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[174.29.1.49]) by smtp.gmail.com with ESMTPSA id 586e51a60fabf-4a838287e9csm351794fac.7.2026.10.23.08.40.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 23 Oct 2026 08:40:56 -0700 (PDT) From: James Hilliard Date: Fri, 23 Oct 2026 09:40:50 -0600 Subject: [PATCH v2] serial: 8250: calculate TX length before preparing DMA flow control X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20261023-submit-serial-8250-dma-tx-length-v1-v2-1-c3106ef4d2df@gmail.com> X-B4-Tracking: v=1; b=H4sIAAAAAAAC/5WO2w6CMAyGX4Xs2iprIgevfA/DxRiVzcAwWyUQw ru74ROYthd/D1//TQTyloK4ZZvwNNtgJxcFnjKhjXI9ge2iFphjIXNECJ92tAzpTA1Q4TWHblT ACwzkejYwS2ifBZZYVhJrJSLp7elpl+PLo/npiHmR5oROG60KBK1XTpvUYl4vscDRwmlsbODJr 4fLWSbMf4Zixqg0aV3VKq/rez8qO5z1NIpm3/cv81FP/gcBAAA= X-Change-ID: 20261022-submit-serial-8250-dma-tx-length-v1-bf627278129a To: Greg Kroah-Hartman , Jiri Slaby , Raul E Rangel , =?utf-8?q?Ilpo_J=C3=A4rvinen?= , Andy Shevchenko , Heikki Krogerus , Miquel Raynal Cc: linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org, stable@vger.kernel.org, James Hilliard X-Mailer: b4 0.15.2 The Renesas RZ/N1 preparation hook programs the UART DMA block size from dma->tx_size. The generic TX path calls that hook before calculating the length of the new transfer, so it programs zero on the first transfer and the previous length on subsequent transfers. Calculate the scatterlist length before calling the preparation hook so the UART flow controller and DMA descriptor use the same transfer size. Fixes: 1788cf6a91d9 ("tty: serial: switch from circ_buf to kfifo") Cc: stable@vger.kernel.org Signed-off-by: James Hilliard --- Changes in v2: - Correct the Fixes tag to the kfifo conversion, as pointed out by Miquel Raynal. - Add Cc: stable@vger.kernel.org. - Link to v1: https://patch.msgid.link/20261022-submit-serial-8250-dma-tx-length-v1-v1-1-18cecc89a099@gmail.com --- drivers/tty/serial/8250/8250_dma.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/drivers/tty/serial/8250/8250_dma.c b/drivers/tty/serial/8250/8250_dma.c index 1da605cc8601..2dc47adafec0 100644 --- a/drivers/tty/serial/8250/8250_dma.c +++ b/drivers/tty/serial/8250/8250_dma.c @@ -110,8 +110,6 @@ int serial8250_tx_dma(struct uart_8250_port *p) return 0; } - serial8250_do_prepare_tx_dma(p); - sg_init_table(sgl, ARRAY_SIZE(sgl)); ret = kfifo_dma_out_prepare_mapped(&tport->xmit_fifo, sgl, ARRAY_SIZE(sgl), @@ -122,6 +120,8 @@ int serial8250_tx_dma(struct uart_8250_port *p) for_each_sg(sgl, sg, ret, i) dma->tx_size += sg_dma_len(sg); + serial8250_do_prepare_tx_dma(p); + desc = dmaengine_prep_slave_sg(dma->txchan, sgl, ret, DMA_MEM_TO_DEV, DMA_PREP_INTERRUPT | DMA_CTRL_ACK); --- base-commit: 1184f87e2790e8af9be3116f608d2e86b4792d7d change-id: 20261022-submit-serial-8250-dma-tx-length-v1-bf627278129a Best regards, -- James Hilliard