path: root/Documentation/DMA-API.txt
diff options
authorRobin Murphy <robin.murphy@arm.com>2018-12-10 14:00:33 (GMT)
committerChristoph Hellwig <hch@lst.de>2018-12-11 13:32:13 (GMT)
commitad78dee0b630527bdfed809d1f5ed95c601886ae (patch)
tree5f71279e0474d43da345329c8a51def891a007af /Documentation/DMA-API.txt
parent0cb0e25e421436a83ee39857923e4213b983e463 (diff)
dma-debug: Batch dma_debug_entry allocation
DMA debug entries are one of those things which aren't that useful individually - we will always want some larger quantity of them - and which we don't really need to manage the exact number of - we only care about having 'enough'. In that regard, the current behaviour of creating them one-by-one leads to a lot of unwarranted function call overhead and memory wasted on alignment padding. Now that we don't have to worry about freeing anything via dma_debug_resize_entries(), we can optimise the allocation behaviour by grabbing whole pages at once, which will save considerably on the aforementioned overheads, and probably offer a little more cache/TLB locality benefit for traversing the lists under normal operation. This should also give even less reason for an architecture-level override of the preallocation size, so make the definition unconditional - if there is still any desire to change the compile-time value for some platforms it would be better off as a Kconfig option anyway. Since freeing a whole page of entries at once becomes enough of a challenge that it's not really worth complicating dma_debug_init(), we may as well tweak the preallocation behaviour such that as long as we manage to allocate *some* pages, we can leave debugging enabled on a best-effort basis rather than otherwise wasting them. Signed-off-by: Robin Murphy <robin.murphy@arm.com> Tested-by: Qian Cai <cai@lca.pw> Signed-off-by: Christoph Hellwig <hch@lst.de>
Diffstat (limited to 'Documentation/DMA-API.txt')
1 files changed, 3 insertions, 1 deletions
diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt
index 7a7d8a4..016eb69 100644
--- a/Documentation/DMA-API.txt
+++ b/Documentation/DMA-API.txt
@@ -747,7 +747,9 @@ driver afterwards. This filter can be disabled or changed later using debugfs.
When the code disables itself at runtime this is most likely because it ran
out of dma_debug_entries and was unable to allocate more on-demand. 65536
entries are preallocated at boot - if this is too low for you boot with
-'dma_debug_entries=<your_desired_number>' to overwrite the default. The
+'dma_debug_entries=<your_desired_number>' to overwrite the default. Note
+that the code allocates entries in batches, so the exact number of
+preallocated entries may be greater than the actual number requested. The
code will print to the kernel log each time it has dynamically allocated
as many entries as were initially preallocated. This is to indicate that a
larger preallocation size may be appropriate, or if it happens continually