From 3ece7805105ef5967fd88547ac958c6d59329e9c Mon Sep 17 00:00:00 2001 From: Stefan Tatschner Date: Fri, 8 Dec 2017 01:12:09 +0100 Subject: Documentation/driver-api/usb: Replace dead link This link is dead: $ curl -vI http://usb.cs.tum.edu/usbdoc * Could not resolve host: usb.cs.tum.edu * Closing connection 0 curl: (6) Could not resolve host: usb.cs.tum.edu I found the document somewhere else. Let's replace it. Signed-off-by: Stefan Tatschner Signed-off-by: Jonathan Corbet --- Documentation/driver-api/usb/writing_usb_driver.rst | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'Documentation/driver-api') diff --git a/Documentation/driver-api/usb/writing_usb_driver.rst b/Documentation/driver-api/usb/writing_usb_driver.rst index 69f077dcdb78..4fe1c06b6a13 100644 --- a/Documentation/driver-api/usb/writing_usb_driver.rst +++ b/Documentation/driver-api/usb/writing_usb_driver.rst @@ -321,6 +321,6 @@ linux-usb-devel Mailing List Archives: http://marc.theaimsgroup.com/?l=linux-usb-devel Programming Guide for Linux USB Device Drivers: -http://usb.cs.tum.edu/usbdoc +http://lmu.web.psi.ch/docu/manuals/software_manuals/linux_sl/usb_linux_programming_guide.pdf USB Home Page: http://www.usb.org -- cgit v1.2.3 From b6e859f6cdd1686c021dcb3e18591d0b28ce8867 Mon Sep 17 00:00:00 2001 From: Elena Reshetova Date: Tue, 5 Dec 2017 12:46:35 +0200 Subject: docs: refcount_t documentation Some functions from refcount_t API provide different memory ordering guarantees that their atomic counterparts. This adds a document outlining these differences ( Documentation/core-api/refcount-vs-atomic.rst) as well as some other minor improvements. Signed-off-by: Elena Reshetova Signed-off-by: Kees Cook Signed-off-by: Jonathan Corbet --- Documentation/core-api/index.rst | 1 + Documentation/core-api/refcount-vs-atomic.rst | 150 ++++++++++++++++++++++++++ Documentation/driver-api/basics.rst | 21 ++-- include/linux/refcount.h | 2 +- 4 files changed, 167 insertions(+), 7 deletions(-) create mode 100644 Documentation/core-api/refcount-vs-atomic.rst (limited to 'Documentation/driver-api') diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst index d5bbe035316d..d4d54b05d6c5 100644 --- a/Documentation/core-api/index.rst +++ b/Documentation/core-api/index.rst @@ -14,6 +14,7 @@ Core utilities kernel-api assoc_array atomic_ops + refcount-vs-atomic cpu_hotplug local_ops workqueue diff --git a/Documentation/core-api/refcount-vs-atomic.rst b/Documentation/core-api/refcount-vs-atomic.rst new file mode 100644 index 000000000000..83351c258cdb --- /dev/null +++ b/Documentation/core-api/refcount-vs-atomic.rst @@ -0,0 +1,150 @@ +=================================== +refcount_t API compared to atomic_t +=================================== + +.. contents:: :local: + +Introduction +============ + +The goal of refcount_t API is to provide a minimal API for implementing +an object's reference counters. While a generic architecture-independent +implementation from lib/refcount.c uses atomic operations underneath, +there are a number of differences between some of the ``refcount_*()`` and +``atomic_*()`` functions with regards to the memory ordering guarantees. +This document outlines the differences and provides respective examples +in order to help maintainers validate their code against the change in +these memory ordering guarantees. + +The terms used through this document try to follow the formal LKMM defined in +github.com/aparri/memory-model/blob/master/Documentation/explanation.txt + +memory-barriers.txt and atomic_t.txt provide more background to the +memory ordering in general and for atomic operations specifically. + +Relevant types of memory ordering +================================= + +.. note:: The following section only covers some of the memory + ordering types that are relevant for the atomics and reference + counters and used through this document. For a much broader picture + please consult memory-barriers.txt document. + +In the absence of any memory ordering guarantees (i.e. fully unordered) +atomics & refcounters only provide atomicity and +program order (po) relation (on the