Re: [PATCH v5 1/5] arm/arm64: add smccc ARCH32

[Will Deacon <will.deacon@arm.com>]

On Thu, Aug 20, 2015 at 12:37:29PM +0100, Jens Wiklander wrote:
> On Wed, Aug 19, 2015 at 05:50:09PM +0100, Will Deacon wrote:
> > On Wed, Aug 19, 2015 at 09:40:25AM +0100, Jens Wiklander wrote:
> > > Adds helpers to do SMC based on ARM SMC Calling Convention.
> > > CONFIG_HAVE_SMCCC is enabled for architectures that may support
> > > the SMC instruction. It's the responsibility of the caller to
> > > know if the SMC instruction is supported by the platform.
> > 
> > [...]
> > > diff --git a/arch/arm64/kernel/smccc-call.S b/arch/arm64/kernel/smccc-call.S
> > > new file mode 100644
> > > index 0000000..3ce7fe8
> > > --- /dev/null
> > > +++ b/arch/arm64/kernel/smccc-call.S
> > > @@ -0,0 +1,34 @@
> > > +/*
> > > + * Copyright (c) 2015, Linaro Limited
> > > + *
> > > + * This program is free software; you can redistribute it and/or modify
> > > + * it under the terms of the GNU General Public License Version 2 as
> > > + * published by the Free Software Foundation.
> > > + *
> > > + * This program is distributed in the hope that it will be useful,
> > > + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> > > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> > > + * GNU General Public License for more details.
> > > + *
> > > + */
> > > +#include <linux/linkage.h>
> > > +
> > > +#define SMC_PARAM_W0_OFFS      0
> > > +#define SMC_PARAM_W2_OFFS      8
> > > +#define SMC_PARAM_W4_OFFS      16
> > > +#define SMC_PARAM_W6_OFFS      24
> > > +
> > > +/* void smccc_call32(struct smccc_param32 *param) */
> > > +ENTRY(smccc_call32)
> > > +       stp     x28, x30, [sp, #-16]!
> > 
> > Why are you saving lr?
> 
> Agree, no point in saving lr, but I still need to decrease sp with 16 to
> maintain correct alignment. I'll do it with an str instruction instead.

That or pad out with xzr

> > 
> > > +       mov     x28, x0
> > > +       ldp     w0, w1, [x28, #SMC_PARAM_W0_OFFS]
> > > +       ldp     w2, w3, [x28, #SMC_PARAM_W2_OFFS]
> > > +       ldp     w4, w5, [x28, #SMC_PARAM_W4_OFFS]
> > > +       ldp     w6, w7, [x28, #SMC_PARAM_W6_OFFS]
> > > +       smc     #0
> > > +       stp     w0, w1, [x28, #SMC_PARAM_W0_OFFS]
> > > +       stp     w2, w3, [x28, #SMC_PARAM_W2_OFFS]
> > > +       ldp     x28, x30, [sp], #16
> > > +       ret
> > > +ENDPROC(smccc_call32)
> > 
> > Could we deal with this like we do for PSCI instead? (see
> > __invoke_psci_fn_smc). We could also then rename psci-call.S to fw-call.S
> > and stick this in there too.
> 
> I assume you're referring to when to use "hvc" and "smc".

No, I mean use a C prototype to avoid marshalling the parameters in assembly
like this. As Rutland pointed out, the return value is a bit messy, but
the arguments align nicely with the PCS afaict.

Will