From f4cdf7ca9a1fdcca413157df19753f388a5a224e Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Wed, 19 Aug 2026 10:09:22 -0700 Subject: Merge tag 'media/v7.3-1' of git://git.kernel.org/pub/scm/linux/kernel/git/mchehab/linux-media Pull media updates from Mauro Carvalho Chehab: - v4l2-core: added ISP statistics support and per-block validation - v4l2-core: Allow unknown HDR10 white point and luminance - New camera sensors: Sony IMX678 and IMX471m, Himax HM1092 IR sensor - New codec: Milos: VPU v2.0 codec support - isp driver: gained support for Dreamchip RPPX1 ISP framework - vsp1 driver: gained support for RZ/T2H and RZ/N2H - Novalake driver: gained CVS support for new NVL hardware - dvb-core: fix feed leak on failed DMX_ADD_PID - several driver fixes, cleanups and minor improvements * tag 'media/v7.3-1' of git://git.kernel.org/pub/scm/linux/kernel/git/mchehab/linux-media: (308 commits) media: ipu-bridge: check all DMI entries when overriding sensor rotation media: v4l2-async: avoid deleting unlinked ASC entry on link error media: rzg2l-cru: Align bytesperline to hardware DMA stride requirement media: intel/ipu6: fix async notifier cleanup leak on parse error media: staging/ipu7: fix async notifier UAF on probe error path media: amd: isp4: fix self-deadlock in isp4sd_pwron_and_init() error path media: amd: isp4: release partial allocations in isp4if_alloc_fw_gpumem() media: rcar-isp: Fix VSPX reference leaks media: rcar-isp: Release ISPCORE resources media: i2c: imx415: Release runtime PM reference on VBLANK error media: i2c: imx415: Return test pattern write errors media: renesas: vsp1: Declare index variables in for loop statement media: renesas: vsp1: Make reset control optional to support platforms without a reset line media: dt-bindings: media: renesas,vsp1: Document RZ/T2H and RZ/N2H SoCs media: dt-bindings: media: renesas,fcp: Document RZ/T2H and RZ/N2H SoCs media: nxp: imx8-isi: Add additional 32-bit RGB format support media: nxp: imx8-isi: Add 16-bit raw Bayer format support media: nxp: imx8-isi: Implement per-stream reference counting for multiplexed streams media: nxp: imx8-isi: Use BIT_ULL() for 64-bit stream masks media: nxp: imx8-isi: Correct color map between V4L2 and ISI ... --- Documentation/process/stable-api-nonsense.rst | 204 ++++++++++++++++++++++++++ 1 file changed, 204 insertions(+) create mode 100644 Documentation/process/stable-api-nonsense.rst (limited to 'Documentation/process/stable-api-nonsense.rst') diff --git a/Documentation/process/stable-api-nonsense.rst b/Documentation/process/stable-api-nonsense.rst new file mode 100644 index 000000000..a9625ab1f --- /dev/null +++ b/Documentation/process/stable-api-nonsense.rst @@ -0,0 +1,204 @@ +.. _stable_api_nonsense: + +The Linux Kernel Driver Interface +================================== + +(all of your questions answered and then some) + +Greg Kroah-Hartman + +This is being written to try to explain why Linux **does not have a binary +kernel interface, nor does it have a stable kernel interface**. + +.. note:: + + Please realize that this article describes the **in kernel** interfaces, not + the kernel to userspace interfaces. + + The kernel to userspace interface is the one that application programs use, + the syscall interface. That interface is **very** stable over time, and + will not break. I have old programs that were built on a pre 0.9something + kernel that still work just fine on the latest 2.6 kernel release. + That interface is the one that users and application programmers can count + on being stable. + + +Executive Summary +----------------- +You think you want a stable kernel interface, but you really do not, and +you don't even know it. What you want is a stable running driver, and +you get that only if your driver is in the main kernel tree. You also +get lots of other good benefits if your driver is in the main kernel +tree, all of which has made Linux into such a strong, stable, and mature +operating system which is the reason you are using it in the first +place. + + +Intro +----- + +It's only the odd person who wants to write a kernel driver that needs +to worry about the in-kernel interfaces changing. For the majority of +the world, they neither see this interface, nor do they care about it at +all. + +First off, I'm not going to address **any** legal issues about closed +source, hidden source, binary blobs, source wrappers, or any other term +that describes kernel drivers that do not have their source code +released under the GPL. Please consult a lawyer if you have any legal +questions, I'm a programmer and hence, I'm just going to be describing +the technical issues here (not to make light of the legal issues, they +are real, and you do need to be aware of them at all times.) + +So, there are two main topics here, binary kernel interfaces and stable +kernel source interfaces. They both depend on each other, but we will +discuss the binary stuff first to get it out of the way. + + +Binary Kernel Interface +----------------------- +Assuming that we had a stable kernel source interface for the kernel, a +binary interface would naturally happen too, right? Wrong. Please +consider the following facts about the Linux kernel: + + - Depending on the version of the C compiler you use, different kernel + data structures will contain different alignment of structures, and + possibly include different functions in different ways (putting + functions inline or not.) The individual function organization + isn't that important, but the different data structure padding is + very important. + + - Depending on what kernel build options you select, a wide range of + different things can be assumed by the kernel: + + - different structures can contain different fields + - Some functions may not be implemented at all, (i.e. some locks + compile away to nothing for non-SMP builds.) + - Memory within the kernel can be aligned in different ways, + depending on the build options. + + - Linux runs on a wide range of different processor architectures. + There is no way that binary drivers from one architecture will run + on another architecture