The documentation for the dma_slave_config struct recommends that if a DMA
controller has special configuration options, which can not be configured
through the dma_slave_config struct, the driver should create its own custom
config struct and embed the dma_slave_config struct in it and pass the custom
config struct to dmaengine_slave_config(). This overloads the generic
dmaengine_slave_config() API with custom semantics and any caller of the
dmaengine_slave_config() that is not aware of these special semantics will cause
undefined behavior. This means that it is impossible for generic code to make
use of dmaengine_slave_config(). Such a restriction contradicts the very idea of
having a generic API.
E.g. consider the following case of a DMA controller that has an option to
reverse the field polarity of the DMA transfer with the following implementation
for setting the configuration:
struct my_slave_config {
struct dma_slave_config config;
unsigned int field_polarity;
};
static int my_dma_controller_slave_config(struct dma_chan *chan,
struct dma_slave_config *config)
{
struct my_slave_config *my_cfg = container_of(config,
struct my_slave_config, config);
...
my_dma_set_field_polarity(chan, my_cfg->field_polarity);
...
}
Now a generic user of the dmaengine API might want to configure a DMA channel
for this DMA controller that it obtained using the following code:
struct dma_slave_config config;
config.src_addr = ...;
...
dmaengine_slave_config(chan, &config);
The call to dmaengine_slave_config() will eventually call into
my_dma_controller_slave_config() which will cast from dma_slave_config to
my_slave_config and then tries to access the field_polarity member. Since the
dma_slave_config struct that was passed in was never embedded into a
my_slave_config struct this attempt will just read random stack garbage and use
that to configure the DMA controller. This is bad. Instead, if a DMA controller
really needs to have custom configuration options, the driver should create a
custom API for it. This makes it very clear that there is a direct dependency
of a user of such an API and the implementer. E.g.:
int my_dma_set_field_polarity(struct dma_chan *chan,
unsigned int field_polarity) {
if (chan->device->dev->driver != &my_dma_controller_driver.driver)
return -EINVAL;
...
}
EXPORT_SYMBOL_GPL(my_dma_set_field_polarity);
Signed-off-by: Lars-Peter Clausen <lars@xxxxxxxxxx>
---
include/linux/dmaengine.h | 14 +++++---------
1 file changed, 5 insertions(+), 9 deletions(-)
diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
index c5c92d5..8300fb8 100644
--- a/include/linux/dmaengine.h
+++ b/include/linux/dmaengine.h
@@ -341,15 +341,11 @@ enum dma_slave_buswidth {
* and this struct will then be passed in as an argument to the
* DMA engine device_control() function.
*
- * The rationale for adding configuration information to this struct
- * is as follows: if it is likely that most DMA slave controllers in
- * the world will support the configuration option, then make it
- * generic. If not: if it is fixed so that it be sent in static from
- * the platform data, then prefer to do that. Else, if it is neither
- * fixed at runtime, nor generic enough (such as bus mastership on
- * some CPU family and whatnot) then create a custom slave config
- * struct and pass that, then make this config a member of that
- * struct, if applicable.
+ * The rationale for adding configuration information to this struct is as
+ * follows: if it is likely that more than one DMA slave controllers in
+ * the world will support the configuration option, then make it generic.
+ * If not: if it is fixed so that it be sent in static from the platform
+ * data, then prefer to do that.
*/
struct dma_slave_config {
enum dma_transfer_direction direction;
--
1.8.0
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