dma-debug: Documentation update
Impact: add documentation about DMA-API debugging to DMA-API.txt Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
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Joerg Roedel
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2118d0c548
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@ -609,3 +609,109 @@ size is the size (and should be a page-sized multiple).
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The return value will be either a pointer to the processor virtual
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address of the memory, or an error (via PTR_ERR()) if any part of the
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region is occupied.
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Part III - Debug drivers use of the DMA-API
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-------------------------------------------
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The DMA-API as described above as some constraints. DMA addresses must be
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released with the corresponding function with the same size for example. With
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the advent of hardware IOMMUs it becomes more and more important that drivers
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do not violate those constraints. In the worst case such a violation can
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result in data corruption up to destroyed filesystems.
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To debug drivers and find bugs in the usage of the DMA-API checking code can
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be compiled into the kernel which will tell the developer about those
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violations. If your architecture supports it you can select the "Enable
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debugging of DMA-API usage" option in your kernel configuration. Enabling this
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option has a performance impact. Do not enable it in production kernels.
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If you boot the resulting kernel will contain code which does some bookkeeping
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about what DMA memory was allocated for which device. If this code detects an
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error it prints a warning message with some details into your kernel log. An
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example warning message may look like this:
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------------[ cut here ]------------
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WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448
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check_unmap+0x203/0x490()
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Hardware name:
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forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong
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function [device address=0x00000000640444be] [size=66 bytes] [mapped as
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single] [unmapped as page]
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Modules linked in: nfsd exportfs bridge stp llc r8169
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Pid: 0, comm: swapper Tainted: G W 2.6.28-dmatest-09289-g8bb99c0 #1
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Call Trace:
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<IRQ> [<ffffffff80240b22>] warn_slowpath+0xf2/0x130
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[<ffffffff80647b70>] _spin_unlock+0x10/0x30
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[<ffffffff80537e75>] usb_hcd_link_urb_to_ep+0x75/0xc0
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[<ffffffff80647c22>] _spin_unlock_irqrestore+0x12/0x40
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[<ffffffff8055347f>] ohci_urb_enqueue+0x19f/0x7c0
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[<ffffffff80252f96>] queue_work+0x56/0x60
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[<ffffffff80237e10>] enqueue_task_fair+0x20/0x50
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[<ffffffff80539279>] usb_hcd_submit_urb+0x379/0xbc0
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[<ffffffff803b78c3>] cpumask_next_and+0x23/0x40
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[<ffffffff80235177>] find_busiest_group+0x207/0x8a0
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[<ffffffff8064784f>] _spin_lock_irqsave+0x1f/0x50
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[<ffffffff803c7ea3>] check_unmap+0x203/0x490
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[<ffffffff803c8259>] debug_dma_unmap_page+0x49/0x50
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[<ffffffff80485f26>] nv_tx_done_optimized+0xc6/0x2c0
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[<ffffffff80486c13>] nv_nic_irq_optimized+0x73/0x2b0
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[<ffffffff8026df84>] handle_IRQ_event+0x34/0x70
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[<ffffffff8026ffe9>] handle_edge_irq+0xc9/0x150
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[<ffffffff8020e3ab>] do_IRQ+0xcb/0x1c0
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[<ffffffff8020c093>] ret_from_intr+0x0/0xa
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<EOI> <4>---[ end trace f6435a98e2a38c0e ]---
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The driver developer can find the driver and the device including a stacktrace
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of the DMA-API call which caused this warning.
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Per default only the first error will result in a warning message. All other
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errors will only silently counted. This limitation exist to prevent the code
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from flooding your kernel log. To support debugging a device driver this can
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be disabled via debugfs. See the debugfs interface documentation below for
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details.
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The debugfs directory for the DMA-API debugging code is called dma-api/. In
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this directory the following files can currently be found:
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dma-api/all_errors This file contains a numeric value. If this
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value is not equal to zero the debugging code
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will print a warning for every error it finds
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into the kernel log. Be carefull with this
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option. It can easily flood your logs.
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dma-api/disabled This read-only file contains the character 'Y'
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if the debugging code is disabled. This can
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happen when it runs out of memory or if it was
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disabled at boot time
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dma-api/error_count This file is read-only and shows the total
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numbers of errors found.
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dma-api/num_errors The number in this file shows how many
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warnings will be printed to the kernel log
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before it stops. This number is initialized to
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one at system boot and be set by writing into
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this file
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dma-api/min_free_entries
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This read-only file can be read to get the
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minimum number of free dma_debug_entries the
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allocator has ever seen. If this value goes
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down to zero the code will disable itself
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because it is not longer reliable.
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dma-api/num_free_entries
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The current number of free dma_debug_entries
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in the allocator.
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If you have this code compiled into your kernel it will be enabled by default.
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If you want to boot without the bookkeeping anyway you can provide
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'dma_debug=off' as a boot parameter. This will disable DMA-API debugging.
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Notice that you can not enable it again at runtime. You have to reboot to do
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so.
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When the code disables itself at runtime this is most likely because it ran
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out of dma_debug_entries. These entries are preallocated at boot. The number
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of preallocated entries is defined per architecture. If it is too low for you
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boot with 'dma_debug_entries=<your_desired_number>' to overwrite the
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architectural default.
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