linux-kernel - Re: [PATCH] MIPS: BMIPS: Disable pref 30 for buggy CPUs

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Date:   Wed, 5 Aug 2020 03:06:36 +0100 (BST)
From:   "Maciej W. Rozycki" <macro@....com>
To:     Florian Fainelli <f.fainelli@...il.com>
cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        linux-mips@...ux-mips.org,
        "open list:BROADCOM BMIPS MIPS ARCHITECTURE" 
        <bcm-kernel-feedback-list@...adcom.com>,
        "open list:BROADCOM BMIPS MIPS ARCHITECTURE" 
        <linux-mips@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "Maciej W. Rozycki" <macro@...ux-mips.org>
Subject: Re: [PATCH] MIPS: BMIPS: Disable pref 30 for buggy CPUs

On Mon, 3 Aug 2020, Florian Fainelli wrote:

> >>>> diff --git a/arch/mips/bmips/setup.c b/arch/mips/bmips/setup.c
> >>>> index 19308df5f577..df0efea12611 100644
> >>>> --- a/arch/mips/bmips/setup.c
> >>>> +++ b/arch/mips/bmips/setup.c
> >>>> @@ -110,6 +110,20 @@ static void bcm6368_quirks(void)
> >>>>       bcm63xx_fixup_cpu1();
> >>>>   }
> >>>>   +static void bmips5000_pref30_quirk(void)
> >>>> +{
> >>>> +    __asm__ __volatile__(
> >>>> +    "    li    $8, 0x5a455048\n"
> >>>> +    "    .word    0x4088b00f\n"    /* mtc0 $8, $22, 15 */
> >>>> +    "    nop; nop; nop\n"
> >>>> +    "    .word    0x4008b008\n"    /* mfc0 $8, $22, 8 */
> >>>> +    /* disable "pref 30" on buggy CPUs */
> >>>> +    "    lui    $9, 0x0800\n"
> >>>> +    "    or    $8, $9\n"
> >>>> +    "    .word    0x4088b008\n"    /* mtc0 $8, $22, 8 */
> >>>> +    : : : "$8", "$9");
> >>>> +}
> >>> Hi,
> >>>
> >>> Is there any toolchain issue blocking read_c0_**** family helpers being
> >>> used?
> >>>
> >>> Use .word looks unreasonable.
> >>
> >> Yes, the assembler would be choking on the custom $22 selector, however
> > 
> > I guess you meant selector 8 and 15. If BMIPS has a 4 bit selector field
> > it might be good to do a binutils patch supporting it.
> 
> Yes, sorry that is what I meant. I don't think an assembler patch makes
> sense at this point given this is an isolated use, and there is not just
> binutils these days, the Clang/LLVM integrated assembler would also need
> to be supported, and then we would need to have the kernel say: I
> require this minimum version to support the customer selectors, not
> worth the trouble if you ask me.

 Well, I asked for a GAS patch to add support 4.5 years ago, so by now and 
9 binutils releases later it would have become fairly common.  And then I 
also suggested how to handle it in a robust way.  Cf. 
<https://www.linux-mips.org/cgi-bin/mesg.cgi?a=linux-mips&i=alpine.DEB.2.00.1602092245180.15885%40tp.orcam.me.uk>.

  Maciej