dmaengine: up-level reference counting to the module level
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.
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Maciej Sosnowski <maciej.sosnowski@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
index b900893..d4d9259 100644
--- a/drivers/dma/dmaengine.c
+++ b/drivers/dma/dmaengine.c
@@ -74,6 +74,7 @@
static DEFINE_MUTEX(dma_list_mutex);
static LIST_HEAD(dma_device_list);
static LIST_HEAD(dma_client_list);
+static long dmaengine_ref_count;
/* --- sysfs implementation --- */
@@ -105,19 +106,8 @@
static ssize_t show_in_use(struct device *dev, struct device_attribute *attr, char *buf)
{
struct dma_chan *chan = to_dma_chan(dev);
- int in_use = 0;
- if (unlikely(chan->slow_ref) &&
- atomic_read(&chan->refcount.refcount) > 1)
- in_use = 1;
- else {
- if (local_read(&(per_cpu_ptr(chan->local,
- get_cpu())->refcount)) > 0)
- in_use = 1;
- put_cpu();
- }
-
- return sprintf(buf, "%d\n", in_use);
+ return sprintf(buf, "%d\n", chan->client_count);
}
static struct device_attribute dma_attrs[] = {
@@ -155,6 +145,78 @@
return bitmap_equal(want->bits, has.bits, DMA_TX_TYPE_END);
}
+static struct module *dma_chan_to_owner(struct dma_chan *chan)
+{
+ return chan->device->dev->driver->owner;
+}
+
+/**
+ * balance_ref_count - catch up the channel reference count
+ * @chan - channel to balance ->client_count versus dmaengine_ref_count
+ *
+ * balance_ref_count must be called under dma_list_mutex
+ */
+static void balance_ref_count(struct dma_chan *chan)
+{
+ struct module *owner = dma_chan_to_owner(chan);
+
+ while (chan->client_count < dmaengine_ref_count) {
+ __module_get(owner);
+ chan->client_count++;
+ }
+}
+
+/**
+ * dma_chan_get - try to grab a dma channel's parent driver module
+ * @chan - channel to grab
+ *
+ * Must be called under dma_list_mutex
+ */
+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;
+
+ if (err == 0)
+ chan->client_count++;
+
+ /* allocate upon first client reference */
+ if (chan->client_count == 1 && err == 0) {
+ int desc_cnt = chan->device->device_alloc_chan_resources(chan, NULL);
+
+ if (desc_cnt < 0) {
+ err = desc_cnt;
+ chan->client_count = 0;
+ module_put(owner);
+ } else
+ balance_ref_count(chan);
+ }
+
+ return err;
+}
+
+/**
+ * dma_chan_put - drop a reference to a dma channel's parent driver module
+ * @chan - channel to release
+ *
+ * Must be called under dma_list_mutex
+ */
+static void dma_chan_put(struct dma_chan *chan)
+{
+ if (!chan->client_count)
+ return; /* this channel failed alloc_chan_resources */
+ chan->client_count--;
+ module_put(dma_chan_to_owner(chan));
+ if (chan->client_count == 0)
+ chan->device->device_free_chan_resources(chan);
+}
+
/**
* dma_client_chan_alloc - try to allocate channels to a client
* @client: &dma_client
@@ -165,7 +227,6 @@
{
struct dma_device *device;
struct dma_chan *chan;
- int desc; /* allocated descriptor count */
enum dma_state_client ack;
/* Find a channel */
@@ -178,23 +239,16 @@
list_for_each_entry(chan, &device->channels, device_node) {
if (!dma_chan_satisfies_mask(chan, client->cap_mask))
continue;
+ if (!chan->client_count)
+ continue;
+ ack = client->event_callback(client, chan,
+ DMA_RESOURCE_AVAILABLE);
- desc = chan->device->device_alloc_chan_resources(
- chan, client);
- if (desc >= 0) {
- ack = client->event_callback(client,
- chan,
- DMA_RESOURCE_AVAILABLE);
-
- /* we are done once this client rejects
- * an available resource
- */
- if (ack == DMA_ACK) {
- dma_chan_get(chan);
- chan->client_count++;
- } else if (ack == DMA_NAK)
- return;
- }
+ /* we are done once this client rejects
+ * an available resource
+ */
+ if (ack == DMA_NAK)
+ return;
}
}
}
@@ -224,7 +278,6 @@
void dma_chan_cleanup(struct kref *kref)
{
