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Subject[PATCH 4/5] dmaengine: Add reference counting to dma_device struct
Adding a reference count helps drivers to properly implement the unbind
while in use case.

References are taken and put every time a channel is allocated or freed.

Once the final reference is put, the device is removed from the
dma_device_list and a release callback function is called to signal
the driver to free the memory.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
---
drivers/dma/dmaengine.c | 57 +++++++++++++++++++++++++++++++++------
include/linux/dmaengine.h | 8 +++++-
2 files changed, 56 insertions(+), 9 deletions(-)

diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
index 1f9a6293f15a..e316abe3672d 100644
--- a/drivers/dma/dmaengine.c
+++ b/drivers/dma/dmaengine.c
@@ -342,6 +342,23 @@ static void balance_ref_count(struct dma_chan *chan)
}
}

+static void dma_device_release(struct kref *ref)
+{
+ struct dma_device *device = container_of(ref, struct dma_device, ref);
+
+ list_del_rcu(&device->global_node);
+ dma_channel_rebalance();
+
+ if (device->device_release)
+ device->device_release(device);
+}
+
+static void dma_device_put(struct dma_device *device)
+{
+ lockdep_assert_held(&dma_list_mutex);
+ kref_put(&device->ref, dma_device_release);
+}
+
/**
* dma_chan_get - try to grab a dma channel's parent driver module
* @chan - channel to grab
@@ -362,6 +379,12 @@ static int dma_chan_get(struct dma_chan *chan)
if (!try_module_get(owner))
return -ENODEV;

+ ret = kref_get_unless_zero(&chan->device->ref);
+ if (!ret) {
+ ret = -ENODEV;
+ goto module_put_out;
+ }
+
/* allocate upon first client reference */
if (chan->device->device_alloc_chan_resources) {
ret = chan->device->device_alloc_chan_resources(chan);
@@ -377,6 +400,8 @@ static int dma_chan_get(struct dma_chan *chan)
return 0;

err_out:
+ dma_device_put(chan->device);
+module_put_out:
module_put(owner);
return ret;
}
@@ -402,6 +427,7 @@ static void dma_chan_put(struct dma_chan *chan)
chan->device->device_free_chan_resources(chan);
}

+ dma_device_put(chan->device);
module_put(dma_chan_to_owner(chan));

/* If the channel is used via a DMA request router, free the mapping */
@@ -837,14 +863,14 @@ EXPORT_SYMBOL(dmaengine_get);
*/
void dmaengine_put(void)
{
- struct dma_device *device;
+ struct dma_device *device, *_d;
struct dma_chan *chan;

mutex_lock(&dma_list_mutex);
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_safe(device, _d, &dma_device_list, global_node) {
if (dma_has_cap(DMA_PRIVATE, device->cap_mask))
continue;
list_for_each_entry(chan, &device->channels, device_node)
@@ -906,6 +932,10 @@ static int get_dma_id(struct dma_device *device)
/**
* dma_async_device_register - registers DMA devices found
* @device: &dma_device
+ *
+ * After calling this routine the structure should not be freed except in the
+ * device_release() callback which will be called after
+ * dma_async_device_unregister() is called and no further references are taken.
*/
int dma_async_device_register(struct dma_device *device)
{
@@ -999,6 +1029,12 @@ int dma_async_device_register(struct dma_device *device)
return -EIO;
}

+ if (!device->device_release)
+ dev_warn(device->dev,
+ "WARN: Device release is not defined so it is not safe to unbind this driver while in use\n");
+
+ kref_init(&device->ref);
+
/* note: this only matters in the
* CONFIG_ASYNC_TX_ENABLE_CHANNEL_SWITCH=n case
*/
@@ -1115,13 +1151,8 @@ void dma_async_device_unregister(struct dma_device *device)
{
struct dma_chan *chan;

- mutex_lock(&dma_list_mutex);
- list_del_rcu(&device->global_node);
- dma_channel_rebalance();
- mutex_unlock(&dma_list_mutex);
-
list_for_each_entry(chan, &device->channels, device_node) {
- WARN_ONCE(chan->client_count,
+ WARN_ONCE(!device->device_release && chan->client_count,
"%s called while %d clients hold a reference\n",
__func__, chan->client_count);
mutex_lock(&dma_list_mutex);
@@ -1130,6 +1161,16 @@ void dma_async_device_unregister(struct dma_device *device)
device_unregister(&chan->dev->device);
free_percpu(chan->local);
}
+
+ mutex_lock(&dma_list_mutex);
+ /*
+ * setting DMA_PRIVATE ensures the device being torn down will not
+ * be used in the channel_table
+ */
+ dma_cap_set(DMA_PRIVATE, device->cap_mask);
+ dma_channel_rebalance();
+ dma_device_put(device);
+ mutex_unlock(&dma_list_mutex);
}
EXPORT_SYMBOL(dma_async_device_unregister);

diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
index 13aa0abb71de..61e59a544af2 100644
--- a/include/linux/dmaengine.h
+++ b/include/linux/dmaengine.h
@@ -719,9 +719,14 @@ struct dma_filter {
* will just return a simple status code
* @device_issue_pending: push pending transactions to hardware
* @descriptor_reuse: a submitted transfer can be resubmitted after completion
+ * @device_release: called sometime atfer dma_async_device_unregister() is
+ * called and there are no further references to this structure. This
+ * must be implemented to free resources however many existing drivers
+ * do not and are therefore not safe to unbind while in use.
+ *
*/
struct dma_device {
-
+ struct kref ref;
unsigned int chancnt;
unsigned int privatecnt;
struct list_head channels;
@@ -802,6 +807,7 @@ struct dma_device {
dma_cookie_t cookie,
struct dma_tx_state *txstate);
void (*device_issue_pending)(struct dma_chan *chan);
+ void (*device_release)(struct dma_device *dev);
};

static inline int dmaengine_slave_config(struct dma_chan *chan,
--
2.20.1
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