Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757458AbZASOtN (ORCPT ); Mon, 19 Jan 2009 09:49:13 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752434AbZASOrV (ORCPT ); Mon, 19 Jan 2009 09:47:21 -0500 Received: from outbound-va3.frontbridge.com ([216.32.180.16]:54158 "EHLO VA3EHSOBE005.bigfish.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751740AbZASOrR (ORCPT ); Mon, 19 Jan 2009 09:47:17 -0500 X-BigFish: VPS-1(z5edJzzzzzz32i65h) X-Spam-TCS-SCL: 4:0 X-FB-SS: 5, X-WSS-ID: 0KDQ3QE-02-0BB-01 From: Joerg Roedel To: mingo@redhat.com, linux-kernel@vger.kernel.org CC: fujita.tomonori@lab.ntt.co.jp, dwmw2@infradead.org, iommu@lists.linux-foundation.org, Joerg Roedel Subject: [PATCH 16/16] dma-debug: Documentation update Date: Mon, 19 Jan 2009 15:47:03 +0100 Message-ID: <1232376423-11067-17-git-send-email-joerg.roedel@amd.com> X-Mailer: git-send-email 1.5.6.4 In-Reply-To: <1232376423-11067-1-git-send-email-joerg.roedel@amd.com> References: <1232376423-11067-1-git-send-email-joerg.roedel@amd.com> X-OriginalArrivalTime: 19 Jan 2009 14:47:04.0172 (UTC) FILETIME=[CB2AE2C0:01C97A44] MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5892 Lines: 130 Impact: add documentation about DMA-API debugging to DMA-API.txt Signed-off-by: Joerg Roedel --- Documentation/DMA-API.txt | 106 +++++++++++++++++++++++++++++++++++++++++++++ 1 files changed, 106 insertions(+), 0 deletions(-) diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt index 5244169..dca8be3 100644 --- a/Documentation/DMA-API.txt +++ b/Documentation/DMA-API.txt @@ -609,3 +609,109 @@ 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-2.6-iommu/lib/dma-debug.c:448 + check_unmap+0x203/0x490() +Hardware name: +forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong + function [device address=0x00000000640444be] [size=66 bytes] [mapped as +single] [unmapped as page] +Modules linked in: nfsd exportfs bridge stp llc r8169 +Pid: 0, comm: swapper Tainted: G W 2.6.28-dmatest-09289-g8bb99c0 #1 +Call Trace: + [] warn_slowpath+0xf2/0x130 + [] _spin_unlock+0x10/0x30 + [] usb_hcd_link_urb_to_ep+0x75/0xc0 + [] _spin_unlock_irqrestore+0x12/0x40 + [] ohci_urb_enqueue+0x19f/0x7c0 + [] queue_work+0x56/0x60 + [] enqueue_task_fair+0x20/0x50 + [] usb_hcd_submit_urb+0x379/0xbc0 + [] cpumask_next_and+0x23/0x40 + [] find_busiest_group+0x207/0x8a0 + [] _spin_lock_irqsave+0x1f/0x50 + [] check_unmap+0x203/0x490 + [] debug_dma_unmap_page+0x49/0x50 + [] nv_tx_done_optimized+0xc6/0x2c0 + [] nv_nic_irq_optimized+0x73/0x2b0 + [] handle_IRQ_event+0x34/0x70 + [] handle_edge_irq+0xc9/0x150 + [] do_IRQ+0xcb/0x1c0 + [] ret_from_intr+0x0/0xa + <4>---[ end trace f6435a98e2a38c0e ]--- + +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=' to overwrite the +architectural default. -- 1.5.6.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/