properly. + +Now a number of these issues can be addressed by simply compiling your +module for the exact specific kernel configuration, using the same exact +C compiler that the kernel was built with. This is sufficient if you +want to provide a module for a specific release version of a specific +Linux distribution. But multiply that single build by the number of +different Linux distributions and the number of different supported +releases of the Linux distribution and you quickly have a nightmare of +different build options on different releases. Also realize that each +Linux distribution release contains a number of different kernels, all +tuned to different hardware types (different processor types and +different options), so for even a single release you will need to create +multiple versions of your module. + +Trust me, you will go insane over time if you try to support this kind +of release, I learned this the hard way a long time ago... + + +Stable Kernel Source Interfaces +------------------------------- + +This is a much more "volatile" topic if you talk to people who try to +keep a Linux kernel driver that is not in the main kernel tree up to +date over time. + +Linux kernel development is continuous and at a rapid pace, never +stopping to slow down. As such, the kernel developers find bugs in +current interfaces, or figure out a better way to do things. If they do +that, they then fix the current interfaces to work better. When they do +so, function names may change, structures may grow or shrink, and +function parameters may be reworked. If this happens, all of the +instances of where this interface is used within the kernel are fixed up +at the same time, ensuring that everything continues to work properly. + +As a specific examples of this, the in-kernel USB interfaces have +undergone at least three different reworks over the lifetime of this +subsystem. These reworks were done to address a number of different +issues: + + - A change from a synchronous model of data streams to an asynchronous + one. This reduced the complexity of a number of drivers and + increased the throughput of all USB drivers such that we are now + running almost all USB devices at their maximum speed possible. + - A change was made in the way data packets were allocated from the + USB core by USB drivers so that all drivers now needed to provide + more information to the USB core to fix a number of documented + deadlocks. + +This is in stark contrast to a number of closed source operating systems +which have had to maintain their older USB interfaces over time. This +provides the ability for new developers to accidentally use the old +interfaces and do things in improper ways, causing the stability of the +operating system to suffer. + +In both of these instances, all developers agreed that these were +important changes that needed to be made, and they were made, with +relatively little pain. If Linux had to ensure that it will preserve a +stable source interface, a new interface would have been created, and +the older, broken one would have had to be maintained over time, leading +to extra work for the USB developers. Since all Linux USB developers do +their work on their own time, asking programmers to do extra work for no +gain, for free, is not a possibility. + +Security issues are also very important for Linux. When a +security issue is found, it is fixed in a very short amount of time. A +number of times this has caused internal kernel interfaces to be +reworked to prevent the security problem from occurring. When this +happens, all drivers that use the interfaces were also fixed at the +same time, ensuring that the security problem was fixed and could not +come back at some future time accidentally. If the internal interfaces +were not allowed to change, fixing this kind of security problem and +insuring that it could not happen again would not be possible. + +Kernel interfaces are cleaned up over time. If there is no one using a +current interface, it is deleted. This ensures that the kernel remains +as small as possible, and that all potential interfaces are tested as +well as they can be (unused interfaces are pretty much impossible to +test for validity.) + + +What to do +---------- + +So, if you have a Linux kernel driver that is not in the main kernel +tree, what are you, a developer, supposed to do? Releasing a binary +driver for every different kernel version for every distribution is a +nightmare, and trying to keep up with an ever changing kernel interface +is also a rough job. + +Simple, get your kernel driver into the main kernel tree (remember we are +talking about drivers released under a GPL-compatible license here, if your +code doesn't fall under this category, good luck, you are on your own here, +you leech). If your driver is in the tree, and a kernel interface changes, +it will be fixed up by the person who did the kernel change in the first +place. This ensures that your driver is always buildable, and works over +time, with very little effort on your part. + +The very good side effects of having your driver in the main kernel tree +are: + + - The quality of the driver will rise as the maintenance costs (to the + original developer) will decrease. + - Other developers will add features to your driver. + - Other people will find and fix bugs in your driver. + - Other people will find tuning opportunities in your driver. + - Other people will update the driver for you when external interface + changes require it. + - The driver automatically gets shipped in all Linux distributions + without having to ask the distros to add it. + +As Linux supports a larger number of different devices "out of the box" +than any other operating system, and it supports these devices on more +different processor architectures than any other operating system, this +proven type of development model must be doing something right :) + + + +------ + +Thanks to Randy Dunlap, Andrew Morton, David Brownell, Hanna Linder, +Robert Love, and Nishanth Aravamudan for their review and comments on +early drafts of this paper. -- cgit v1.3.1