From mboxrd@z Thu Jan 1 00:00:00 1970 From: cyril@ti.com (Cyril Chemparathy) Date: Mon, 4 Feb 2013 16:54:45 -0500 Subject: [PATCH v7 01/10] ARM: davinci: move private EDMA API to arm/common In-Reply-To: References: <510C2A47.1090607@mvista.com> <20130201205600.GA31762@arwen.pp.htv.fi> <20130201213003.GW2637@n2100.arm.linux.org.uk> <20130204154153.GA18237@arwen.pp.htv.fi> <510FF1A6.403@mvista.com> <20130204164712.GB4269@arwen.pp.htv.fi> <510FF5C9.3030600@mvista.com> <20130204170216.GC4269@arwen.pp.htv.fi> <51100A72.6030909@ti.com> <20130204203358.GX4720@opensource.wolfsonmicro.com> Message-ID: <51102E25.8040107@ti.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 02/04/2013 04:11 PM, Linus Walleij wrote: > On Mon, Feb 4, 2013 at 9:33 PM, Mark Brown > wrote: >> On Mon, Feb 04, 2013 at 09:29:46PM +0100, Linus Walleij wrote: >>> On Mon, Feb 4, 2013 at 8:22 PM, Cyril Chemparathy wrote: >> >>>> Based on our experience with fitting multiple subsystems on top of this >>>> DMA-Engine driver, I must say that the DMA-Engine interface has proven >>>> to be a less than ideal fit for the network driver use case. >> >>>> The first problem is that the DMA-Engine interface expects to "push" >>>> completed traffic up into the upper layer as a part of its callback. >>>> This doesn't fit cleanly with NAPI, which expects to "pull" completed >>>> traffic from below in the NAPI poll. We've somehow kludged together a >>>> solution around this, but it isn't very elegant. >> >>> I cannot understand the actual technical problem from the above >>> paragraphs though. dmaengine doesn't have a concept of pushing >>> nor polling, it basically copies streams of words from A to B, where >>> A/B can be a device or a buffer, nothing else. >> >>> The thing you're looking for sounds more like an adapter on top >>> of dmaengine, which can surely be constructed, some >>> drivers/dma/dmaengine-napi.c or whatever. >> >> Broadly speaking what NAPI wants is to never get any callbacks from the >> hardware (or DMAs). It wants to wake up periodically, take a look at >> what packets have been read by the hardware and process them. The goal >> is to have the DMAs sitting and running without disturbing the processor >> at all after the first packet has been handled. > > OK we should definately be able to encompass that in dmaengine > quite easily. > > So I think the above concerns are moot. The callback we can > set on cookies is entirely optional, and it's even implemented by > each DMA engine, and some may not even support it but *require* > polling, and then it won't even be implemented by the driver. > > Which probably stems from the original design of the dmaengine > API, which was for TCP networking acceleration, mainly. > > Cyril, just stack up the cookies and take a sweep over them to see > which ones are baked when the NAPI poll comes in -> problem > solved. > You're assuming that cookies complete in order. That is not necessarily true. Thanks -- Cyril.