path: root/Documentation/DMA-API.txt
diff options
authorRobin Murphy <robin.murphy@arm.com>2018-12-10 14:00:29 (GMT)
committerChristoph Hellwig <hch@lst.de>2018-12-11 13:31:17 (GMT)
commit2b9d9ac02b9d8d32c515c82bb17401c429f160ab (patch)
tree8f224b9fe62c452686c7b781f8a742c78e619605 /Documentation/DMA-API.txt
parentf737b095c60c635db260e02fdb9f0efb9f3360c4 (diff)
dma-debug: Dynamically expand the dma_debug_entry pool
Certain drivers such as large multi-queue network adapters can use pools of mapped DMA buffers larger than the default dma_debug_entry pool of 65536 entries, with the result that merely probing such a device can cause DMA debug to disable itself during boot unless explicitly given an appropriate "dma_debug_entries=..." option. Developers trying to debug some other driver on such a system may not be immediately aware of this, and at worst it can hide bugs if they fail to realise that dma-debug has already disabled itself unexpectedly by the time their code of interest gets to run. Even once they do realise, it can be a bit of a pain to emprirically determine a suitable number of preallocated entries to configure, short of massively over-allocating. There's really no need for such a static limit, though, since we can quite easily expand the pool at runtime in those rare cases that the preallocated entries are insufficient, which is arguably the least surprising and most useful behaviour. To that end, refactor the prealloc_memory() logic a little bit to generalise it for runtime reallocations as well. 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, 5 insertions, 6 deletions
diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt
index 6bdb095..0fcb756 100644
--- a/Documentation/DMA-API.txt
+++ b/Documentation/DMA-API.txt
@@ -717,8 +717,8 @@ dma-api/num_errors The number in this file shows how many
dma-api/min_free_entries This read-only file can be read to get the
minimum number of free dma_debug_entries the
allocator has ever seen. If this value goes
- down to zero the code will disable itself
- because it is not longer reliable.
+ down to zero the code will attempt to increase
+ nr_total_entries to compensate.
dma-api/num_free_entries The current number of free dma_debug_entries
in the allocator.
@@ -745,10 +745,9 @@ driver filter at boot time. The debug code will only print errors for that
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. These entries are preallocated at boot. The number
-of preallocated entries is defined per architecture. If it is too low for you
-boot with 'dma_debug_entries=<your_desired_number>' to overwrite the
-architectural default.
+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.