Re: [PATCH V2 1/5] arm: mvebu: Added support for coherency fabric in mach-mvebu

[Will Deacon <will.deacon@arm.com>]

On Mon, Nov 12, 2012 at 08:21:07PM +0000, Gregory CLEMENT wrote:
> On 11/05/2012 03:02 PM, Will Deacon wrote:
> > On Mon, Oct 29, 2012 at 09:11:44PM +0000, Gregory CLEMENT wrote:
> >> +int set_cpu_coherent(unsigned int hw_cpu_id, int smp_group_id)
> >> +{
> >> +	int reg;
> >> +
> >> +	if (!coherency_base) {
> >> +		pr_warn("Can't make CPU %d cache coherent.\n", hw_cpu_id);
> >> +		pr_warn("Coherency fabric is not initialized\n");
> >> +		return 1;
> >> +	}
> >> +
> >> +	/* Enable the CPU in coherency fabric */
> >> +	reg = readl(coherency_base + COHERENCY_FABRIC_CTL_OFFSET);
> >> +	reg |= 1 << (24 + hw_cpu_id);
> >> +	writel(reg, coherency_base + COHERENCY_FABRIC_CTL_OFFSET);
> >> +
> >> +	/* Add CPU to SMP group */
> >> +	reg = readl(coherency_base + COHERENCY_FABRIC_CFG_OFFSET);
> >> +	reg |= 1 << (16 + hw_cpu_id + (smp_group_id == 0 ? 8 : 0));
> >> +	writel(reg, coherency_base + COHERENCY_FABRIC_CFG_OFFSET);
> >> +
> >> +	return 0;
> >> +}
> > 
> > These writels may expand to code containing calls to outer_sync(), which
> > will attempt to take a spinlock for the aurora l2. Given that the CPU isn't
> > coherent, how does this play out with the exclusive store instruction in the
> > lock?
> 
> I dug a little this subject: and I am not sure there is problem. In SMP mode,
> only the system cache mode of Aurora is used. In this mode, outer_cache.sync
> is void then outer_sync() won't call any function, so there will be no
> access to any spinlock.

Hmm, that is pretty subtle and it doesn't really solve the bigger picture.
printk takes logbuf_lock, for example, and I'm sure that by the time you get
to this code you will have relied on exclusives behaving correctly.

Will