Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754829AbcKYOkm convert rfc822-to-8bit (ORCPT ); Fri, 25 Nov 2016 09:40:42 -0500 Received: from unicorn.mansr.com ([81.2.72.234]:42536 "EHLO unicorn.mansr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754673AbcKYOkY (ORCPT ); Fri, 25 Nov 2016 09:40:24 -0500 From: =?iso-8859-1?Q?M=E5ns_Rullg=E5rd?= To: Russell King - ARM Linux 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 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> <20161125141708.GM14217@n2100.armlinux.org.uk> Date: Fri, 25 Nov 2016 14:40:21 +0000 In-Reply-To: <20161125141708.GM14217@n2100.armlinux.org.uk> (Russell King's message of "Fri, 25 Nov 2016 14:17:08 +0000") Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1808 Lines: 37 Russell King - ARM Linux writes: > 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... So what do you suggest? Stick our heads in the sand and pretend everything is perfect? -- M?ns Rullg?rd