From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E5E1322D4C8 for ; Tue, 24 Feb 2026 00:39:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771893550; cv=none; b=pJs4u8M/ob6fqQ5yDirrDpYD7zk5D/0rien9QKfLfBzfDr05BradI+qwHUoQnp3Ugm1ss2EMBQY96NVyoxSxVueqKuIH3nMfa+11LtdMIsH1n+4D7ehDDsgegIJxDcRV7SRWjhrZqgNX6BHydcfMaz4lK7xXATbqSYSDPleaSOo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771893550; c=relaxed/simple; bh=qNlHGqjTHH8LyHyOr4HPiUphZTLtvXz+dccGz01OT8E=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=cKJQWvvPj4jDjvptfxVpdYRHoevvAqNaEPqoDMkgzr5LkOc/fsddFP8qi77AP0YvtwbaOHDSfO+PF06H1FGIMgAcKapj5ir/9rHT55QrALXVqmWVHOyw76r2hOawHkgS0mhYHo3Cf7MWnmecq9vgs5O8zg3sTAdg80RkHcOoza0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=Sn5MrbII; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="Sn5MrbII" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5DF29C116C6; Tue, 24 Feb 2026 00:39:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1771893549; bh=qNlHGqjTHH8LyHyOr4HPiUphZTLtvXz+dccGz01OT8E=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Sn5MrbIIcf08Go5AXhGthK3sAmARaLIQAuZYrhbYsksjgNC2A/UtI6Hc4Ga/kmlu0 QlTyKQh41QfcWdaNcK/eLM/ehXH4i8/S0Rl/nFjj35L8QCHzDDFpkBx29kvOK2NyEC JA8EU/zR43V9I+I2SCqDr0e/8QEeSZ4wDD0GzFy8= Date: Tue, 24 Feb 2026 01:39:06 +0100 From: Greg KH To: Forest Cc: linux-serial@vger.kernel.org Subject: Re: Seeking help diagnosing serial port failure to transmit (while receive works) Message-ID: <2026022418-stellar-overwrite-bf17@gregkh> References: <7imppktb51fdnlm8jaovts1tkhr6r1751c@sonic.net> Precedence: bulk X-Mailing-List: linux-serial@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7imppktb51fdnlm8jaovts1tkhr6r1751c@sonic.net> On Mon, Feb 23, 2026 at 03:19:03PM -0800, Forest wrote: > Hi, folks. > > I'm seeing puzzling behavior from a recent AMD motherboard serial port: > It receives data, but doesn't appear to send, and doesn't report any > errors. Does this problem ring a bell for anyone here? > > dmesg reports this: > Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled > 00:04: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A > > The /proc/tty/driver/serial tx count rises as I type in picocom, but the > terminal at the other end of a null-modem cable (a laptop with a > USB/serial adapter) doesn't see the typed characters. The other > direction of the same connection works just fine. > > Things that have not helped: > - Replacing the motherboard's cables and connectors > - Replacing the null modem adapter with a different model > - Replacing the remote USB/serial adapter with a different model > - Switching picocom's flow control between RTS/CTS, xon/xoff, none > - Changing the serial port's IRQ and address in the BIOS > - Changing between 9600 and 115200 bps > > I'm approaching my wit's end. Could this be a known problem with the > kernel's serial driver and the B650 chipset (socket AM5), or with recent > AGESA versions? Based on experience, it's always flow control being set somewhere (on one side or the other), so watch out for that. good luck! greg k-h