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Date: Fri, 9 Jan 2009 17:19:30 +0100 From: Joerg Roedel <joerg.roedel@....com> To: linux-kernel@...r.kernel.org CC: mingo@...hat.com, dwmw2@...radead.org, fujita.tomonori@....ntt.co.jp, netdev@...r.kernel.org, iommu@...ts.linux-foundation.org, Joerg Roedel <joerg.roedel@....com> Subject: [PATCH 16/16] dma-debug: Documentation update Impact: add documentation about DMA-API debugging to DMA-API.txt Signed-off-by: Joerg Roedel <joerg.roedel@....com> --- Documentation/DMA-API.txt | 117 +++++++++++++++++++++++++++++++++++++++++++++ 1 files changed, 117 insertions(+), 0 deletions(-) diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt index b462bb1..e36e85a 100644 --- a/Documentation/DMA-API.txt +++ b/Documentation/DMA-API.txt @@ -610,3 +610,120 @@ size is the size (and should be a page-sized multiple). The return value will be either a pointer to the processor virtual address of the memory, or an error (via PTR_ERR()) if any part of the region is occupied. + +Part III - Debug drivers use of the DMA-API +------------------------------------------- + +The DMA-API as described above as some constraints. DMA addresses must be +released with the corresponding function with the same size for example. With +the advent of hardware IOMMUs it becomes more and more important that drivers +do not violate those constraints. In the worst case such a violation can +result in data corruption up to destroyed filesystems. + +To debug drivers and find bugs in the usage of the DMA-API checking code can +be compiled into the kernel which will tell the developer about those +violations. If your architecture supports it you can select the "Enable +debugging of DMA-API usage" option in your kernel configuration. Enabling this +option has a performance impact. Do not enable it in production kernels. + +If you boot the resulting kernel will contain code which does some bookkeeping +about what DMA memory was allocated for which device. If this code detects an +error it prints a warning message with some details into your kernel log. An +example warning message may look like this: + +------------[ cut here ]------------ +WARNING: at /data2/repos/linux.trees.git/lib/dma-debug.c:231 + check_unmap+0xab/0x3d9() +Hardware name: Toonie +bnx2 0000:01:00.0: DMA-API: device driver tries to free DMA + memory it has not allocated [device address=0x00000000011] +Modules linked in: +Pid: 0, comm: swapper Not tainted 2.6.28 #174 +Call Trace: + <IRQ> [<ffffffff8105af3a>] warn_slowpath+0xd3/0xf2 + [<ffffffff8107c36f>] ? find_usage_backwards+0xe2/0x116 + [<ffffffff8107c36f>] ? find_usage_backwards+0xe2/0x116 + [<ffffffff812efd16>] ? usb_hcd_link_urb_to_ep+0x94/0xa0 + [<ffffffff8107c52b>] ? mark_lock+0x1c/0x364 + [<ffffffff8107d8f7>] ? __lock_acquire+0xaec/0xb55 + [<ffffffff8107c52b>] ? mark_lock+0x1c/0x364 + [<ffffffff811e2b4b>] ? get_hash_bucket+0x28/0x33 + [<ffffffff814b25a5>] ? _spin_lock_irqsave+0x69/0x75 + [<ffffffff811e2b4b>] ? get_hash_bucket+0x28/0x33 + [<ffffffff811e2ff2>] check_unmap+0xab/0x3d9 + [<ffffffff8107c9ed>] ? trace_hardirqs_on_caller+0x108/0x14a + [<ffffffff8107ca3c>] ? trace_hardirqs_on+0xd/0xf + [<ffffffff811e3433>] debug_unmap_single+0x3e/0x40 + [<ffffffff8128d2d8>] dma_unmap_single+0x3d/0x60 + [<ffffffff8128d335>] pci_unmap_page+0x1c/0x1e + [<ffffffff81290759>] bnx2_poll_work+0x626/0x8cb + [<ffffffff8107d8f7>] ? __lock_acquire+0xaec/0xb55 + [<ffffffff81070100>] ? run_posix_cpu_timers+0x49c/0x603 + [<ffffffff81070000>] ? run_posix_cpu_timers+0x39c/0x603 + [<ffffffff8107c52b>] ? mark_lock+0x1c/0x364 + [<ffffffff8107d8f7>] ? __lock_acquire+0xaec/0xb55 + [<ffffffff81292804>] bnx2_poll_msix+0x33/0x81 + [<ffffffff813b6478>] net_rx_action+0x8a/0x139 + [<ffffffff8105ff39>] __do_softirq+0x8b/0x147 + [<ffffffff8102933c>] call_softirq+0x1c/0x34 + [<ffffffff8102a611>] do_softirq+0x39/0x90 + [<ffffffff8105fde8>] irq_exit+0x4e/0x98 + [<ffffffff8102a5c2>] do_IRQ+0x11f/0x135 + [<ffffffff81028b93>] ret_from_intr+0x0/0xf + <EOI> <4>---[ end trace 4339d58302097423 ]--- + +The driver developer can find the driver and the device including a stacktrace +of the DMA-API call which caused this warning. + +Per default only the first error will result in a warning message. All other +errors will only silently counted. This limitation exist to prevent the code +from flooding your kernel log. To support debugging a device driver this can +be disabled via debugfs. See the debugfs interface documentation below for +details. + +The debugfs directory for the DMA-API debugging code is called dma-api/. In +this directory the following files can currently be found: + + dma-api/all_errors This file contains a numeric value. If this + value is not equal to zero the debugging code + will print a warning for every error it finds + into the kernel log. Be carefull with this + option. It can easily flood your logs. + + dma-api/disabled This read-only file contains the character 'Y' + if the debugging code is disabled. This can + happen when it runs out of memory or if it was + disabled at boot time + + dma-api/error_count This file is read-only and shows the total + numbers of errors found. + + dma-api/num_errors The number in this file shows how many + warnings will be printed to the kernel log + before it stops. This number is initialized to + one at system boot and be set by writing into + this file + + 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. + + dma-api/num_free_entries + The current number of free dma_debug_entries + in the allocator. + +If you have this code compiled into your kernel it will be enabled by default. +If you want to boot without the bookkeeping anyway you can provide +'dma_debug=off' as a boot parameter. This will disable DMA-API debugging. +Notice that you can not enable it again at runtime. You have to reboot to do +so. + +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. + -- 1.5.6.4 -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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