Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon Djtag driver

[John Garry]

On 08/06/2017 17:35, Mark Rutland wrote:
> Hi,
>
> On Mon, May 22, 2017 at 08:48:32PM +0800, Shaokun Zhang wrote:
> > +/*
> > + * hisi_djtag_lock_v2: djtag lock to avoid djtag access conflict b/w kernel
> > + * and UEFI.
>
> The mention of UEFI here worries me somewhat, and I have a number of
> questions specifically relating to how we interact with UEFI here.

Hi Mark,

This djtag locking mechanism is an advisory software-only policy. The 
problem is the hardware designers made an interface which does not 
consider multiple agents in the system concurrently accessing the djtag 
registers.

System wide, djtag is used as an interface to other HW modules, but we 
only use for perf HW in the kernel.

> When precisely does UEFI need to touch the djtag hardware? e.g. does
> this happen in runtime services? ... or completely asynchronously?

Actually it's trusted firmware which accesses for L3 cache management in 
CPU hotplug

> What does UEFI do with djtag when it holds the lock?

As mentioned, cache management

> Are there other software agents (e.g. secure firmware) which try to
> take this lock?

No

> Can you explain how the locking scheme works? e.g. is this an advisory
> software-only policy, or does the hardware prohibit accesses from other
> agents somehow?

The locking scheme is a software solution to spinlock. It's uses djtag 
module select register as the spinlock flag, to avoid using some shared 
memory.

The tricky part is that there is no test-and-set hardware support, so we 
use this algorithm:
- precondition: flag initially set unlocked

a. agent reads flag
    - if not unlocked, continues to poll
    - otherwise, writes agent's unique lock value to flag
b. agent waits defined amount of time *uninterrupted* and then checks 
the flag
    - if it is unchanged, it has the lock -> continue
    - if it is changed, it means other agent is trying to access the 
lock and got it, so it goes back to a.
c. has lock, so safe to access djtag
d. to unlock, release by writing "unlock" value to flag

> What happens if the kernel takes the lock, but doesn't release it?

This should not happen. We use spinlock_irqsave() when locking. However 
I have noted that we can BUG if djtag access timeout, so we need to 
release the lock at this point. I don't think the code handles this 
properly now.

> What happens if UEFI takes the lock, but doesn't release it?

Again, we would not expect this to happen; but, if it does, Kernel 
access should timeout.

> Are there any points at which UEFI needs to hold the locks of several
> djtag units simultaneously?

No

Thanks,
John

> > + * @reg_base:	djtag register base address
> > + *
> > + * Return - none.
> > + */
> > +static void hisi_djtag_lock_v2(void __iomem *regs_base)
> > +{
> > +	u32 rd, wr, mod_sel;
> > +
> > +	/* Continuously poll to ensure the djtag is free */
> > +	while (1) {
> > +		rd = readl(regs_base + SC_DJTAG_MSTR_CFG_EX);
> > +		mod_sel = ((rd >> DEBUG_MODULE_SEL_SHIFT_EX) & 0xFF);
> > +		if (mod_sel == SC_DJTAG_V2_UNLOCK) {
> > +			wr = ((rd & SC_DJTAG_V2_MODULE_SEL_MASK) |
> > +			      (SC_DJTAG_V2_KERNEL_LOCK <<
> > +			       DEBUG_MODULE_SEL_SHIFT_EX));
> > +			writel(wr, regs_base + SC_DJTAG_MSTR_CFG_EX);
> > +			udelay(10); /* 10us */
> > +
> > +			rd = readl(regs_base + SC_DJTAG_MSTR_CFG_EX);
> > +			mod_sel = ((rd >> DEBUG_MODULE_SEL_SHIFT_EX) & 0xFF);
> > +			if (mod_sel == SC_DJTAG_V2_KERNEL_LOCK)
> > +				break;
> > +		}
> > +		udelay(10); /* 10us */
> > +	}
> > +}
> > +
> > +/*

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