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

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Message-ID: <20170216181407.GA28497@linaro.org>
Date:   Thu, 16 Feb 2017 11:14:07 -0700
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Leo Yan <leo.yan@...aro.org>, Rob Herring <robh+dt@...nel.org>,
        Wei Xu <xuwei5@...ilicon.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Daniel Thompson <daniel.thompson@...aro.org>
Subject: Re: [PATCH RFC 2/3] coresight: add support for debug module

On Wed, Feb 15, 2017 at 11:43:27AM +0000, Mark Rutland wrote:
> 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).
> 
> > 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.
> 
> > 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?
> 
> [...]
> 
> > +static struct debug_drvdata *debug_drvdata[NR_CPUS];
> 
> A per-cpu variable is preferred to an NR_CPUS sized array.
> 
> > +
> > +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.

Mark has a point - isb() sounds like an overkill to prevent
re-ordering (same for the ETM4x driver).  smb_wmb() should be sufficient.  

> 
> [...]
> 
> > +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.
> 
> [...]
> 
> > +/*
> > + * 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?
> 
> [...]
> 
> > +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?
> 
> Thanks,
> Mark.
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