linux-kernel - Re: [PATCH] arm64: fix cleared E0POE bit after cpu

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Message-ID: <aV46bBotl53gf3Kb@e129823.arm.com>
Date: Wed, 7 Jan 2026 10:50:20 +0000
From: Yeoreum Yun <yeoreum.yun@....com>
To: Kevin Brodsky <kevin.brodsky@....com>
Cc: linux-pm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, rafael@...nel.org, pavel@...nel.org,
	catalin.marinas@....com, will@...nel.org, anshuman.khandual@....com,
	ryan.roberts@....com, yang@...amperecomputing.com,
	joey.gouly@....com
Subject: Re: [PATCH] arm64: fix cleared E0POE bit after cpu_suspend()/resume()

Hi Kevin,

> On 07/01/2026 10:57, Yeoreum Yun wrote:
> >>> @@ -97,8 +97,11 @@ SYM_FUNC_START(cpu_do_suspend)
> >>>  	mrs	x9, mdscr_el1
> >>>  	mrs	x10, oslsr_el1
> >>>  	mrs	x11, sctlr_el1
> >>> -	get_this_cpu_offset x12
> >>> -	mrs	x13, sp_el0
> >>> +alternative_if ARM64_HAS_TCR2
> >>> +	mrs	x12, REG_TCR2_EL1
> >>> +alternative_else_nop_endif
> >>> +	get_this_cpu_offset x13
> >>> +	mrs	x14, sp_el0
> >>>  	stp	x2, x3, [x0]
> >>>  	stp	x4, x5, [x0, #16]
> >>>  	stp	x6, x7, [x0, #32]
> >>> @@ -109,7 +112,7 @@ SYM_FUNC_START(cpu_do_suspend)
> >>>  	 * Save x18 as it may be used as a platform register, e.g. by shadow
> >>>  	 * call stack.
> >>>  	 */
> >>> -	str	x18, [x0, #96]
> >>> +	stp	x14, x18, [x0, #96]
> >> If TCR2_EL1 isn't supported, we store and reload an unused arbitrary
> >> value. I think it'd be better to make it all conditional and add it at
> >> the end, something like:
> >>
> >> ï¿½ ï¿½ alternative_if ARM64_HAS_TCR2
> >> ï¿½ ï¿½ ï¿½ ï¿½ mrsï¿½ ï¿½ x2, REG_TCR2_EL1
> >> ï¿½ ï¿½ ï¿½ ï¿½ strï¿½ ï¿½ x2, [x0, #104]
> >> ï¿½ ï¿½ alternative_else_nop_endif
> >>
> >> Same idea on the resume path. This also avoids the noise of renaming
> >> existing registers.
> > IMHO, I think it would be better to sustain the change since
> > it seems more simpler to maintain  and x12 is temporary regsiter
> > so leaking whatever was in x12 does not really feel like a concern...
>
> Leaking is not a concern, but I don't think it's really easier to
> maintain. We can have all the conditional registers grouped together,
> like DISR_EL1 and soon SCTLR2_EL1. This avoids renaming a bunch of
> registers every time we save/restore a new register here.

Oh. I overlooked that point.
I'll follow your suggestion.

Thanks!

--
Sincerely,
Yeoreum Yun