Re: [PATCH 03/13] dmaengine: up-level reference counting to the module level

[Guennadi Liakhovetski <g.liakhovetski@gmx.de>]

On Thu, 4 Dec 2008, Dan Williams wrote:

> On Thu, Dec 4, 2008 at 9:56 AM, Guennadi Liakhovetski
> <g.liakhovetski@gmx.de> 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()".

Ok, but why not postpone calling .device_alloc_chan_resources() until a 
channel is _found_? Calling it for all channels just because one client 
has once called dmaengine_get() and _probably_ will poll if any channels 
become available - doesn't it look like an overkill?

> >> @@ -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().

Hm, interesting, so that's why you want dma_find_channel() as light-weight 
as possible?

Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer