lkml.org 
[lkml]   [2008]   [Nov]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 03/13] dmaengine: up-level reference counting to the module level
On Fri, 14 Nov 2008 14:34:32 -0700 Dan Williams <dan.j.williams@intel.com> wrote:

> Simply, if a client wants any dmaengine channel then prevent all dmaengine
> modules from being removed. Once the clients are done re-enable module
> removal.
>
> Why?, beyond reducing complication:
> 1/ Tracking reference counts per-transaction in an efficient manner, as
> is currently done, requires a complicated scheme to avoid cache-line
> bouncing effects.
> 2/ Per-transaction ref-counting gives the false impression that a
> dma-driver can be gracefully removed ahead of its user (net, md, or
> dma-slave)
> 3/ None of the in-tree dma-drivers talk to hot pluggable hardware, but
> if such an engine were built one day we still would not need to notify
> clients of remove events. The driver can simply return NULL to a
> ->prep() request, something that is much easier for a client to handle.
>
> ...
>
> +static struct module *dma_chan_to_owner(struct dma_chan *chan)
> +{
> + return chan->device->dev->driver->owner;
> +}

Has this all been tested with CONFIG_MODULES=n?

It looks like we have a lot of unneeded code if CONFIG_MODULES=n.
However that might not be a case which is worth bothering about.

> +/**
> + * balance_ref_count - catch up the channel reference count
> + */
> +static void balance_ref_count(struct dma_chan *chan)

Forgot to kerneldocument the argument.

> +{
> + struct module *owner = dma_chan_to_owner(chan);
> +
> + while (chan->client_count < dmaengine_ref_count) {
> + __module_get(owner);
> + chan->client_count++;
> + }
> +}

The locking for ->client_count is undocumented.

> +/**
> + * dma_chan_get - try to grab a dma channel's parent driver module
> + * @chan - channel to grab
> + */
> +static int dma_chan_get(struct dma_chan *chan)
> +{
> + int err = -ENODEV;
> + struct module *owner = dma_chan_to_owner(chan);
> +
> + if (chan->client_count) {
> + __module_get(owner);
> + err = 0;
> + } else if (try_module_get(owner))
> + err = 0;

I wonder if try_module_get() could be used in both cases (migt not make
sense to do so though).

> + if (err == 0)
> + chan->client_count++;

Locking for this?

> + /* allocate upon first client reference */
> + if (chan->client_count == 1 && err == 0) {
> + int desc = chan->device->device_alloc_chan_resources(chan, NULL);
> +
> + if (desc < 0) {
> + chan->client_count = 0;
> + module_put(owner);
> + err = -ENOMEM;

Shouldn't we just propagate the ->device_alloc_chan_resources() return value?

> + } else
> + balance_ref_count(chan);
> + }
> +
> + return err;
> +}
> +
> +static void dma_chan_put(struct dma_chan *chan)
> +{
> + if (!chan->client_count)
> + return; /* this channel failed alloc_chan_resources */

Or we had a bug ;)

> + chan->client_count--;

Undocumented locking..

>
> ...
>


\
 
 \ /
  Last update: 2008-11-15 07:11    [from the cache]
©2003-2014 Jasper Spaans. Advertise on this site