linux-kernel - Re: [RFC PATCH 0/15] x86: Remove support for TSC-less and CX8-less CPUs

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Message-ID: <aAs8fuPmXz7kK1l4@gmail.com>
Date: Fri, 25 Apr 2025 09:40:46 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Arnd Bergmann <arnd@...db.de>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	kernel test robot <oliver.sang@...el.com>, oe-lkp@...ts.linux.dev,
	kernel test robot <lkp@...el.com>, linux-kernel@...r.kernel.org,
	John Stultz <jstultz@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Stephen Boyd <sboyd@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH 0/15] x86: Remove support for TSC-less and CX8-less
 CPUs


* Arnd Bergmann <arnd@...db.de> wrote:

> >       x86/platform: Remove CONFIG_X86_RDC321X support
> >       arch/x86, gpio: Remove GPIO_RDC321X support
> >       arch/x86, watchdog: Remove the RDC321X_WDT watchdog driver
> >       arch/x86, mfd: Remove MFD_RDC321X support
> >       x86/reboot: Remove the RDC321X reboot quirk
> 
> I'm not sure about the RDC321X bits. Obviously the original
> 321x/861x/vortex86sx chips are obsolete and can be removed,
> but the product line is still actively developed by RDC and 
> DM&P, and I suspect that some of the drivers are still used
> on 586tsc-class (vortex86dx, vortex86mx) and 686-class
> (vortex86dx3, vortex86ex) SoCs that do run modern kernels and
> get updates.

So CONFIG_X86_RDC321X actively selects M486:

+++ b/arch/x86/Kconfig
 
 config X86_RDC321X
        bool "RDC R-321x SoC"
        depends on X86_32
        depends on X86_EXTENDED_PLATFORM
        select M486
        ^^^^^^^^^^^
        select X86_REBOOTFIXUPS

But indeed the other drivers are not dependent on M486, at least 
overtly:

  arch/x86, mfd: Remove MFD_RDC321X support
  arch/x86, watchdog: Remove the RDC321X_WDT watchdog driver
  arch/x86, gpio: Remove GPIO_RDC321X support

Although the watchdog driver has this indirect dependency:

   drivers/watchdog/Kconfig:       depends on X86_RDC321X || COMPILE_TEST

But the 486 kernel would work on any 586/686 upgraded boards as well.

Anyway, I've dropped the mfd/watchdog/gpio removal patches, no harm in 
keeping these drivers, and I've switched the watchdog driver over to 
X86_32:

  config RDC321X_WDT
        tristate "RDC R-321x SoC watchdog"
        depends on X86_32 || COMPILE_TEST

There's also no harm in keeping the southbridge reboot quirk I suppose, 
so I've dropped this as well:

  x86/reboot: Remove the RDC321X reboot quirk


> >       x86/cpu: Remove CPU_SUP_UMC_32 support
> >       x86/cpu: Remove TSC-less CONFIG_M586 support
> 
> I think Winchip6 (486-class, no tsc, no cx8) and Winchip3D
> (486-class, with tsc but no cx8) need to go as well then.

Okay, agreed, I've added this patch to the tree:

  bf82539ad9f6 x86/cpu: Remove CONFIG_MWINCHIP3D/MWINCHIPC6

  arch/x86/Kconfig.cpu            | 28 ++++------------------------
  arch/x86/Makefile_32.cpu        |  2 --
  arch/x86/include/asm/vermagic.h |  4 ----
  3 files changed, 4 insertions(+), 30 deletions(-)

> At this point, maybe we can consider removing CONFIG_X86_GENERIC and 
> just always build kernels that work across a wide set of CPUs: Only 
> CMOV and PAE still require a CPU with the hardware support, and 
> X86_L1_CACHE_SHIFT needs to be at least 6 (64 byte) for 
> compatibility, but everything else should just be a tuning option.

Agreed.

Thanks,

	Ingo