Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752683AbcLISYH (ORCPT ); Fri, 9 Dec 2016 13:24:07 -0500 Received: from smtp2-g21.free.fr ([212.27.42.2]:31791 "EHLO smtp2-g21.free.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752586AbcLISYG (ORCPT ); Fri, 9 Dec 2016 13:24:06 -0500 Subject: Re: Tearing down DMA transfer setup after DMA client has finished To: Vinod Koul Cc: Sebastian Frias , Russell King , dmaengine@vger.kernel.org, Linus Walleij , Dan Williams , LKML , Linux ARM , Jon Mason , Mark Brown , Lars-Peter Clausen , Lee Jones , Laurent Pinchart , Arnd Bergmann , Maxime Ripard , Dave Jiang , Peter Ujfalusi , Bartlomiej Zolnierkiewicz , Thibaud Cornic References: <20161207164341.GX6408@localhost> <20161208103921.GC6408@localhost> <91b0d10c-1bc2-c3e1-4088-f4ad9adcd6c0@free.fr> <20161208163755.GH6408@localhost> <20161209065955.GJ6408@localhost> <6ce1ea97-1d68-2203-c7b4-73315e801655@laposte.net> <20161209171727.GK6408@localhost> <20161209175606.GM6408@localhost> From: Mason Message-ID: <7900ae24-6803-a271-944c-8830f718fef0@free.fr> Date: Fri, 9 Dec 2016 19:23:17 +0100 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:50.0) Gecko/20100101 Firefox/50.0 SeaMonkey/2.47 MIME-Version: 1.0 In-Reply-To: <20161209175606.GM6408@localhost> Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1799 Lines: 44 [ Dropping Mans to preserve his peace-of-mind ] On 09/12/2016 18:56, Vinod Koul wrote: > On Fri, Dec 09, 2016 at 06:34:15PM +0100, Mason wrote: >> On 09/12/2016 18:17, Vinod Koul wrote: >> >>> On Fri, Dec 09, 2016 at 11:25:57AM +0100, Sebastian Frias wrote: >>>> >>>> What concrete solution do you propose? >>> >>> I have already proposed two solutions. >>> >>> A) Request a channel only when you need it. Obviously we can't do virtual >>> channels with this (though we should still use virt-channels framework). >>> The sbox setup and teardown can be done as part of channel request and >>> freeup. PL08x already does this. >>> >>> Downside is that we can only have as many consumers at a time as channels. >>> >>> I have not heard any technical reason for not doing this apart from drivers >>> grab the channel at probe, which is incorrect and needs to be fixed >>> irrespective of the problem at hand. >>> >>> This is my preferred option. >> >> There is one important drawback with this solution. If a driver calls >> dma_request_chan() when no channels are currently available, it will >> get -EBUSY. If there were a flag in dma_request_chan to be put to >> sleep (with timeout) until a channel is available, then it would >> work. But busy waiting in the client driver is a waste of power. > > Right, but in that case the fallback would be PIO mode, and if that is > not availble (IIRC some f your devices don't) then reject the usage with > EAGAIN. Maybe I'm missing something, but I don't see how that would help. Take the NAND Flash controller driver, for instance. PIO is not an option, because the ECC engine is tied to DMA. And failing with -EAGAIN doesn't help the busy looping situation. The caller should be put on some kind of queue to wait for a "channel ready" event. Regards.