linux-kernel - Re: [PATCH 1/1] ARM: imx: build suspend-imx6.S with arm instruction set

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Message-ID: <147692b37063057ce4128894db112caf9320b65b.camel@gmail.com>
Date:   Mon, 11 Jan 2021 19:30:51 +0100
From:   Max Krummenacher <max.oss.09@...il.com>
To:     Ahmad Fatoum <a.fatoum@...gutronix.de>
Cc:     Max Krummenacher <max.krummenacher@...adex.com>,
        Lucas Stach <l.stach@...gutronix.de>,
        linux-arm-kernel@...ts.infradead.org,
        Fabio Estevam <festevam@...il.com>,
        Rouven Czerwinski <r.czerwinski@...gutronix.de>,
        linux-kernel@...r.kernel.org,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Russell King <linux@...linux.org.uk>,
        Shawn Guo <shawnguo@...nel.org>,
        NXP Linux Team <linux-imx@....com>
Subject: Re: [PATCH 1/1] ARM: imx: build suspend-imx6.S with arm instruction
 set

Resent due to gmail adding HTML, sorry for the noise.

Am Montag, den 11.01.2021, 18:49 +0100 schrieb
Ahmad Fatoum:
> 
> On 11.01.21 16:17, Max Krummenacher wrote:
> > When the kernel is configured to use the Thumb-2 instruction set
> > "suspend-to-memory" fails to resume. Observed on a Colibri iMX6ULL
> > (i.MX 6ULL) and Apalis iMX6 (i.MX 6Q).
> > 
> > It looks like the CPU resumes unconditionally in ARM instruction mode
> > and then chokes on the presented Thumb-2 code it should execute.
> > 
> > Fix this by using the arm instruction set for all code in
> > suspend-imx6.S.
> > 
> > Signed-off-by: Max Krummenacher <max.krummenacher@...adex.com>
> > 
> > ---
> > 
> >  arch/arm/mach-imx/suspend-imx6.S | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/arch/arm/mach-imx/suspend-imx6.S b/arch/arm/mach-imx/suspend-imx6.S
> > index 1eabf2d2834be..e06f946b75b96 100644
> > --- a/arch/arm/mach-imx/suspend-imx6.S
> > +++ b/arch/arm/mach-imx/suspend-imx6.S
> > @@ -67,6 +67,7 @@
> >  #define MX6Q_CCM_CCR	0x0
> >  
> >  	.align 3
> > +	.arm
> 
> You had a return to thumb at the end of this subroutine in the cover letter,
> yet here it's omitted. Why?

Now the whole subroutine is compiled for ARM and the return address has bit
0 set so that on jumping back to the caller the CPU will switch back to
Thumb-2.

Probably the return to Thumb-2 isn't needed in the cover letter solution
and it would also work to finish the subroutine in ARM instruction set.
However it looks strange to me if a function which begins with the ARM
instruction set would come to the return in Thumb-2.

> 
> >  
> >  	.macro  sync_l2_cache
> >  
> >