ARM: Ensure PTE modifications via dma_alloc_coherent are visible
authorRussell King <rmk+kernel@arm.linux.org.uk>
Wed, 8 Sep 2010 15:27:56 +0000 (16:27 +0100)
committerRussell King <rmk+kernel@arm.linux.org.uk>
Wed, 8 Sep 2010 15:27:56 +0000 (16:27 +0100)
commit2be23c475af8ae4e25f8bab08d815b17593bd547
tree864cf9dca5faaa51c9346a8835c8f101bd5c8fa2
parenta387f0f5409276c5cf75eecb61ef6e6896f851b4
ARM: Ensure PTE modifications via dma_alloc_coherent are visible

Dave Hylands reports:
| We've observed a problem with dma_alloc_writecombine when the system
| is under heavy load (heavy bus traffic).  We've managed to reduce the
| problem to the following snippet, which is run from a kthread in a
| continuous loop:
|
|   void *virtAddr;
|   dma_addr_t physAddr;
|   unsigned int numBytes = 256;
|
|   for (;;) {
|       virtAddr = dma_alloc_writecombine(NULL,
|             numBytes, &physAddr, GFP_KERNEL);
|       if (virtAddr == NULL) {
|          printk(KERN_ERR "Running out of memory\n");
|          break;
|       }
|
|       /* access DMA memory allocated */
|       tmp = virtAddr;
|       *tmp = 0x77;
|
|       /* free DMA memory */
|       dma_free_writecombine(NULL,
|             numBytes, virtAddr, physAddr);
|
|         ...sleep here...
|     }
|
| By itself, the code will run forever with no issues. However, as we
| increase our bus traffic (typically using DMA) then the *tmp = 0x77
| line will eventually cause a page fault. If we add a small delay (a
| few microseconds) before the *tmp = 0x77, then we don't see a page
| fault, even under heavy load.

A dsb() is required after modifying the PTE entries to ensure that they
will always be visible.  Add this dsb().

Reported-by: Dave Hylands <dhylands@gmail.com>
Tested-by: Dave Hylands <dhylands@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
arch/arm/mm/dma-mapping.c