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Fri, 06 Mar 2026 22:52:04 -0800 (PST) Date: Sat, 7 Mar 2026 14:51:57 +0800 From: Kevin Hao To: Jakub Kicinski Cc: edumazet@google.com, claudiu.beznea@tuxon.dev, davem@davemloft.net, stable@vger.kernel.org, netdev@vger.kernel.org, andrew+netdev@lunn.ch, linux@armlinux.org.uk, pabeni@redhat.com, nicolas.ferre@microchip.com, quanyang.wang@windriver.com Subject: Re: [net] net: macb: Shuffle the tx ring before enabling tx Message-ID: References: <20260305-zynqmp-v1-1-5de72254d56b@gmail.com> <20260307025638.1345906-1-kuba@kernel.org> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="VsGoh7P8rs3RZGT3" Content-Disposition: inline In-Reply-To: <20260307025638.1345906-1-kuba@kernel.org> --VsGoh7P8rs3RZGT3 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Mar 06, 2026 at 06:56:38PM -0800, Jakub Kicinski wrote: > Does this memcpy corrupt the descriptor fields on systems with 64-bit DMA? > The macb_tx_desc() macro uses macb_adj_dma_desc_idx() to properly space t= he > descriptor pointers in the ring according to the descriptor size reported= by > macb_dma_desc_get_size(bp), which can be 8 bytes (base), 16 bytes (with > DMA64), or 24 bytes (with DMA64 + PTP). However, all three memcpy calls in > this function use sizeof(struct macb_dma_desc), which is only 8 bytes and > covers only the addr and ctrl fields. >=20 > On systems where macb_dma64(bp) =3D=3D true, such as the AMD ZynqMp platf= orm > this patch targets, each descriptor has an additional macb_dma_desc_64 > struct containing the upper 32 bits of the DMA address (addrh field). >=20 > When the shuffle copies descriptors using only 8 bytes, it moves the lower > 32-bit address (desc->addr) and control word (desc->ctrl) to new positions > but leaves the upper 32-bit address (addrh) unmoved in the old location. > After the shuffle, hardware reads a composite 64-bit DMA address from > mismatched descriptor slots: the old slot's addrh combined with the new > slot's addr. This is indeed an issue. Coincidentally, all the upper 32-bit addresses in = the tx ring were identical, which is why it went undetected during testing. My apologies for this oversight; I will address it in the v2 version. 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