Re: [PATCH RFC 2/3] coresight: add support for debug module

[Leo Yan]

Hi Mark,

On Wed, Feb 15, 2017 at 11:44:16AM +0000, Mark Rutland wrote:
> [resending due to a mail server snafu]
> 
> On Mon, Feb 13, 2017 at 02:11:37PM +0800, Leo Yan wrote:
> > Coresight includes debug module and usually the module connects with CPU
> > debug logic. ARMv8 architecture reference manual (ARMv8-ARM) has defined
> > the debug registers in the chapter "H9: External Debug Register
> > Descriptions".
> 
> This should have been in the binding description also.
> 
> The layout of the ARM ARM can change over time, so please refer to the
> full document number, which can be found at the bottom of each page
> (e.g. ARM DDI 0487A.j).

Good to know this. Will use this way to connect code with spec.

> > After enable the debug module we can check CPU state and PC value, etc.
> > So this is helpful for some CPU lockup bugs, e.g. if one CPU has run
> > into infinite loop with IRQ disabled. So the CPU cannot switch context
> > and handle any interrupt, so it cannot handle SMP call for stack dump,
> > etc. Furthermore, now ARMv8 introduces some other runtime firmwares like
> > ARM trusted firmware BL31, so sometime CPU hard lock may happen in the
> > firmware and cannot return back to kernel.
> 
> I would generally expect that the secure world would lock down
> debugging, as this poses a security risk.
> 
> I take it that this is only unlocked on development firmware.
> 
> Given that cores can be powered down outside of our control, I'm not
> sure that accesses to these registers is safe in general.

Mike Leach has gave method in another email to check CPU power state in
register EDPRSR.PU before access EDPCSR register.

For security state, I will check if EDSCR.NS bit can be use to
distinguish CPU is secure or non-secure state. And also follow Mike's
suggestion to use EDPCSR = 0xFFFF_FFFF as hint that the CPU is in
secure state.

> > This patch is to enable coresight debug module and register callback
> > notifier for panic; so when system detect the CPU lockup we can utilize
> > debug module registers to get to know PC value for all CPUs; so we can
> > quickly know the hang address for CPUs.
> > 
> > This is initial driver for coresight debug module and could enhance it
> > later according to debugging requirement.
> 
> How does this interact with an external debugger making use of these
> registers?

IIUC, external debugger also will access these registers, like use
EDPCSR to display PC value. I have no idea for the connection between
this driver with external debugger, except I think we can use this
driver to enable clock for CPU debug module.

> [...]
> 
> > +static struct debug_drvdata *debug_drvdata[NR_CPUS];
> 
> A per-cpu variable is preferred to an NR_CPUS sized array.

Will do.

> > +
> > +static void debug_os_unlock(struct debug_drvdata *drvdata)
> > +{
> > +	/* Unlocks the debug registers */
> > +	writel_relaxed(0x0, drvdata->base + EDOSLAR);
> > +	isb();
> > +}
> 
> I do not believe this barrier is correct.

I copied the code piece from coresight-etm4x.c.

I guess here is to ensure the os unlock operations is finished before
any sequential register accessing. I think isb() is redundant for
debug module and we can only use writel_relaxed() will be enough for
accessing EDOSLAR and EDPCSR, these two registers are in the same
endpoint so we don't need add extra barrier for them. Is this correct?

> [...]
> 
> > +static void debug_read_pcsr(struct debug_drvdata *drvdata)
> > +{
> > +	u32 pcsr_hi, pcsr_lo;
> > +
> > +	CS_UNLOCK(drvdata->base);
> > +
> > +	debug_os_unlock(drvdata);
> > +
> > +#ifdef CONFIG_64BIT
> > +	pcsr_lo = readl_relaxed(drvdata->base + EDPCSR_LO);
> > +	pcsr_hi = readl_relaxed(drvdata->base + EDPCSR_HI);
> > +
> > +	pr_emerg("CPU[%d]: PSCR=0x%lx\n", drvdata->cpu,
> > +		 ((unsigned long)pcsr_hi << 32 | (unsigned long)pcsr_lo));
> > +#else
> > +	pcsr_lo = readl_relaxed(drvdata->base + EDPCSR_LO);
> > +
> > +	pr_emerg("CPU[%d]: PSCR=0x%lx\n", drvdata->cpu,	pcsr_lo);
> > +#endif
> > +
> > +	CS_LOCK(drvdata->base);
> > +}
> 
> Per ARM DDI 0487A.k_iss10775, H9.2.32, "EDPCSR, External Debug Program
> Counter Sample Register":
> 
> 	Implemented only if the OPTIONAL PC Sample-based Profiling
> 	Extension is implemented.
> 
> So even if we have access to an MMIO debug interface, we cannot
> necessarily acecess this register.

Eventually this is a hardware feature, right? If so we can use one DT
property to describe this.

> [...]
> 
> > +/*
> > + * Dump out memory limit information on panic.
> > + */
> > +static int dump_debug(struct notifier_block *self, unsigned long v, void *p)
> > +{
> > +	int i;
> > +
> > +	pr_emerg("Coresight debug module:\n");
> > +
> > +	for_each_possible_cpu(i) {
> > +
> > +		if (!debug_drvdata[i])
> > +			continue;
> > +
> > +		debug_read_pcsr(debug_drvdata[i]);
> > +	}
> 
> Is there no potential for deadlock with a CPU reading its own debug
> interface registers?

Have checked this, does not introduce deadlock.

> [...]
> 
> > +static struct amba_id debug_ids[] = {
> > +	{       /* Debug for Cortex-A53 */
> > +		.id	= 0x000bbd03,
> > +		.mask	= 0x000fffff,
> > +		.data	= "debug",
> > +	},
> > +	{ 0, 0},
> > +};
> 
> The DT binding said nothing about Cortex-A53.
> 
> How variable are the MMIO registers in practice? Do we need to know the
> particular CPU?

We don't need know particular CPU for current driver; but I think the
amba id is different for different CPU variants. Should I remove
related binding for this?

Thanks,
Leo Yan