Linux-Kernel Archive: Re: [PATCH] MIPS: Fix CP0 counter erratum detection for R4k CPUs Re: [PATCH] MIPS: Fix CP0 counter erratum detection for R4k CPUs From: Thomas Bogendoerfer Date: Fri Apr 29 2022 - 10:03:25 EST Next message: Zi Yan: "Re: [PATCH v11 3/6] mm: make alloc_contig_range work at pageblock granularity" Previous message: Paolo Bonzini: "Re: [PATCH for-5.18] KVM: fix bad user ABI for KVM_EXIT_SYSTEM_EVENT" In reply to: Philippe Mathieu-Daudé: "Re: [PATCH] MIPS: Fix CP0 counter erratum detection for R4k CPUs" Messages sorted by: [ date ] [ thread ] [ subject ] [ author ] On Sun, Apr 24, 2022 at 12:46:23PM +0100, Maciej W. Rozycki wrote: > Fix the discrepancy between the two places we check for the CP0 counter > erratum in along with the incorrect comparison of the R4400 revision > number against 0x30 which matches none and consistently consider all > R4000 and R4400 processors affected, as documented in processor errata > publications[1][2][3], following the mapping between CP0 PRId register > values and processor models: > > PRId | Processor Model > ---------+-------------------- > 00000422 | R4000 Revision 2.2 > 00000430 | R4000 Revision 3.0 > 00000440 | R4400 Revision 1.0 > 00000450 | R4400 Revision 2.0 > 00000460 | R4400 Revision 3.0 > > No other revision of either processor has ever been spotted. > > Contrary to what has been stated in commit ce202cbb9e0b ("[MIPS] Assume > R4000/R4400 newer than 3.0 don't have the mfc0 count bug") marking the > CP0 counter as buggy does not preclude it from being used as either a > clock event or a clock source device. It just cannot be used as both at > a time, because in that case clock event interrupts will be occasionally > lost, and the use as a clock event device takes precedence. > > Compare against 0x4ff in `can_use_mips_counter' so that a single machine > instruction is produced. > > References: > > [1] "MIPS R4000PC/SC Errata, Processor Revision 2.2 and 3.0", MIPS > Technologies Inc., May 10, 1994, Erratum 53, p.13 > > [2] "MIPS R4400PC/SC Errata, Processor Revision 1.0", MIPS Technologies > Inc., February 9, 1994, Erratum 21, p.4 > > [3] "MIPS R4400PC/SC Errata, Processor Revision 2.0 & 3.0", MIPS > Technologies Inc., January 24, 1995, Erratum 14, p.3 > > Signed-off-by: Maciej W. Rozycki > Fixes: ce202cbb9e0b ("[MIPS] Assume R4000/R4400 newer than 3.0 don't have the mfc0 count bug") > Cc: stable@xxxxxxxxxxxxxxx # v2.6.24+ > --- > Thomas, > > Please review the requirements for SNI platforms. In the case of an > erratic CP0 timer we give precedence to the use as a clock event rather > than clock source device; see `time_init' in arch/mips/kernel/time.c. > Therefore if SNI systems have no alternative timer interrupt source, then > the CP0 timer is supposed to still do regardless of the erratum. > > Conversely a system can do without a high-precision clock source, in > which case jiffies will be used. Of course such a system will suffer if > used for precision timekeeping, but such is the price for broken hardware. > Don't SNI systems have any alternative timer available, not even the > venerable 8254? > > Long-term I think this code should be factored out and rewritten so that > it lives in one place and can take advantage of compile-time constants, > which will be the case for the majority of platforms. For the time being > fix the immediate breakage however. > > With the considerations above in mind, please apply. > > Maciej > --- > arch/mips/include/asm/timex.h | 8 ++++---- > arch/mips/kernel/time.c | 11 +++-------- > 2 files changed, 7 insertions(+), 12 deletions(-) applied to mips-fixes. Thomas. -- Crap can work. Given enough thrust pigs will fly, but it's not necessarily a good idea. [ RFC1925, 2.3 ] Next message: Zi Yan: "Re: [PATCH v11 3/6] mm: make alloc_contig_range work at pageblock granularity" Previous message: Paolo Bonzini: "Re: [PATCH for-5.18] KVM: fix bad user ABI for KVM_EXIT_SYSTEM_EVENT" In reply to: Philippe Mathieu-Daudé: "Re: [PATCH] MIPS: Fix CP0 counter erratum detection for R4k CPUs" Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
