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From: Sascha Hauer <s.hauer@pengutronix.de>
To: Trent Piepho <tpiepho@kymetacorp.com>
Cc: barebox <barebox@lists.infradead.org>
Subject: Re: [PATCH] ARM: Remove kernel booting call for thumb2 mode
Date: Fri, 4 Dec 2015 08:30:48 +0100	[thread overview]
Message-ID: <20151204073048.GV11966@pengutronix.de> (raw)
In-Reply-To: <1449196125.26955.13.camel@rtred1test09.kymeta.local>

On Fri, Dec 04, 2015 at 02:28:40AM +0000, Trent Piepho wrote:
> On Thu, 2015-12-03 at 01:24 +0000, Trent Piepho wrote:
> > The asm code to do the actual call into the kernel (or another
> > barebox) when compiled in thumb2 mode isn't necessary.  gcc generates
> > a perfectly good calling sequence from a normal function pointer call.
> > If it didn't, the code in bootstrap_boot() that uses a function
> > pointer to jump to the 2nd stage barebox from an xloader wouldn't
> > work.
> 
> The same commit that originally added this code also added
> thumb2_execute(), which is used by the go command in a thumb2 build.
> 
> This doesn't seem necessary for the same reason as the code in
> start_linux.  THUMB2 selects AEABI, which in turn turns on the aapcs
> abi, which requires interworking safe indirect calls.  And again, the
> bootstrap code doesn't use a special call sequence so if it didn't work
> that would be broken too.
> 
> The code for thumb2_execute() is also odd in that it subtracts one from
> argc, drops the 1st argv entry, and supplies zero for the first function
> argument.  I.e., "go" in arm mode (and all non-arm arches) does this:
> 	func(argc - 1, &argv[1]);
> and in thumb2 mode does this:
> 	func(0, argc - 2, &argv[2]);
> 
> I don't see why one would want this difference.

This is a bug. We want to shift argv[] by one, not by two. I must
accidently have done this twice.
As you note the thumb2 execution code is unnecessary anyway, we can
remove the bug by removing the code.

Sascha

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  reply	other threads:[~2015-12-04  7:31 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-12-03  1:24 Trent Piepho
2015-12-04  2:28 ` Trent Piepho
2015-12-04  7:30   ` Sascha Hauer [this message]
2015-12-04 19:19     ` Trent Piepho
2015-12-04  7:20 ` Sascha Hauer

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