Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754518AbcKYO2r (ORCPT ); Fri, 25 Nov 2016 09:28:47 -0500 Received: from pandora.armlinux.org.uk ([78.32.30.218]:35626 "EHLO pandora.armlinux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753981AbcKYO23 (ORCPT ); Fri, 25 Nov 2016 09:28:29 -0500 Date: Fri, 25 Nov 2016 14:17:08 +0000 From: Russell King - ARM Linux To: =?iso-8859-1?Q?M=E5ns_Rullg=E5rd?= Cc: Vinod Koul , Mason , Lars-Peter Clausen , Dave Jiang , Arnd Bergmann , Mark Brown , Linus Walleij , Bartlomiej Zolnierkiewicz , LKML , Laurent Pinchart , dmaengine@vger.kernel.org, Dan Williams , Jon Mason , Lee Jones , Maxime Ripard , Linux ARM Subject: Re: Tearing down DMA transfer setup after DMA client has finished Message-ID: <20161125141708.GM14217@n2100.armlinux.org.uk> References: <58356EA8.2010806@free.fr> <20161125045549.GC2698@localhost> <20161125124528.GJ14217@n2100.armlinux.org.uk> <20161125133436.GK14217@n2100.armlinux.org.uk> <20161125135830.GL14217@n2100.armlinux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1779 Lines: 34 On Fri, Nov 25, 2016 at 02:03:20PM +0000, M?ns Rullg?rd wrote: > Russell King - ARM Linux writes: > > > On Fri, Nov 25, 2016 at 01:50:35PM +0000, M?ns Rullg?rd wrote: > >> Russell King - ARM Linux writes: > >> > It would be unfair to augment the API and add the burden on everyone > >> > for the new API when 99.999% of the world doesn't require it. > >> > >> I don't think making this particular dma driver wait for the descriptor > >> callback to return before reusing a channel quite amounts to a horrid > >> hack. It certainly wouldn't burden anyone other than the poor drivers > >> for devices connected to it, all of which are specific to Sigma AFAIK. > > > > Except when you stop to think that delaying in a tasklet is exactly > > the same as randomly delaying in an interrupt handler - the tasklet > > runs on the return path back to the parent context of an interrupt > > handler. Even if you sleep in the tasklet, you're sleeping on behalf > > of the currently executing thread - if it's a RT thread, you effectively > > destroy the RT-ness of the thread. Let's hope no one cares about RT > > performance on that hardware... > > That's why I suggested to do this only if the needed delay is known to > be no more than a few bus cycles. The completion callback is currently > the only post-transfer interaction we have between the dma and device > drivers. To handle an arbitrarily long delay, some new interface will > be required. And now we're back at the point I made a few emails ago about undue burden which is just about quoted above... -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net.