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From: ard.biesheuvel@linaro.org (Ard Biesheuvel)
Date: Tue, 21 Apr 2015 15:28:14 +0200
Subject: your mail
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To: linux-arm-kernel@lists.infradead.org
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On 21 April 2015 at 15:21, Dave P Martin <Dave.Martin@arm.com> wrote:
> On Tue, Apr 21, 2015 at 02:10:42PM +0100, Ard Biesheuvel wrote:
>> On 21 April 2015 at 14:50, Russell King - ARM Linux
>> <linux@arm.linux.org.uk> wrote:
>> > On Tue, Apr 21, 2015 at 12:24:20PM +0100, Russell King - ARM Linux wrote:
>> >> We should probably create a badr macro to complement the adr pseudo-op
>> >> which incorporates the BSYM thing so make this clearer - and a comment
>> >> before it.  This is really the case where BSYM should be used.
>> >
>> > Something like this.  Note that I've also removed the BSYM() usage in
>> > the KVM code.
>> >
>>
>> Yes, that is much better. It is a pity that we still can't use '| 1'
>> but the fact that you are forced to use 'adr' now probably mostly
>> eliminates the risk regarding that.
>>
>> I did notice that are are 4 or 5 instances (commented inline) of an
>> ARM to thumb mode switch which can just as easily be implemented as
>> 'blx 1f' instead of using this badr macro (whose use we want to
>> discourage, I assume, since the address arithmetic is still slightly
>> dodgy). Do you think we should do something about that as well?
>
> Err, probably.  That just looks like an oversight -- I think I'm
> responsible for at least some of those.
>
> There's no good reason not to replace adr+BSYM+bx.
>
> For switches from ARM, this could be replaced with bx <label> which
> should work just fine.  There shouldn't be any instances of this from
> Thumb, because switching instruction set is the whole point here.
>
> (Thumb doesn't have bx <label>, but blx <label> is available at the cost
> of clobbering lr).
>

It appears neither have 'bx <label>', but 'add pc, pc, #1; nop' does
the job nicely as well.
And as you say, there are no instances of Thumb->ARM mode switches.