Linux Kernel Development
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New Sound Drivers Coming In Linux 4.16 Kernel
Due to longtime SUSE developer Takashi Iwai going on holiday the next few weeks, he has already sent in the sound driver feature updates targeting the upcoming Linux 4.16 kernel cycle.
The sound subsystem in Linux 4.16 sees continued changes to the ASoC code, clean-ups to the existing drivers, and a number of new drivers.
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Varlink: a protocol for IPC
One of the motivations behind projects like kdbus and bus1, both of which have fallen short of mainline inclusion, is to have an interprocess communication (IPC) mechanism available early in the boot process. The D-Bus IPC mechanism has a daemon that cannot be started until filesystems are mounted and the like, but what if the early boot process wants to perform IPC? A new project, varlink, was recently announced; it aims to provide IPC from early boot onward, though it does not really address the longtime D-Bus performance complaints that also served as motivation for kdbus and bus1.
The announcement came from Harald Hoyer, but he credited Kay Sievers and Lars Karlitski with much of the work. At its core, varlink is simply a JSON-based protocol that can be used to exchange messages over any connection-oriented transport. No kernel "special sauce" (such as kdbus or bus1) is needed to support it as TCP or Unix-domain sockets will provide the necessary functionality. The messages can be used as a kind of remote procedure call (RPC) using an API defined in an interface file.
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Statistics for the 4.15 kernel
The 4.15 kernel is likely to require a relatively long development cycle as a result of the post-rc5 merge of the kernel page-table isolation patches. That said, it should be in something close to its final form, modulo some inevitable bug fixes. The development statistics for this kernel release look fairly normal, but they do reveal an unexpectedly busy cycle overall.
This development cycle was supposed to be relatively calm after the anticipated rush to get work into the 4.14 long-term-support release. But, while 4.14 ended up with 13,452 non-merge changesets at release, 4.15-rc6 already has 14,226, making it one of the busiest releases in the kernel project's history. Only 4.9 (16,214 changesets) and 4.12 (14,570) brought in more work, and 4.15 may exceed 4.12 by the time it is finished. So far, 1,707 developers have contributed to this kernel; they added 725,000 lines of code while removing 407,000, for a net growth of 318,000 lines of code.
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A new kernel polling interface
Polling a set of file descriptors to see which ones can perform I/O without blocking is a useful thing to do — so useful that the kernel provides three different system calls (select(), poll(), and epoll_wait() — plus some variants) to perform it. But sometimes three is not enough; there is now a proposal circulating for a fourth kernel polling interface. As is usually the case, the motivation for this change is performance.
On January 4, Christoph Hellwig posted a new polling API based on the asynchronous I/O (AIO) mechanism. This may come as a surprise to some, since AIO is not the most loved of kernel interfaces and it tends not to get a lot of attention. AIO allows for the submission of I/O operations without waiting for their completion; that waiting can be done at some other time if need be. The kernel has had AIO support since the 2.5 days, but it has always been somewhat incomplete. Direct file I/O (the original use case) works well, as does network I/O. Many other types of I/O are not supported for asynchronous use, though; attempts to use the AIO interface with them will yield synchronous behavior. In a sense, polling is a natural addition to AIO; the whole point of polling is usually to avoid waiting for operations to complete.
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