linux-kernel - [REGRESSION][BISECTED] tpm: Popping noise in USB headphones since 1b6d7f9eb150

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Message-ID: <7d052744-4bfa-40bc-ba06-1b4c47a5eb87@heusel.eu>
Date: Fri, 25 Oct 2024 02:11:04 +0200
From: Christian Heusel <christian@...sel.eu>
To: Peter Huewe <peterhuewe@....de>, Jarkko Sakkinen <jarkko@...nel.org>, 
	Jason Gunthorpe <jgg@...pe.ca>, James Bottomley <James.Bottomley@...senpartnership.com>
Cc: linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org, 
	regressions@...ts.linux.dev
Subject: [REGRESSION][BISECTED] tpm: Popping noise in USB headphones since
 1b6d7f9eb150 

Hello everyone,

Adam reports that since switching to the 6.10 kernel they have a popping
sound from time to time (roughly every 10 minues) on their headphones
which are connected via a [UGREEN's 5-in-1 USB-C Hub][0].

We have then done a [lengthy bisection][1] on the mainline kernel which
came to the conclusion that the following commit is the culprit of the
issue:

    1b6d7f9eb150 ("tpm: add session encryption protection to tpm2_get_random()")

Since I thought that these results are likely not correct Adam has
verified our results again by re-testing the bisection steps and again
came to the same conclusion. Additionally I have shot the same question
[at Jarkko on Mastodon] to find out if it's possible that changes in the
TPM subsystem can have such an effect.. Without knowing the exact commit
we had bisected to he pointed to the same functionality:

> Jarkko Sakkinen (@jarkko@...ial.kernel.org):
>
> @gromit i might actually might have hunch for this but have been
> unmotivated to move forward before these patches are merged. One thing
> at a time IMHO, especially with performance fixes.
> NOTE: this is a hyphothesis.
>
> tpm2_get_random() needs to be reconsidered with encryption as hwrng
> calls it often enough and there's now more overhead.
>
> It pulls entropy in small chunks subtracting the length in the look
> like you would use read() syscall from user space. This is not right.
> This leads to small and variable size exchanges and yeah generally
> that equates to an inefficient implemenation with bad latency
> properties.

If there are any patches to test or if further input is needed from the
initial reporter on our Bugtracker we can do that!

Thank you all for your hard work on the kernel!

Cheers,
Chris


[0]: https://uk.ugreen.com/collections/hubs-1/products/ugreen-5-in-1-4k-hdmi-usb-c-hub
[1]: https://gitlab.archlinux.org/archlinux/packaging/packages/linux/-/issues/79#note_210599
[2]: https://chaos.social/@gromit/113345582873908273

---
#regzbot introduced: 1b6d7f9eb150 
#regzbot link: https://gitlab.archlinux.org/archlinux/packaging/packages/linux/-/issues/79
#regzbot link: https://chaos.social/@gromit/113345582873908273
#regzbot title: tpm: popping sound in headphones connected via USB-C

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