On Wed, Jan 14, 2015 at 10:28:54PM +0000, Paul Zimmerman wrote: > > From: Felipe Balbi [mailto:balbi@xxxxxx] > > Sent: Wednesday, January 14, 2015 1:46 PM > > > > On Wed, Jan 14, 2015 at 04:41:23PM -0500, Alan Stern wrote: > > > > > > This is really, really odd. Register accesses are atomic, so the lock > > > > > > isn't really doing anything. Besides, you're calling > > > > > > dwc2_is_controller_alive() from within the IRQ handler, so IRQs are > > > > > > already disabled. > > > > > > > > > > Spinlocks sometimes do more than you think. For instance, here the > > > > > lock prevents the register access from happening while some other CPU > > > > > is holding the lock. If a silicon quirk causes the register access to > > > > > interfere with other activities, this could be important. > > > > > > > > readl() (which is used by dwc2_is_controller_alive()) adds a memory > > > > barrier to the register accesses, that should force all register > > > > accesses the be correctly ordered. > > > > > > Memory barriers will order accesses that are all made on the same CPU > > > with respect to each other. They do not order these accesses against > > > accesses made from another CPU -- that's why we have spinlocks. :-) > > > > a fair point :-) The register is still read-only, so that shouldn't > > matter either :-) > > > > > > I fail to see how a silicon quirk > > > > could cause this and if, indeed, it does, I'd be more comfortable with a > > > > proper STARS tickect number from synopsys :-s > > > > > > Maybe accessing this register somehow resets something else. I don't > > > know. It seems unlikely, but at least it explains how adding a > > > spinlock could fix the problem. > > > > I would really need Paul (or someone at Synopsys) to confirm this > > somehow. Maybe it has something to do with how the register is > > implemented, dunno. > > > > Paul, do you have any idea what could cause this ? Could the HW into > > some weird state if we read GSNPSID at random locations or when data is > > being transferred, or anything like that ? > > Only thing I can think of is that there is some silicon bug in Robert's > platform. But I am not aware of any STARs that mention accesses to the > GSNPSID register as being problematic. > > Funny thing is, this code has been basically the same since at least > November 2013. So I think some other recent change must have modified > the timing of the register accesses, or something like that. But that's > just handwaving, really. Alright, I'll apply this patch but for 3.20 with a stable tag as I have already sent my last pull request to Greg. Unless someone has a really big complaint about doing things as such. -- balbi
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