Messages in this thread | | | Subject | Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon Djtag driver | From | John Garry <> | Date | Wed, 14 Jun 2017 12:35:07 +0100 |
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On 14/06/2017 12:01, Will Deacon wrote: > On Wed, Jun 14, 2017 at 11:42:30AM +0100, Mark Rutland wrote: >> On Wed, Jun 14, 2017 at 11:06:58AM +0100, Will Deacon wrote: >>> Apologies, I misunderstood your algorithm (I thought step (a) was on one CPU >>> and step (b) was on another). Still, I don't understand the need for the >>> timeout. If you instead read back the flag immediately, wouldn't it still >>> work? e.g. >>> >>> >>> lock: >>> Readl_relaxed flag >>> if (locked) >>> goto lock; >>> >>> Writel_relaxed unique ID to flag >>> Readl flag >>> if (locked by somebody else) >>> goto lock; >>> >>> <critical section> >>> >>> unlock: >>> Writel unlocked value to flag >>> >>> >>> Given that we're dealing with iomem, I think it will work, but I could be >>> missing something obvious. >> >> Don't we have the race below where both threads can enter the critical >> section? >> >> // flag f initial zero (unlocked) >> >> // t1, flag 1 // t2, flag 2 >> readl(f); // reads 0 l = readl(f); // reads 0 >> >> <thinks lock is free> <thinks lock is free> >> >> writel(1, f); >> readl(f); // reads 1 >> <thinks lock owned> >> writel(2, f); >> readl(f) // reads 2 >> <thinks lock owned> >> >> <crticial section> <critical section> > > Urgh, yeah, of course and *that's* what the udelay is trying to avoid, > by "ensuring" that the <thinks lock is free> time and subsequent write > propagation is all over before we re-read the flag. > > John -- how much space do you have on this device? Do you have, e.g. a byte > for each CPU?
Hi Will,
To be clear, the agents in our case are the kernel and UEFI. Within the kernel, we use a kernel spinlock to lock the same djtag between threads, for these reasons: - kernel has a native spinlock - we are limited in locking values, as the lock flag is only a 8b field in v2 hw (called module select)
Thanks, John
> > Will > > . >
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