Subject: [PATCH V2] ARM: BCM5301X: Implement SMP support

From: [email-masked] (Hauke Mehrtens)
Date: Wed, 14 Oct 2015 00:29:19 +0200

Hi,

I tested this patch on my device now.

What does the loader do before Linux gets started on the second CPU and
what is ensured?

On 03/26/2015 01:00 PM, Russell King - ARM Linux wrote:
> On Sun, Mar 22, 2015 at 02:20:15PM +0100, Rafa? Mi?ecki wrote:
>> +/*
>> + * BCM5301X specific entry point for secondary CPUs.
>> + */
>> +ENTRY(bcm5301x_secondary_startup)
>> +	mrc	p15, 0, r0, c0, c0, 5
>> +	and	r0, r0, #15
>> +	adr	r4, 1f
>> +	ldmia	r4, {r5, r6}
>> +	sub	r4, r4, r5
>> +	add	r6, r6, r4
>> +pen:	ldr	r7, [r6]
>> +	cmp	r7, r0
>> +	bne	pen
>> +
>> +	/*
>> +	 * In case L1 cache has unpredictable contents at power-up
>> +	 * clean its contents without flushing.
>> +	 */
>> +	bl      v7_invalidate_l1
>> +
>> +	mov	r0, #0
>> +	mcr	p15, 0, r0, c7, c5, 0	/* Invalidate icache */
>> +	dsb
>> +	isb
> 
> So if your I-cache contains unpredictable contents, how do you execute
> the code to this point?  Shouldn't the I-cache invalidate be the very
> first instruction you execute followed by the dsb and isb (oh, and iirc
> it ignores the value in the register).
> 
> In the case where a CPU has unpredictable contents at power up, the
> ARM ARM requires that an implementation specific sequence is followed
> to initialise the caches.  I doubt that such a sequence includes testing
> a pen value.

When I remove the test for the pen value the CPU does not come up any
more, I get this log output:

[ 0.132292] CPU: Testing write buffer coherency: ok
[ 0.137635] CPU0: thread -1, cpu 0, socket 0, mpidr 80000000
[ 0.143675] Setting up static identity map for 0x82a0 - 0x82d4
[ 10.149786] CPU1: failed to boot: -38
[ 10.153651] Brought up 1 CPUs

This was caused just by removing the "cmp r7, r0" and "bne pen"
instructions.

With these instructions are added it works and I get this:

[    0.132329] CPU: Testing write buffer coherency: ok
[    0.137682] CPU0: thread -1, cpu 0, socket 0, mpidr 80000000
[    0.143708] Setting up static identity map for 0x82a0 - 0x82d4
[    0.189788] CPU1: thread -1, cpu 1, socket 0, mpidr 80000001
[    0.189892] Brought up 2 CPUs
[    0.198889] SMP: Total of 2 processors activated (3188.32 BogoMIPS).

Currently this is 100% reproducible.

Could it be that the second CPU needs some time till it is synchronised
correctly?

I do not know if and why the cache clearing is needed, I do not have
access to the SoC documentation or the ASIC/firmware developer.

> 
>> +	sysram_base_addr = of_iomap(node, 0);
>> +	if (!sysram_base_addr) {
>> +		pr_warn("Failed to map sysram\n");
>> +		return;
>> +	}
>> +
>> +	writel(virt_to_phys(entry_point), sysram_base_addr + SOC_ROM_LUT_OFF);
>> +
>> +	dsb_sev();	/* Exit WFI */
> 
> Which WFI?  This seems to imply that you have some kind of initial
> firmware.  If so, that should be taking care of the cache initialisation,
> not the kernel.
> 
>> +	mb();		/* make sure write buffer is drained */
> 
> writel() already ensures that.
> 
>> +	/*
>> +	 * The secondary processor is waiting to be released from
>> +	 * the holding pen - release it, then wait for it to flag
>> +	 * that it has been released by resetting pen_release.
>> +	 *
>> +	 * Note that "pen_release" is the hardware CPU ID, whereas
>> +	 * "cpu" is Linux's internal ID.
>> +	 */
>> +	write_pen_release(cpu_logical_map(cpu));
>> +
>> +	 /* Send the secondary CPU SEV */
>> +	dsb_sev();
> 
> If you even need any of the pen code, if you're having to send a SEV here,
> wouldn't having a WFE in the pen assembly loop above be a good idea?
> 

I have to read more on how WFE and co works.

Hauke