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Date: Mon, 18 May 2015 17:52:04 +0800
Message-ID: <CAFP4FLo3=2J4pwn5nFCtbHon6ZSh+W1JUrgrGXNCpWG6=R+AuQ@mail.gmail.com>
Subject: Re: [RFC V2] arm:consider THUMB and BE endian kernel build
From: yalin wang <yalin.wang2010@gmail.com>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: will.deacon@arm.com, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>, viro@zeniv.linux.org.uk,
        Neil.Gao@sonymobile.com
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2015-05-18 17:40 GMT+08:00 Russell King - ARM Linux <linux@arm.linux.org.uk>:
> On Mon, May 18, 2015 at 03:36:10PM +0800, yalin wang wrote:
>> this patch fix the function in kernel_thread(),
>> when kernel is build as THUMB2 or BE8 endian, we should
>> also set the correct bit in CPSR, so that kernel can return to
>> the correct state to execute.
>
> Why do you think any of this is needed?
>
> When a kernel thread is created via kernel_thread(), copy_thread() is
> called with the function pointer in stack_start, and the functions
> argument in stk_size.
>
> When the scheduler switches to the thread, it reads the register state
> from thread->cpu_context, thereby loading r4 and r5 with the function
> argument and function pointer, and directing the PC to ret_from_fork.
> (For normal user clones and forks, r4 and r5 in kernel space will be
> zero.)
>
> The scheduler switch preserves the CPSR from the previous task, so if
> we're running a T2 BE8 kernel, the new thread will have its ret_from_fork
> called in T2 BE8 mode.
>
> ret_from_fork checks for a non-zero r5, and if so, calls that function,
> which will also see the CPSR set appropriately for the kernel mode.
>
> Functions called from kernel_thread() are not permitted to return, so
> we will never read the "childregs" off the top of the kernel stack.
> Childregs are initialised because we expect them to be at the top of
> every kernel stack.
>
> Ergo, this patch is not needed at all.
Genius explanation,
i see your idea,
i really make a mistake about user thread / kernel thread return method .
it is really a trick for kernel thread return implemented like this.  :)

Thank you !
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