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charset="utf-8" Content-Transfer-Encoding: 7bit X-B4-Tracking: v=1; b=H4sIAPKGu2oC/3XPwU7DMAwG4FeZciYotpMm4cR7IA5ek7FI24oSF DpNfXe8cRhoqnz6LfmzfVEt15KbetlcVM29tDKdJLinjRr3fPrIuiTJCg0OxgHpI88EZta12eB 0+9YpH/ismcyQfKDkMCoZ/qx5V+Yb/PYueV/a11TPtz0drt1f0pu4RnbQoK2zPILZRvL82rk0P vDzOB3VFe14hwLAKoQCZbIYCI3zIT5CdIciuFWIBPIJDVuprcNHyP6FhlXICpTkpSFauWvc/Ye WZfkBQgApj5kBAAA= X-Change-ID: 20260513-max310x-rs485-sw-delay-a306d783d529 To: Greg Kroah-Hartman , Jiri Slaby Cc: linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org, Hugo Villeneuve , Tapio Reijonen , Tapio Reijonen X-Mailer: b4 0.14.2 X-Developer-Signature: v=1; a=ed25519-sha256; t=1790674682; l=5027; i=tapio.reijonen@vaisala.com; s=20250903; h=from:subject:message-id; bh=xr3HMZq/cCA68Q3MWoaSY+3f5H9EEnj2ia9uPFFAENg=; b=T2pjdnUPAsSSniJQ5OQYWTpOzvVad9Qks7CyaoVizMhqRJOBatLrEhHB1WXMg0Mc0G+OJqobi EHwjanelMZjDezr10cAlWGvZFFA6sJpkkAPGCvduxfXAqRLRSWCWlTx X-Developer-Key: i=tapio.reijonen@vaisala.com; a=ed25519; pk=jWBz3VD84WbWgfEgIqB5iFFiyVIHZr52zVBPOm7qiGo= X-ClientProxiedBy: OS6P279CA0089.NORP279.PROD.OUTLOOK.COM (2603:10a6:e10:3e::11) To AM9PR06MB7907.eurprd06.prod.outlook.com (2603:10a6:20b:3a6::23) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: AM9PR06MB7907:EE_|AS8PR06MB8258:EE_ X-MS-Office365-Filtering-Correlation-Id: c9b5f3da-eec0-41ce-571c-08df1e0d5bde X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|23010399003|366016|1800799024|52116014|376014|18002099003|38350700014|10067099003|260925021911599003|260925022911599003|260925021311599003|56012099006|11063799006|5023799004; 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The driver rejected anything above 0x0f with -ERANGE, upon which uart_rs485_config() wipes port->rs485 and silently disables RS485 - a device tree asking for a 20 ms setup delay boots with RS485 off and an unusable bus. The values that were accepted got written into HDPIXDELAY unconverted, milliseconds as bit-times. Patches 1-4 fix pre-existing bugs found on the way: a termios write clobbering an active break; breaks never reaching the wire on RS485 ports because auto-RTS only drives the transceiver for FIFO data; the milliseconds-as-bit-times unit bug; and close() truncating the final character because tx_empty() does not cover the transmit shift register. Patch 5 adds active-low RTS on the hardware path via IRDA.RTSINVERT. Patch 6 is preparation, and patch 7 adds the software-timed RTS path that takes over whenever the hardware cannot represent the requested timing, clamping the delays to the UART core's maximum instead of rejecting them. Patch 8 fixes a reconfigure-versus- write race the asynchronous rs485 config application has had since 2016, which the software path would have made worse. v4 was all of this in a single patch; Greg asked for it to be broken up into one change at a time [1]. Splitting it meant re-verifying each patch in isolation on hardware, and that re-verification found two bugs v4 contained: a set_termios() or TIOCSRS485 during an active break released the transceiver mid-break while the break bookkeeping still looked correct (prevented by the tx_break ownership guard in patches 2 and 3), and the patch-8 race, where a TIOCSRS485 followed immediately by a write could put an entire transfer on the wire with the transceiver released. Tested on a MAX14830 (SPI, i.MX6SX) driving RS485 transceivers: for each patch the bug it fixes was first reproduced on the wire with a logic analyzer against the kernel one patch earlier, then shown fixed. The complete series additionally passed an automated 25-scenario regression matrix covering both RTS paths, both polarities, RS485/RS232 mode round-trips, close-during-transmission, and termios/ TIOCSRS485 disturbances landing in every envelope phase (setup, data, hold, break), each scenario checked both on the wire and against the driver's reported state. Changes in v5, beyond the split: - teardown interlock (tx_teardown): shutdown() and the rs485-disable path set it under port->lock, and start_tx() checks it on entry and again after retaking the dropped lock, so a racing write can no longer re-arm the delay timer or queue RTS work against a port being torn down (addresses the remaining review-bot findings on v4) - shutdown() also cancels tx_work, previously only cancelled in remove() - the per-character duration is stored as unsigned int microseconds instead of ktime_t: single-copy atomic on 32-bit, so a torn read of the 64-bit value is gone by construction - the TXEMPTY handling documents that the interrupt latches on the FIFO becoming empty, so a stale interrupt cannot pump data during an RTS setup delay - new in v5: the tx_break ownership guard (patches 2/3) and the reconfigure-pending gate (patch 8), both found during the per-patch hardware re-testing described above - also new in v5, from a review pass over the split series: startup() clears a latched break (nothing clears TXBREAK when a port is closed with a break still asserted - 8250 does the same); a reconfigure arriving during a break is now deferred and applied at break-end instead of partially dropped; the rs485-config worker runs under port->mutex so its break-guarded register writes cannot straddle a break edge; the termios-path idle settle re-checks tx_state after writing and requeues rts_work if an envelope started meanwhile; and the hardware-delay ceiling is computed in u64 [1] https://lore.kernel.org/all/2026092326-truth-unweave-c773@gregkh/ --- Tapio Reijonen (8): serial: max310x: don't clobber the TX break bit in set_termios serial: max310x: assert the transceiver during a break serial: max310x: convert RS485 delays from milliseconds to bit-times serial: max310x: wait for TX to drain before powering down in shutdown serial: max310x: support active-low RTS on the hardware path serial: max310x: schedule tx_work directly from the IRQ handler serial: max310x: drive RTS in software when hardware delays are too short serial: max310x: don't transmit while an RS485 reconfigure is pending drivers/tty/serial/max310x.c | 511 ++++++++++++++++++++++++++++++++++++++++--- 1 file changed, 477 insertions(+), 34 deletions(-) --- base-commit: 9505146e885b1a842118aa6410f737290c4a5a32 change-id: 20260513-max310x-rs485-sw-delay-a306d783d529 Best regards, -- Tapio Reijonen