From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E26B0606A327 for ; Fri, 30 Oct 2026 17:54:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=lists.infradead.org header.i=@lists.infradead.org header.a=rsa-sha256 header.s=bombadil.20210309 header.b=YHVlibe1; dkim=pass (2048-bit key, unprotected) header.d=intel.com header.i=@intel.com header.a=rsa-sha256 header.s=Intel header.b=jwxyrNsV DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To: Content-Transfer-Encoding:Content-Type:MIME-Version:References:Message-ID: Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Unsubscribe-Post:List-Owner; bh=vFtK92loJ0xr1IzJVV1xwMCKKp55jpNQBYy00GgLoCA=; b=YHVlibe1VpE7JDGWsZ6/KmO1XF IEqm+o7ax9zedt94HoImhamYiteRNF40uUQB/lf1S1N4dkKUdpKywd2L8Da8QT7al3y6CllFzgWCn JnovJk5y4VuCUEVVgodFJadVrle70oDMc9aC8y83Ax9P1O5N+74jf71jcELhod/sR620oNy1oxReZ mlVsqqH+hxYDBVcVC/nEQBU/U58t+w89N0meAHIeH1POgtCi0qSMzv37cEgbcnvXJlKRUWnW1pOd1 B1V0EJPDXpidDnnSXmUnAgKk6LkGY5PKGkm6AZ5jGfJHhiCfiakJTMwH71Lr6e7UvUWNjK08J1k+n P5mduYdA==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.100.1 #2 (Red Hat Linux)) id 1xMqnu-0000000CKBj-0tNr; Fri, 30 Oct 2026 17:54:14 +0000 Received: from mgamail.intel.com ([192.198.163.17]) by bombadil.infradead.org with esmtps (Exim 4.100.1 #2 (Red Hat Linux)) id 1xMqnr-0000000CKAq-3Izn for linux-arm-kernel@lists.infradead.org; Fri, 30 Oct 2026 17:54:13 +0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1793382852; x=1824918852; h=date:from:to:cc:subject:message-id:references: mime-version:content-transfer-encoding:in-reply-to; bh=IzqHLossDmxKbFc+aGxUKjOQbcSxP3c6buURWcuw2Tc=; b=jwxyrNsVCRRHE5mQY9GhsfqnkBThSiBT//ZjKQKsy+4yT7W//DJdJXVd LjswIJlqBKtCb76dFNB0wgdqc1WxrArKqF7od6TTjxRf2khyWai1TtCOC qNISN5Zg7HddG66wYHZgUFTwwlr1wspsNdmcXUOAjCKMj/F0lDtixnEPr yjBCAAy3FWhFY/nPYQ8o9dics1B0qf11vhl3QTmyvPQ9eNuGYmLYserVn ZkH/FNSM2iB2DNO4atD2HzHdmdfpnb3MZgilaqm4wusZPIEqZK38asMNx 1S8ZOSas74XJUOvXw9llfiSxF6w74KYc/LXYg6D4xv9c8eVivZIGXyQet A==; X-CSE-ConnectionGUID: VBlZJOfyQa+3CNm+nNQq/g== X-CSE-MsgGUID: HrpmTU2XQZO+pjofD2QTXw== X-IronPort-AV: E=McAfee;i="6800,10657,11951"; a="2170304" X-IronPort-AV: E=Sophos;i="6.27,186,1787036400"; d="scan'208";a="2170304" 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-Archived-By: 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 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20261030_105411_887208_B9F8ABD1 X-CRM114-Status: GOOD ( 30.99 ) X-Spam-Score: -4.2 (----) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" X-Spam-Status: No, score=-3.9 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_EF,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 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