From mboxrd@z Thu Jan  1 00:00:00 1970
From: linux@arm.linux.org.uk (Russell King - ARM Linux)
Date: Wed, 18 Mar 2015 10:35:57 +0000
Subject: [PATCH 2/2] ARM: psci: move psci firmware calls out of line
In-Reply-To: <20150318103014.GD18951@arm.com>
References: <1424866236-12482-1-git-send-email-will.deacon@arm.com>
 <1424866236-12482-2-git-send-email-will.deacon@arm.com>
 <20150318102029.GC8656@n2100.arm.linux.org.uk>
 <20150318103014.GD18951@arm.com>
Message-ID: <20150318103556.GD8656@n2100.arm.linux.org.uk>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On Wed, Mar 18, 2015 at 10:30:14AM +0000, Will Deacon wrote:
> On Wed, Mar 18, 2015 at 10:20:29AM +0000, Russell King - ARM Linux wrote:
> > Why not convert these to:
> > 
> > static int __naked __invoke_psci_fn_hvc(u32 function_id, u32 arg0, u32 arg1,
> > 					u32 arg2)
> > {
> > 	asm(
> > 		__HVC(0)
> > 		"bx lr");
> > }
> > 
> > ?
> 
> I tried this, but the compiler printed a diagnostic along the lines of
> "Ignoring __naked attribute", so we moved the functions out-of-line like
> we have on arm64 instead. Assuming it worked reliably, what's the
> advantage of __naked over having these out-of-line?

Note that the above isn't using any asm constraints/arguments.  The GCC
manual states what can be safely included:

`naked'
     Use this attribute on the ARM, AVR, IP2K, RX and SPU ports to
     indicate that the specified function does not need
     prologue/epilogue sequences generated by the compiler.  It is up
     to the programmer to provide these sequences. The only statements

     that can be safely included in naked functions are `asm'

     statements that do not have operands.  All other statements,
                ^^^^^^^^^^^^^^^^^^^^^^^^^^

     including declarations of local variables, `if' statements, and so
     forth, should be avoided.  Naked functions should be used to
     implement the body of an assembly function, while allowing the
     compiler to construct the requisite function declaration for the
     assembler.

The advantage is that you have locality of code, and we don't need to
spring up lots of assembly files along side their .c files.

I'd guess that if you try to declare local variables etc, the compiler
will just ignore the naked attribute, because it then can't give the
guarantees that the function will be truely naked.

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