From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Dan Williams" Subject: Re: [PATCH 03/13] dmaengine: up-level reference counting to the module level Date: Thu, 4 Dec 2008 11:51:53 -0700 Message-ID: References: <20081114213300.32354.1154.stgit@dwillia2-linux.ch.intel.com> <20081114213432.32354.2427.stgit@dwillia2-linux.ch.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org, maciej.sosnowski@intel.com, hskinnemoen@atmel.com, nicolas.ferre@atmel.com To: "Guennadi Liakhovetski" Return-path: Received: from qw-out-2122.google.com ([74.125.92.27]:47342 "EHLO qw-out-2122.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755613AbYLDSvz (ORCPT ); Thu, 4 Dec 2008 13:51:55 -0500 Received: by qw-out-2122.google.com with SMTP id 3so1005453qwe.37 for ; Thu, 04 Dec 2008 10:51:53 -0800 (PST) In-Reply-To: Content-Disposition: inline Sender: netdev-owner@vger.kernel.org List-ID: On Thu, Dec 4, 2008 at 9:56 AM, Guennadi Liakhovetski wrote: >> + >> + /* try to grab channels */ >> + list_for_each_entry_safe(device, _d, &dma_device_list, global_node) >> + list_for_each_entry(chan, &device->channels, device_node) { >> + err = dma_chan_get(chan); > > Dan, could you please explain this: dma_chan_get() takes a reference on > the channel _and_ calls .device_alloc_chan_resources() on first invocation > for a specific channel. I now see three locations in dmaengine.c, where > dma_chan_get() is called: in dma_request_channel() - logical, but also in > dmaengine_get() and dma_async_device_register(), and these latter two I > don't understand. I do not understand why we have to grab references and > allocate resources for all (public) channels on all controllers in the > system if someone just called dmaengine_get()? Consider the case where a subsystem that consumes engines like async_tx or net_dma loads before engines are available in the system. They will have taken a reference against dmaengine, but calls to dma_find_channel will return NULL. Once a channel driver is loaded dma_find_channel() can start returning results, and it is at this point that resources must be allocated and the backing module pinned. So dmaengine_get() means "I am interested in offloading stuff, if you see an offload resource grab it and prep it so I can discover it with dma_find_channel()". > >> @@ -420,6 +443,19 @@ int dma_async_device_register(struct dma_device *device) >> } >> >> mutex_lock(&dma_list_mutex); >> + list_for_each_entry(chan, &device->channels, device_node) { >> + /* if clients are already waiting for channels we need to >> + * take references on their behalf >> + */ >> + if (dmaengine_ref_count && dma_chan_get(chan) == -ENODEV) { >> + /* note we can only get here for the first >> + * channel as the remaining channels are >> + * guaranteed to get a reference >> + */ > > This is the second location - where and how are clients waiting for > channels? In the old implementation clients had notification callbacks, > which were called as new channels became available. Now clients are gone, > so, what is meant here? > The assumption is that mem-to-mem offload clients poll dma_find_channel(). Regards, Dan