Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755038AbcKYPMd convert rfc822-to-8bit (ORCPT ); Fri, 25 Nov 2016 10:12:33 -0500 Received: from unicorn.mansr.com ([81.2.72.234]:42682 "EHLO unicorn.mansr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754602AbcKYPM0 (ORCPT ); Fri, 25 Nov 2016 10:12:26 -0500 From: =?iso-8859-1?Q?M=E5ns_Rullg=E5rd?= To: Mason Cc: Russell King - ARM Linux , Vinod Koul , 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 15:12:24 +0000 In-Reply-To: (Mason's message of "Fri, 25 Nov 2016 16:02:44 +0100") 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: 2404 Lines: 51 Mason writes: > On 25/11/2016 15:17, Russell King - ARM Linux wrote: >> 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... > > I've had several talks with the HW dev, and I don't think they > anticipated the need to mux the 3 channels. In their minds, > customers would choose at most 3 devices to support, and > assign one channel to each device statically. > > In fact, in tango4, supported devices are: > A) NAND Flash controllers 0 and 1 > NB: the upstream driver only uses controller 0 > B) IDE or SATA controllers 0 and 1 > C) a few crypto HW blocks which do not work as expected (unused) > > Customers typically use 1 channel for NAND, maybe 1 for SATA, > and 1 channel remains unused. The hardware has two sata controllers, and I have a board that uses both. -- M?ns Rullg?rd