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Date:   Sun, 24 Apr 2022 00:33:44 +0100 (BST)
From:   "Maciej W. Rozycki" <macro@orcam.me.uk>
To:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>
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Subject: Re: [PATCH v4 04/11] mips: use fallback for random_get_entropy()
 instead of zero
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On Mon, 18 Apr 2022, Thomas Bogendoerfer wrote:

> >  Also the systems I have in mind and that lack a counter in the chipset 
> > actually can make use of the buggy CP0 timer, because it's only when CP0 
> > timer interrupts are used that the erratum matters, but they use a DS1287 
> > RTC interrupt instead unconditionally as the clock event (see the comment 
> > at the bottom of arch/mips/dec/time.c).  But this has not been factored in 
> > with `can_use_mips_counter' (should it just check for `mips_hpt_frequency' 
> > being zero perhaps, meaning the timer interrupt not being used?).
> > 
> >  Thomas, do you happen to know if any of the SGI systems that we support 
> > had buggy early R4k chips?
> 
> IP22 has probably seen all buggy MIPS chips produced, so yes I even own
> Indy/Indigo2 CPU boards with early R4k chips.

 Do they actually use the CP0 timer as a clock event device?  Do they have 
an alternative high-precision timer available?

 In the course of verifying this change I have noticed my DECstation
5000/260, which has a high-precision timer in the chipset available as a 
clock source device, does register the CP0 timer as a clock source device 
regardless.  Upon a closer inspection I have noticed that the CP0 timer 
interrupt is non-functional in this machine, which I have then confirmed 
as a valid CPU hardware configuration via the TimIntDis/TimerIntDis (the 
R4k CPU manual is inconsistent in naming here) boot-mode bit.  It allows 
IP7 to be used as an external interrupt source instead.  I used not to be 
aware of the presence of this boot-mode bit.

 I find this arrangement odd, because IP7 used to be wired internally as 
the FPU interrupt with the 5000/240's CPU module, so it's not usable as an 
external interrupt anyway with this system's mainboard.

 That means however that this machine (and possibly the 5000/150 as well, 
but I'll have to verify that once I get at the KN04 CPU module I have in a 
drawer at my other place) can use the CP0 timer as a clock source device 
unconditionally.  I think this discovery asks for code optimisation, which 
I'll try to cook up sometime.

 I don't expect the IP22 to have a similar arrangement with the CP0 timer 
interrupt given that the CPU was an in-house design at SGI, but who knows?  
Do you?

  Maciej