From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753799AbcGVLBR (ORCPT ); Fri, 22 Jul 2016 07:01:17 -0400 Received: from devils.ext.ti.com ([198.47.26.153]:60337 "EHLO devils.ext.ti.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753197AbcGVLBP (ORCPT ); Fri, 22 Jul 2016 07:01:15 -0400 Subject: Re: [PATCH 2/7] dmaengine: omap-dma: Complete the cookie first on transfer completion To: Russell King - ARM Linux References: <20160714124242.7579-1-peter.ujfalusi@ti.com> <20160714124242.7579-3-peter.ujfalusi@ti.com> <20160718103409.GH5783@n2100.arm.linux.org.uk> <923b54d6-c7fc-66c2-1c20-d8d74ebed912@ti.com> <877fcg25c4.fsf@belgarion.home> <20160721094704.GL5783@n2100.arm.linux.org.uk> CC: Robert Jarzmik , , , , , , From: Peter Ujfalusi Message-ID: Date: Fri, 22 Jul 2016 14:00:33 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0 MIME-Version: 1.0 In-Reply-To: <20160721094704.GL5783@n2100.arm.linux.org.uk> Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/21/16 12:47, Russell King - ARM Linux wrote: > On Thu, Jul 21, 2016 at 12:33:12PM +0300, Peter Ujfalusi wrote: >> On 07/20/16 09:26, Robert Jarzmik wrote: >>> Speaking of which, from a purely design point of view, as long as you think >>> beforehand what is your sequence, ie. what is the sequence of your link >>> chaining, completion handling, etc ..., both marking before or after next tx >>> start should be fine IMHO. >> >> Yes, it might be a bit better from performance point of view if we first start >> the pending descriptor (if there is one) then do the vchan_cookie_complete(). >> On the other hand if we care more about latency and accuracy we should >> complete the transfer first then look for pending descriptors. But since >> virt_dma is using a tasklet for the real completion, the latency is always >> going to be when the tasklet is given the chance to execute. > > I think this shows a slight misunderstanding of the DMA engine API. The > DMA completion is defined by the API to always happen in tasklet context, > which is why the virt-dma stuff does it that way - and all other DMA > engine drivers. It's one of the fundamentals of the API. > > As it happens in tasklet context, tasklets can be scheduled to run with > variable latency, so any use of the DMA engine API which has a predictable > latency around the completion handling is going to be unreliable. > > Remember also that with circular buffers, there's no guarantee of getting > period-based completion callbacks - several periods can complete and you > are only guaranteed to get one completion callback. > > So, the idea that completion callbacks can have anything to do with low > latency or accuracy is totally incorrect. Thanks for refreshing my memory, you are absolutely right. -- Péter