same CPU). It guarantees that +each ``atomic_*()`` and ``refcount_*()`` operation is atomic and instructions +are executed in program order on a single CPU. +This is implemented using :c:func:`READ_ONCE`/:c:func:`WRITE_ONCE` and +compare-and-swap primitives. + +A strong (full) memory ordering guarantees that all prior loads and +stores (all po-earlier instructions) on the same CPU are completed +before any po-later instruction is executed on the same CPU. +It also guarantees that all po-earlier stores on the same CPU +and all propagated stores from other CPUs must propagate to all +other CPUs before any po-later instruction is executed on the original +CPU (A-cumulative property). This is implemented using :c:func:`smp_mb`. + +A RELEASE memory ordering guarantees that all prior loads and +stores (all po-earlier instructions) on the same CPU are completed +before the operation. It also guarantees that all po-earlier +stores on the same CPU and all propagated stores from other CPUs +must propagate to all other CPUs before the release operation +(A-cumulative property). This is implemented using +:c:func:`smp_store_release`. + +A control dependency (on success) for refcounters guarantees that +if a reference for an object was successfully obtained (reference +counter increment or addition happened, function returned true), +then further stores are ordered against this operation. +Control dependency on stores are not implemented using any explicit +barriers, but rely on CPU not to speculate on stores. This is only +a single CPU relation and provides no guarantees for other CPUs. + + +Comparison of functions +======================= + +case 1) - non-"Read/Modify/Write" (RMW) ops +------------------------------------------- + +Function changes: + + * :c:func:`atomic_set` --> :c:func:`refcount_set` + * :c:func:`atomic_read` --> :c:func:`refcount_read` + +Memory ordering guarantee changes: + + * none (both fully unordered) + + +case 2) - increment-based ops that return no value +-------------------------------------------------- + +Function changes: + + * :c:func:`atomic_inc` --> :c:func:`refcount_inc` + * :c:func:`atomic_add` --> :c:func:`refcount_add` + +Memory ordering guarantee changes: + + * none (both fully unordered) + +case 3) - decrement-based RMW ops that return no value +------------------------------------------------------ + +Function changes: + + * :c:func:`atomic_dec` --> :c:func:`refcount_dec` + +Memory ordering guarantee changes: + + * fully unordered --> RELEASE ordering + + +case 4) - increment-based RMW ops that return a value +----------------------------------------------------- + +Function changes: + + * :c:func:`atomic_inc_not_zero` --> :c:func:`refcount_inc_not_zero` + * no atomic counterpart --> :c:func:`refcount_add_not_zero` + +Memory ordering guarantees changes: + + * fully ordered --> control dependency on success for stores + +.. note:: We really assume here that necessary ordering is provided as a + result of obtaining pointer to the object! + + +case 5) - decrement-based RMW ops that return a value +----------------------------------------------------- + +Function changes: + + * :c:func:`atomic_dec_and_test` --> :c:func:`refcount_dec_and_test` + * :c:func:`atomic_sub_and_test` --> :c:func:`refcount_sub_and_test` + * no atomic counterpart --> :c:func:`refcount_dec_if_one` + * ``atomic_add_unless(&var, -1, 1)`` --> ``refcount_dec_not_one(&var)`` + +Memory ordering guarantees changes: + + * fully ordered --> RELEASE ordering + control dependency + +.. note:: :c:func:`atomic_add_unless` only provides full order on success. + + +case 6) - lock-based RMW +------------------------ + +Function changes: + + * :c:func:`atomic_dec_and_lock` --> :c:func:`refcount_dec_and_lock` + * :c:func:`atomic_dec_and_mutex_lock` --> :c:func:`refcount_dec_and_mutex_lock` + +Memory ordering guarantees changes: + + * fully ordered --> RELEASE ordering + control dependency + hold + :c:func:`spin_lock` on success diff --git a/Documentation/driver-api/basics.rst b/Documentation/driver-api/basics.rst index 73fa7d42bbba..826e85d50a16 100644 --- a/Documentation/driver-api/basics.rst +++ b/Documentation/driver-api/basics.rst @@ -13,12 +13,6 @@ Driver device table .. kernel-doc:: include/linux/mod_devicetable.h :internal: -Atomic and pointer manipulation -------------------------------- - -.. kernel-doc:: arch/x86/include/asm/atomic.h - :internal: - Delaying, scheduling, and timer routines ---------------------------------------- @@ -85,6 +79,21 @@ Internal Functions .. kernel-doc:: kernel/kthread.c :export: +Reference counting +------------------ + +.. kernel-doc:: include/linux/refcount.h + :internal: + +.. kernel-doc:: lib/refcount.c + :export: + +Atomics +------- + +.. kernel-doc:: arch/x86/include/asm/atomic.h + :internal: + Kernel objects manipulation --------------------------- diff --git a/include/linux/refcount.h b/include/linux/refcount.h index e8286585e149..4193c41e383a 100644 --- a/include/linux/refcount.h +++ b/include/linux/refcount.h @@ -8,7 +8,7 @@ #include /** - * refcount_t - variant of atomic_t specialized for reference counts + * struct refcount_t - variant of atomic_t specialized for reference counts * @refs: atomic_t counter field * * The counter saturates at UINT_MAX and will not move once -- cgit v1.2.3 From 90b2a01e68167665a3f86c477414e795cd4a0139 Mon Sep 17 00:00:00 2001 From: Lu Baolu Date: Wed, 13 Dec 2017 13:07:44 +0800 Subject: usb: doc: Update document for USB3 debug port usage Update Documentation/driver-api/usb/usb3-debug-port.rst. This update includes the guide for using xHCI debug capability based TTY serial link. Cc: Mathias Nyman Cc: Greg Kroah-Hartman Signed-off-by: Lu Baolu Signed-off-by: Jonathan Corbet --- Documentation/driver-api/usb/usb3-debug-port.rst | 52 ++++++++++++++++++++++++ 1 file changed, 52 insertions(+) (limited to 'Documentation/driver-api') diff --git a/Documentation/driver-api/usb/usb3-debug-port.rst b/Documentation/driver-api/usb/usb3-debug-port.rst index feb1a36a65b7..b9fd131f4723 100644 --- a/Documentation/driver-api/usb/usb3-debug-port.rst +++ b/Documentation/driver-api/usb/usb3-debug-port.rst @@ -98,3 +98,55 @@ you to check the sanity of the setup. cat /dev/ttyUSB0 done ===== end of bash scripts =============== + +Serial TTY +========== + +The DbC support has been added to the xHCI driver. You can get a +debug device provided by the DbC at runtime. + +In order to use this, you need to make sure your kernel has been +configured to support USB_XHCI_DBGCAP. A sysfs attribute under +the xHCI device node is used to enable or disable DbC. By default, +DbC is disabled:: + + root@target:/sys/bus/pci/devices/0000:00:14.0# cat dbc + disabled + +Enable DbC with the following command:: + + root@target:/sys/bus/pci/devices/0000:00:14.0# echo enable > dbc + +You can check the DbC state at anytime:: + + root@target:/sys/bus/pci/devices/0000:00:14.0# cat dbc + enabled + +Connect the debug target to the debug host with a USB 3.0 super- +speed A-to-A debugging cable. You can see /dev/ttyDBC0 created +on the debug target. You will see below kernel message lines:: + + root@target: tail -f /var/log/kern.log + [ 182.730103] xhci_hcd 0000:00:14.0: DbC connected + [ 191.169420] xhci_hcd 0000:00:14.0: DbC configured + [ 191.169597] xhci_hcd 0000:00:14.0: DbC now attached to /dev/ttyDBC0 + +Accordingly, the DbC state has been brought up to:: + + root@target:/sys/bus/pci/devices/0000:00:14.0# cat dbc + configured + +On the debug host, you will see the debug device has been enumerated. +You will see below kernel message lines:: + + root@host: tail -f /var/log/kern.log + [ 79.454780] usb 2-2.1: new SuperSpeed USB device number 3 using xhci_hcd + [ 79.475003] usb 2-2.1: LPM exit latency is zeroed, disabling LPM. + [ 79.475389] usb 2-2.1: New USB device found, idVendor=1d6b, idProduct=0010 + [ 79.475390] usb 2-2.1: New USB device strings: Mfr=1, Product=2, SerialNumber=3 + [ 79.475391] usb 2-2.1: Product: Linux USB Debug Target + [ 79.475392] usb 2-2.1: Manufacturer: Linux Foundation + [ 79.475393] usb 2-2.1: SerialNumber: 0001 + +The debug device works now. You can use any communication or debugging +program to talk between the host and the target. -- cgit v1.2.3