From 034dd340b08be1f2f0477ad16131d609f9dbd53c Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sun, 30 Aug 2026 09:22:00 -0700 Subject: Merge tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace Pull tracing fixes from Steven Rostedt: - Fix error output of boot instance creation failure Currently if a boot instance creation fails, instead of printing out the name of the instance that failed, it prints "(null)". That is because it prints "cur_str" that had already been processed by strsep(). Print the saved name instead. While at it, print the error code of the failure. - Fix use-after-free for same named historgrams Histograms can be named so that they can be used in multiple events. But if the named histogram has a variable attached, the second event that uses the named histogram which duplicates it and needs to free the original after duplication leaves the old variable in place and still visible. If another histogram uses than variable, it will use the stale one which will try to reference the freed duplicate histogram and crash the kernel. Free the duplicate variables along with the duplicated histogram data. - Check return value of kthread_run() in event self test The events self tests uses a kthread for testing but does not check if it succeeded in creating a kthread. If the kthread creation were to fail, the code will still try to call kthread_stop() on the error returned. - Fix race between reading trace_pipe and updating subbuffer size If a user is reading the trace_pipe file at the same time they update the ring buffer sub-buffer size, can cause the trace_pipe read to read stale data. Add trace_access_lock() around updating the ring buffer sub-buffer size. - Fix eventfs_inode on failure path in creation of the events directory In the creation of the "events" directory, if after allocating the eventfs_inode a failure is detected, it calls cleanup_ei() which calls free_ei(). The free_ei() will test if eventfs_inode being freed has no children. It is a bug if it does. But on the failure case of the creation of the "events" directory, the children lists have not yet been initialized and the free will trigger a warning because list_empty() on an uninitialized list returns false. Move the initialization into init_ei() where it makes more sense and makes sure that a created eventfs_inode has its lists initialized upon creation. - Check return value of kthread_run() in ftrace direct sample code The sample code that shows how to use the ftrace direct calls does not test the return of kthread_run() to see if it succeeds. Return a failure if the kthread_run() doesn't succeed. - Clear user events state on fork in case of alloc failure On fork, the child gets a pointer to the parent's user events state. It makes a copy of it then updates the child's pointer to it. But if the allocation fails, the duplication function leaves the child with a pointer to its parent's descriptor. When the child cleans up its data, it will free the parent's descriptor while the parent is still using it. In the duplication function, set the child's user_event_mm to NULL before testing if the allocation succeeded, and when it exits it will not free the parent's descriptor. - Fix retry exhaustion in simple ring buffer reader swap simple_ring_buffer_swap_reader_page() starts with retry set to 8 and post-decrements it only after a failed link replacement. On the final attempt, a successful replacement leaves retry at zero, while a failed replacement leaves it at -1. But the check for success expects the retry value to be non-zero and exits with an error on zero. This is the opposite result. Fix it. - Fail nicely when the remote swap_reader_page() returns an error Currently, if the swap_reader_page() of a remote buffer fails, it triggers a WARN_ON_ONCE() and continues normally. Instead, have it exit with an error and a pr_warn() print instead of a full WARNING. * tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace: ring-buffer: Stop remote reader update when page swap fails tracing: Fix retry exhaustion in simple ring buffer reader swap tracing/user_events: Clear copied tracing state before fork duplication samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-multi-modify samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-modify eventfs: Initialize ei->children and ei->list in init_ei() tracing: Fix use-after-free in trace_pipe read on sub-buffer order change tracing: Fix crash passing ERR_PTR to kthread_stop() tracing: Fix use-after-free with same-name named triggers tracing: Fix logged instance name on creation failure --- Documentation/i2c/summary.rst | 82 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 82 insertions(+) create mode 100644 Documentation/i2c/summary.rst (limited to 'Documentation/i2c/summary.rst') diff --git a/Documentation/i2c/summary.rst b/Documentation/i2c/summary.rst new file mode 100644 index 000000000..579a1c7df --- /dev/null +++ b/Documentation/i2c/summary.rst @@ -0,0 +1,82 @@ +============================= +Introduction to I2C and SMBus +============================= + +I²C (pronounce: I squared C and written I2C in the kernel documentation) is +a protocol developed by Philips. It is a two-wire protocol with variable +speed (typically up to 400 kHz, high speed modes up to 5 MHz). It provides +an inexpensive bus for connecting many types of devices with infrequent or +low bandwidth communications needs. I2C is widely used with embedded +systems. Some systems use variants that don't meet branding requirements, +and so are not advertised as being I2C but come under different names, +e.g. TWI (Two Wire Interface), IIC. + +The latest official I2C specification is the `"I²C-bus specification and user +manual" (UM10204) `_ +published by NXP Semiconductors, version 7 as of this writing. + +SMBus (System Management Bus) is based on the I2C protocol, and is mostly +a subset of I2C protocols and signaling. Many I2C devices will work on an +SMBus, but some SMBus protocols add semantics beyond what is required to +achieve I2C branding. Modern PC mainboards rely on SMBus. The most common +devices connected through SMBus are RAM modules configured using I2C EEPROMs, +and hardware monitoring chips. + +Because the SMBus is mostly a subset of the generalized I2C bus, we can +use its protocols on many I2C systems. However, there are systems that don't +meet both SMBus and I2C electrical constraints; and others which can't +implement all the common SMBus protocol semantics or messages. + + +Terminology +=========== + +The I2C bus connects one or more controller chips and one or more target chips. + +.. kernel-figure:: i2c_bus.svg + :alt: Simple I2C bus with one controller and 3 targets + + Simple I2C bus + +A **controller** chip is a node that starts communications with targets. In the +Linux kernel implementation it is also called an "adapter" or "bus". Controller +drivers are usually in the ``drivers/i2c/busses/`` subdirectory. + +An **algorithm** contains general code that can be used to implement a whole +class of I2C controllers. Each specific controller driver either depends on an +algorithm driver in the ``drivers/i2c/algos/`` subdirectory, or includes its +own implementation. + +A **target** chip is a node that responds to communications when addressed by a +controller. In the Linux kernel implementation it is also called a "client". +While targets are usually separate external chips, Linux can also act as a +target (needs hardware support) and respond to another controller on the bus. +This is then called a **local target**. In contrast, an external chip is called +a **remote target**. + +Target drivers are kept in a directory specific to the feature they provide, +for example ``drivers/gpio/`` for GPIO expanders and ``drivers/media/i2c/`` for +video-related chips. + +For the example configuration in the figure above, you will need one driver for +the I2C controller, and drivers for your I2C targets. Usually one driver for +each target. + +Synonyms +-------- + +As mentioned above, the Linux I2C implementation historically uses the terms +"adapter" for controller and "client" for target. A number of data structures +have these synonyms in their name. So, when discussing implementation details, +you should be aware of these terms as well. The official wording is preferred, +though. + +Outdated terminology +-------------------- + +In earlier I2C specifications, controller was named "master" and target was +named "slave". These terms have been obsoleted with v7 of the specification and +their use is also discouraged by the Linux Kernel Code of Conduct. You may +still find them in references to documentation which has not been updated. The +general attitude, however, is to use the inclusive terms: controller and +target. Work to replace the old terminology in the Linux Kernel is on-going. -- cgit v1.3.1