[PATCH] arm/start.S: Do not use a scratch register to hold the stack pointer

[Sebastian Huber]

On 26/07/2019 07:37, Chris Johns wrote:
> On 26/7/19 3:06 pm, Sebastian Huber wrote:
>> Hello Chris,
>>
>> I am not sure, if using r8 is the right thing to do since r8..r14 are banked in
>> FIQ mode.
> 
> The ARM docs I referenced say the register is general purpose. There is other
> docs they say something else?

This is a standard ARM feature, see for example "B1.3.2
ARM core registers" in the ARMv7-AR reference manual.

> 
> This is early boot code who why would FIQ banks be an issue? Which BSPs would
> that effect?

You set the stack in FIQ mode in start.S:

	/* Initialize stack pointer registers for the various modes */

	/* Enter FIQ mode and set up the FIQ stack pointer */
	mov	r0, #(ARM_PSR_M_FIQ | ARM_PSR_I | ARM_PSR_F)
	msr	cpsr, r0
	ldr	r1, =bsp_stack_fiq_size
	mov	sp, r8
	sub	r8, r8, r1

#ifdef BSP_START_NEEDS_REGISTER_INITIALIZATION
	bl bsp_start_init_registers_banked_fiq
#endif

> 
>> I think the bsp_start_arm_drop_hyp_mode needs to be changed to not
>> touch r3, it can use r1 instead.
> 
> That is a bad idea. The registers are documented as scratch and it's use here
> across a call is wrong.

We are here in the low level startup code, so we have to be a bit 
flexible if it comes to the ABI. We should move these low level 
functions to start.S.

-- 
Sebastian Huber, embedded brains GmbH

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