From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from wr-out-0506.google.com (wr-out-0506.google.com [64.233.184.224]) by ozlabs.org (Postfix) with ESMTP id 59364DDEDF for ; Fri, 14 Mar 2008 11:06:20 +1100 (EST) Received: by wr-out-0506.google.com with SMTP id 67so2980005wri.3 for ; Thu, 13 Mar 2008 17:06:19 -0700 (PDT) Message-ID: Date: Thu, 13 Mar 2008 17:06:19 -0700 From: "Dan Williams" Sender: dan.j.williams@gmail.com To: "Olof Johansson" Subject: Re: [PATCH] pasemi_dma: Driver for PA Semi PWRficient on-chip DMAengine In-Reply-To: <20080313231442.GA4639@lixom.net> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 References: <20080306233900.GA3969@lixom.net> <1205255051.26723.19.camel@dwillia2-linux.ch.intel.com> <20080313195431.GA1449@lixom.net> <20080313231442.GA4639@lixom.net> Cc: linuxppc-dev@ozlabs.org, pasemi-linux@ozlabs.org, "Nelson, Shannon" , linux-kernel@vger.kernel.org, hskinnemoen@atmel.com List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Thu, Mar 13, 2008 at 4:14 PM, Olof Johansson wrote: > > Dummy descriptors that do nothing but cause an interrupt is the intent. > > Well, it'd be slightly more efficient to do add the interrupt attribute > to the last issued descriptor when it's known in advance. If the > underlying driver doesn't support it, adding a separate descriptor would > be a good fallback. > When it is known in advance the interrupt attribute *is* set, otherwise the descriptor may already be in flight. > Anyway, this isn't likely to be a performance bottleneck. If it turns > out to be, I'll refactor it and submit patches. > Would not hurt to have another pair of eyes on this part of the code. A rewrite of the channel switch mechanism is currently pending in async_tx.git#upstream. [..] > > > Can't do that if it's called both from the polling as well as the IRQ > > > context, it'd need to hold off irqs. I.e. once I add the DMA_INTERRUPT > > > support it will be needed. > > > > > > > ...do it in a tasklet. > > Why? That just adds overhead and latency. > The original ioat_dma code used nothing heavier than spin_lock_bh. Async_tx now assumes that local_bh_disable prevents races with any channel's cleanup routine. Clients can place the same kind of code in an async_tx callback as they would in a timer callback. The assumption is that code using async_tx can afford its extra overhead, which is true for raid. This is also why you don't see async_memcpy calls in net_dma. -- Dan