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commitd75a170fd8upstream. We've got a regression report about M-Audio Fast Track C400 device, and the git bisection resulted in the commite0ccdef926("ALSA: usb-audio: Clean up check_input_term()"). This commit was about the rewrite of the input terminal parser, and it's not too obvious from the change what really broke. The answer is: it's the interpretation of UAC2/3 effect units. In the original code, UAC2 effect unit is as if through UAC1 processing unit because both UAC1 PU and UAC2/3 EU share the same number (0x07). The old code went through a complex switch-case fallthrough, finally bailing out in the middle: if (protocol == UAC_VERSION_2 && hdr[2] == UAC2_EFFECT_UNIT) { /* UAC2/UAC1 unit IDs overlap here in an * uncompatible way. Ignore this unit for now. */ return 0; } ... and this special handling was missing in the new code; the new code treats UAC2/3 effect unit as if it were equivalent with the processing unit. Actually, the old code was too confusing. The effect unit has an incompatible unit description with the processing unit, so we shouldn't have dealt with EU in the same way. This patch addresses the regression by changing the effect unit handling to the own parser function. The own parser function makes the clear distinct with PU, so it improves the readability, too. The EU parser just sets the type and the id like the old kernels. Once when the proper effect unit support is added, we can revisit this parser function, but for now, let's keep this simple setup as is. Fixes:e0ccdef926("ALSA: usb-audio: Clean up check_input_term()") Cc: <stable@vger.kernel.org> BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=206147 Link: https://lore.kernel.org/r/20200211160521.31990-1-tiwai@suse.de Signed-off-by: Takashi Iwai <tiwai@suse.de> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.