linux-kernel - Re: [PATCH v2 6/7] efi: runtime-wrapper: get rid of the rtc

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Message-ID: <20151208124854.GC2518@codeblueprint.co.uk>
Date:	Tue, 8 Dec 2015 12:48:54 +0000
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	linux-efi@...r.kernel.org,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	rtc-linux@...glegroups.com
Subject: Re: [PATCH v2 6/7] efi: runtime-wrapper: get rid of the rtc_lock
 spinlock

On Tue, 01 Dec, at 11:50:19AM, Ard Biesheuvel wrote:
> The rtc_lock spinlock aims to serialize access to the CMOS RTC between
> the UEFI firmware and the kernel drivers that use it directly. However,
> x86 is the only arch that performs such direct accesses, and that never
> uses the time related UEFI runtime services. Since no other UEFI enlightened
> architectures have a legcay CMOS RTC anyway, we can remove the rtc_lock
> spinlock entirely.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> ---
>  drivers/firmware/efi/runtime-wrappers.c | 32 +++++---------------
>  1 file changed, 8 insertions(+), 24 deletions(-)

Is this really true? It's not possible, for instance, for 32-bit ARM
systems to use the rtc-cmos driver which would access the same
physical device that UEFI would with the GetTime() service?

With the pending 32-bit ARM UEFI support coming, this needs to be
considered carefully.
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