x does the exact same thing, increasing > rx_bytes after headers are pulled. The undercount is real, but not visible to > userspace. Both implementations (platform and PCI) read hardware counters directly I'm a little unsure, but if it's consistent then I guess that is ok. It's an old interface anyway.