From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=192.198.163.17 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793382853; b=HX834kn910U3yCtazV6XlfeOsEHwRUvfnbEPJ3mbR51wTx+Z1lnKhlH1QH167zBak+MP bXzqDnhavd9/xE01qVzyRlfRPT1ij0AltAunflyZCDz3rcm2FHGjyd0hi/N3vzwII4R/z 5idsea7T5kihWtlK3rdh3x5/MjF0rphqf0jWE2lPu70RQLPOgKDfGvhu59vUO+1SDzri/ YfCYfnV6EEf0/5/6Xi5Fd7e4p+rqNaNuqWQfenmdkD26nFPYyu191m8+BHtImCjaXVLvY XATHdGGE6wN7fWrqQAyIyQeBxi47j4OmAUwQymgWUejKNR4acOE4+ghRSkcHDWCXFtQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793382853; h=DMARC-Filter:DKIM-Signature:X-CSE-ConnectionGUID:X-CSE-MsgGUID: X-IronPort-AV:X-IronPort-AV:Received:X-CSE-ConnectionGUID: X-CSE-MsgGUID:X-ExtLoop1:X-IronPort-AV:Received:Date:From:To:Cc: Subject:Message-ID:References:MIME-Version:Content-Type: Content-Disposition:Content-Transfer-Encoding:In-Reply-To: Organization; bh=vFtK92loJ0xr1IzJVV1xwMCKKp55jpNQBYy00GgLoCA=; b=pMux3f9towrPAoUjMDPjh5RZrOve8/ZlvLAbR0r7VFZEm3D9CfCTDRJRPLxiL42Z8bIU 9Zpyd1PTa5Jkj+TC2DrrQWTWVewMkK3earuXkBaHlzcJoOo2R2H7wBpVCxBvRrIwmk8H2 K8nVJbA6+Mj8ARrgHtOmMLuoX2nB8EvVabJFm2T2tWMgSUBkJ1oNdtex6FNwY7smP/MSs JHVfmM+O4oQU+DTpS1e2jOHlEYoymcClxqW2Ho3rLs6BTS9j7o/mJTvF2sy8B2pfHY1fw BUqb3QJaS9X7bJgwhCjpLYBJqKPGi1S7D2lpTMurtC91JOufGlIaY8ofGLTmPg2cccg== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=linux.intel.com; spf=pass smtp.mailfrom=linux.intel.com; dkim=pass header.d=intel.com header.i=@intel.com header.a=rsa-sha256 header.s=Intel header.b=HXiv2R96; arc=none smtp.remote-ip=192.198.163.17 Received: from mgamail.intel.com (mgamail.intel.com [192.198.163.17]) (using TLSv1.2 with cipher ECDHE-ECDSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 47178606A31C; Fri, 30 Oct 2026 17:54:12 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 47178606A31C Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.intel.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.intel.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=intel.com header.i=@intel.com header.a=rsa-sha256 header.s=Intel header.b=HXiv2R96 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1793382853; x=1824918853; h=date:from:to:cc:subject:message-id:references: mime-version:content-transfer-encoding:in-reply-to; bh=IzqHLossDmxKbFc+aGxUKjOQbcSxP3c6buURWcuw2Tc=; b=HXiv2R96nL23J3TEKA7EuVFwW+y+FvRBx+gSYbOTbPqpzFN5jsQDBY8N W7ICTJLAGMeNUaqwkGXma5686S6TO8G6DJ5/xi0q4GZyEy96BSjzOZ5kc 7sHOAItt8rntURs4qiKoNHvo2vo8P2zg8N8TZWKVCj5wXcPUUTnBJl9cQ y6uRLRgupKrrU6Cv9DmAmrijK7Ky6s2zyOtiWZqcjnNaj2ydhKFdcQsYa uLSH4BI2Xrw3TTT7NSgc/AFc/TSjY4S4/ISHIoDl+rSvLxp70cmxB/vV0 AQF0zPNiz/yt2Fcdxy9uoCdytrWqMQyS17BkCx9RQlA3eHykxQKNW+2hK Q==; X-CSE-ConnectionGUID: ZmrW4xZrSeeV/9XF1B1VpA== X-CSE-MsgGUID: w8HWwnfBSrCGCi7D3XA9tA== X-IronPort-AV: E=McAfee;i="6800,10657,11951"; a="2170307" X-IronPort-AV: E=Sophos;i="6.27,186,1787036400"; d="scan'208";a="2170307" Received: from fmviesa004.fm.intel.com ([10.60.135.144]) by fmvoesa111.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 30 Oct 2026 10:54:07 -0700 X-CSE-ConnectionGUID: 4l33Hy4iTb6mNx52iAPxHQ== X-CSE-MsgGUID: aoBFLn+ZRDe7lWiftDeSQg== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.27,186,1787036400"; d="scan'208";a="5516478" Received: from ettammin-mobl3.ger.corp.intel.com (HELO localhost) ([10.245.245.247]) by smtpauth.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 30 Oct 2026 10:54:01 -0700 Date: Fri, 30 Oct 2026 19:53:58 +0200 From: Andy Shevchenko To: James Hilliard Cc: Greg Kroah-Hartman , Jiri Slaby , Raul E Rangel , Ilpo =?iso-8859-1?Q?J=E4rvinen?= , Heikki Krogerus , Tony Lindgren , Sebastian Andrzej Siewior , Richard Genoud , Nicolas Ferre , Alexandre Belloni , Claudiu Beznea , Jacky Huang , Shan-Chun Hung , Pali =?iso-8859-1?Q?Roh=E1r?