struct dma_chan *chan = container_of(kref, struct dma_chan, refcount);
- chan->device->device_free_chan_resources(chan);
kref_put(&chan->device->refcount, dma_async_device_cleanup);
}
EXPORT_SYMBOL(dma_chan_cleanup);
@@ -232,18 +285,12 @@
static void dma_chan_free_rcu(struct rcu_head *rcu)
{
struct dma_chan *chan = container_of(rcu, struct dma_chan, rcu);
- int bias = 0x7FFFFFFF;
- int i;
- for_each_possible_cpu(i)
- bias -= local_read(&per_cpu_ptr(chan->local, i)->refcount);
- atomic_sub(bias, &chan->refcount.refcount);
+
kref_put(&chan->refcount, dma_chan_cleanup);
}
static void dma_chan_release(struct dma_chan *chan)
{
- atomic_add(0x7FFFFFFF, &chan->refcount.refcount);
- chan->slow_ref = 1;
call_rcu(&chan->rcu, dma_chan_free_rcu);
}
@@ -263,43 +310,36 @@
}
/**
- * dma_chans_notify_available - tell the clients that a channel is going away
- * @chan: channel on its way out
- */
-static void dma_clients_notify_removed(struct dma_chan *chan)
-{
- struct dma_client *client;
- enum dma_state_client ack;
-
- mutex_lock(&dma_list_mutex);
-
- list_for_each_entry(client, &dma_client_list, global_node) {
- ack = client->event_callback(client, chan,
- DMA_RESOURCE_REMOVED);
-
- /* client was holding resources for this channel so
- * free it
- */
- if (ack == DMA_ACK) {
- dma_chan_put(chan);
- chan->client_count--;
- }
- }
-
- mutex_unlock(&dma_list_mutex);
-}
-
-/**
* dma_async_client_register - register a &dma_client
* @client: ptr to a client structure with valid 'event_callback' and 'cap_mask'
*/
void dma_async_client_register(struct dma_client *client)
{
+ struct dma_device *device, *_d;
+ struct dma_chan *chan;
+ int err;
+
/* validate client data */
BUG_ON(dma_has_cap(DMA_SLAVE, client->cap_mask) &&
!client->slave);
mutex_lock(&dma_list_mutex);
+ dmaengine_ref_count++;
+
+ /* 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);
+ if (err == -ENODEV) {
+ /* module removed before we could use it */
+ list_del_init(&device->global_node);
+ break;
+ } else if (err)
+ pr_err("dmaengine: failed to get %s: (%d)\n",
+ dev_name(&chan->dev), err);
+ }
+
+
list_add_tail(&client->global_node, &dma_client_list);
mutex_unlock(&dma_list_mutex);
}
@@ -315,23 +355,17 @@
{
struct dma_device *device;
struct dma_chan *chan;
- enum dma_state_client ack;
if (!client)
return;
mutex_lock(&dma_list_mutex);
- /* free all channels the client is holding */
+ dmaengine_ref_count--;
+ BUG_ON(dmaengine_ref_count < 0);
+ /* drop channel references */
list_for_each_entry(device, &dma_device_list, global_node)
- list_for_each_entry(chan, &device->channels, device_node) {
- ack = client->event_callback(client, chan,
- DMA_RESOURCE_REMOVED);
-
- if (ack == DMA_ACK) {
- dma_chan_put(chan);
- chan->client_count--;
- }
- }
+ list_for_each_entry(chan, &device->channels, device_node)
+ dma_chan_put(chan);
list_del(&client->global_node);
mutex_unlock(&dma_list_mutex);
@@ -423,6 +457,21 @@
}
mutex_lock(&dma_list_mutex);
+ if (dmaengine_ref_count)
+ 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 (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
+ */
+ rc = -ENODEV;
+ mutex_unlock(&dma_list_mutex);
+ goto err_out;
+ }
+ }
list_add_tail(&device->global_node, &dma_device_list);
mutex_unlock(&dma_list_mutex);
@@ -456,7 +505,7 @@
}
/**
- * dma_async_device_unregister - unregisters DMA devices
+ * dma_async_device_unregister - unregister a DMA device
* @device: &dma_device
*/
void dma_async_device_unregister(struct dma_device *device)
@@ -468,7 +517,9 @@
mutex_unlock(&dma_list_mutex);
list_for_each_entry(chan, &device->channels, device_node) {
- dma_clients_notify_removed(chan);
+ WARN_ONCE(chan->client_count,
+ "%s called while %d clients hold a reference\n",
+ __func__, chan->client_count);
device_unregister(&chan->dev);
dma_chan_release(chan);
}