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-rw-r--r--Documentation/DMA-API-HOWTO.txt126
1 files changed, 126 insertions, 0 deletions
diff --git a/Documentation/DMA-API-HOWTO.txt b/Documentation/DMA-API-HOWTO.txt
index a0b6250..4a4fb29 100644
--- a/Documentation/DMA-API-HOWTO.txt
+++ b/Documentation/DMA-API-HOWTO.txt
@@ -468,11 +468,46 @@ To map a single region, you do:
size_t size = buffer->len;
dma_handle = dma_map_single(dev, addr, size, direction);
+ if (dma_mapping_error(dma_handle)) {
+ /*
+ * reduce current DMA mapping usage,
+ * delay and try again later or
+ * reset driver.
+ */
+ goto map_error_handling;
+ }
and to unmap it:
dma_unmap_single(dev, dma_handle, size, direction);
+You should call dma_mapping_error() as dma_map_single() could fail and return
+error. Not all dma implementations support dma_mapping_error() interface.
+However, it is a good practice to call dma_mapping_error() interface, which
+will invoke the generic mapping error check interface. Doing so will ensure
+that the mapping code will work correctly on all dma implementations without
+any dependency on the specifics of the underlying implementation. Using the
+returned address without checking for errors could result in failures ranging
+from panics to silent data corruption. Couple of example of incorrect ways to
+check for errors that make assumptions about the underlying dma implementation
+are as follows and these are applicable to dma_map_page() as well.
+
+Incorrect example 1:
+ dma_addr_t dma_handle;
+
+ dma_handle = dma_map_single(dev, addr, size, direction);
+ if ((dma_handle & 0xffff != 0) || (dma_handle >= 0x1000000)) {
+ goto map_error;
+ }
+
+Incorrect example 2:
+ dma_addr_t dma_handle;
+
+ dma_handle = dma_map_single(dev, addr, size, direction);
+ if (dma_handle == DMA_ERROR_CODE) {
+ goto map_error;
+ }
+
You should call dma_unmap_single when the DMA activity is finished, e.g.
from the interrupt which told you that the DMA transfer is done.
@@ -489,6 +524,14 @@ Specifically:
size_t size = buffer->len;
dma_handle = dma_map_page(dev, page, offset, size, direction);
+ if (dma_mapping_error(dma_handle)) {
+ /*
+ * reduce current DMA mapping usage,
+ * delay and try again later or
+ * reset driver.
+ */
+ goto map_error_handling;
+ }
...
@@ -496,6 +539,12 @@ Specifically:
Here, "offset" means byte offset within the given page.
+You should call dma_mapping_error() as dma_map_page() could fail and return
+error as outlined under the dma_map_single() discussion.
+
+You should call dma_unmap_page when the DMA activity is finished, e.g.
+from the interrupt which told you that the DMA transfer is done.
+
With scatterlists, you map a region gathered from several regions by:
int i, count = dma_map_sg(dev, sglist, nents, direction);
@@ -578,6 +627,14 @@ to use the dma_sync_*() interfaces.
dma_addr_t mapping;
mapping = dma_map_single(cp->dev, buffer, len, DMA_FROM_DEVICE);
+ if (dma_mapping_error(dma_handle)) {
+ /*
+ * reduce current DMA mapping usage,
+ * delay and try again later or
+ * reset driver.
+ */
+ goto map_error_handling;
+ }
cp->rx_buf = buffer;
cp->rx_len = len;
@@ -658,6 +715,75 @@ failure can be determined by:
* delay and try again later or
* reset driver.
*/
+ goto map_error_handling;
+ }
+
+- unmap pages that are already mapped, when mapping error occurs in the middle
+ of a multiple page mapping attempt. These example are applicable to
+ dma_map_page() as well.
+
+Example 1:
+ dma_addr_t dma_handle1;
+ dma_addr_t dma_handle2;
+
+ dma_handle1 = dma_map_single(dev, addr, size, direction);
+ if (dma_mapping_error(dev, dma_handle1)) {
+ /*
+ * reduce current DMA mapping usage,
+ * delay and try again later or
+ * reset driver.
+ */
+ goto map_error_handling1;
+ }
+ dma_handle2 = dma_map_single(dev, addr, size, direction);
+ if (dma_mapping_error(dev, dma_handle2)) {
+ /*
+ * reduce current DMA mapping usage,
+ * delay and try again later or
+ * reset driver.
+ */
+ goto map_error_handling2;
+ }
+
+ ...
+
+ map_error_handling2:
+ dma_unmap_single(dma_handle1);
+ map_error_handling1:
+
+Example 2: (if buffers are allocated a loop, unmap all mapped buffers when
+ mapping error is detected in the middle)
+
+ dma_addr_t dma_addr;
+ dma_addr_t array[DMA_BUFFERS];
+ int save_index = 0;
+
+ for (i = 0; i < DMA_BUFFERS; i++) {
+
+ ...
+
+ dma_addr = dma_map_single(dev, addr, size, direction);
+ if (dma_mapping_error(dev, dma_addr)) {
+ /*
+ * reduce current DMA mapping usage,
+ * delay and try again later or
+ * reset driver.
+ */
+ goto map_error_handling;
+ }
+ array[i].dma_addr = dma_addr;
+ save_index++;
+ }
+
+ ...
+
+ map_error_handling:
+
+ for (i = 0; i < save_index; i++) {
+
+ ...
+
+ dma_unmap_single(array[i].dma_addr);
}
Networking drivers must call dev_kfree_skb to free the socket buffer