= , Frank Li , Sascha Hauer , Pengutronix Kernel Team , Fabio Estevam , "Maciej W. Rozycki" , Timur Tabi , linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org, linux-arm-kernel@lists.infradead.org, imx@lists.linux.dev, linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH v8 6/6] serial: 8250: retain DMA resources when termination fails Message-ID: References: <20261029-submit-serial-8250-dma-cancel-v1-v8-0-3fd56ba352ed@gmail.com> <20261029-submit-serial-8250-dma-cancel-v1-v8-6-3fd56ba352ed@gmail.com> 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-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Organization: Intel Finland Oy - BIC 0357606-4 - c/o Alberga Business Park, 6 krs, Bertel Jungin Aukio 5, 02600 Espoo On Fri, Oct 30, 2026 at 11:09:53AM -0600, James Hilliard wrote: > On Fri, Oct 30, 2026 at 3:40 AM Andy Shevchenko > wrote: > > On Thu, Oct 29, 2026 at 11:19:59AM -0600, James Hilliard wrote: > > > On Thu, Oct 29, 2026 at 8:24 AM Andy Shevchenko > > > wrote: > > > > On Thu, Oct 29, 2026 at 01:37:02AM -0600, James Hilliard wrote: ... > > > > > + while (rx_ret || tx_ret) { > > > > > + dev_err_ratelimited(p->port.dev, > > > > > + "DMA termination failed: RX %d, TX %d; retrying\n", > > > > > + rx_ret, tx_ret); > > > > > > > > > + msleep(100); > > > > > > > > Huh?! > > > > > > > > > + if (rx_ret) > > > > > + rx_ret = dmaengine_terminate_sync(dma->rxchan); > > > > > + if (tx_ret) > > > > > + tx_ret = dmaengine_terminate_sync(dma->txchan); > > > > > + } > > > > > > > > This is unacceptable. Sleeping is okay, but value should really match the > > > > baudspeed and other parameters of the transfer. > > > > > > This sleep is backoff after dmaengine_terminate_sync() has returned > > > an error, rather than an estimate of UART transmission time. Each > > > retry asks the provider to stop the channel again; elapsed time is > > > never treated as evidence that the resources are safe to free. > > > > > > The particular 100 ms interval has no hardware-derived justification. > > > However, I don't yet see a sound way to derive the retry cadence from > > > baud rate either. RX can be waiting for input, TX can be held by flow > > > control, and callback retirement is a separate requirement from > > > moving bytes through the UART. > > > > > > Do you have a particular provider failure mode in mind where retrying > > > termination should follow the serial transfer timing? I'd like to > > > base the interval on the condition that can make termination succeed, > > > rather than substitute a wire-time estimate for an arbitrary delay. > > > > Termination of DMA sometimes require to clean up internal FIFOs and other state > > machines. That's what happens on HW side. In the software waiting for too much > > may lead to the very big undesired latencies. The value of 100ms sounds insane > > to me for the HSUART cases (megabaud range of speed). Also note, not every PCB > > design uses HW flow control, so this may be even worse for them. > > The 100 ms interval can add unnecessary teardown latency if a failed > termination becomes retryable much sooner. There is no hardware-derived > justification for that particular value. > > The FIFO/state-machine draining is part of the provider's stop sequence. > For example, DW performs its FIFO-empty polling inside > dwc_terminate_all(). If termination succeeds, this patch proceeds without > the retry sleep, regardless of baud rate. The sleep is only reached > after an error during DMA release, not during normal RX delivery. > > Baud rate can influence how quickly a transmit FIFO drains, even without > flow control. However, the termination error does not tell 8250 whether > that drain is still progressing, how much remains, or whether the > controller needs a different recovery operation. That distinction matters > when choosing a retry interval: a shorter interval reduces the added > latency, but does not establish that retrying can recover the channel. > > There is also a specific API detail here: dmaengine_terminate_sync() > returns immediately if device_terminate_all() fails, without reaching > dmaengine_synchronize(). Consequently, an error leaves callback draining > unconfirmed as well. The retry cadence can change, but resource release > still requires successful termination and synchronization. Most likely this is AI slop. I can't see the justification for 100ms. Can you explain that? > > > Regardless of the retry interval, shutdown cannot release the buffers > > > until the provider establishes that DMA and its callbacks have stopped. -- With Best Regards, Andy Shevchenko