linux-kernel - Re: [PATCH v2 11/20] mips: MAAR: Use more precise address mask

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Date:   Thu, 7 May 2020 22:13:37 +0300
From:   Serge Semin <Sergey.Semin@...kalelectronics.ru>
To:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>
CC:     Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Paul Burton <paulburton@...nel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Arnd Bergmann <arnd@...db.de>,
        Rob Herring <robh+dt@...nel.org>, <linux-pm@...r.kernel.org>,
        <devicetree@...r.kernel.org>, Zhou Yanjie <zhouyanjie@...o.com>,
        Paul Cercueil <paul@...pouillou.net>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        <linux-mips@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 11/20] mips: MAAR: Use more precise address mask

On Thu, May 07, 2020 at 01:09:51PM +0200, Thomas Bogendoerfer wrote:
> On Wed, May 06, 2020 at 08:42:29PM +0300, Sergey.Semin@...kalelectronics.ru wrote:
> > From: Serge Semin <Sergey.Semin@...kalelectronics.ru>
> > 
> > Indeed according to the P5600/P6000 manual the MAAR pair register
> > address field either takes [12:31] bits for 32-bits non-XPA systems
> > and [12:35] otherwise. In any case the current address mask is just
> > wrong for 64-bit and 32-bits XPA chips. So lets extend it to 39-bits
> > value. This shall cover the 64-bits architecture and systems with XPA
> > enabled, and won't cause any problem for non-XPA 32-bit systems, since
> > the value will be just truncated when written to the 32-bits register.
> 
> according to MIPS32 Priveleged Resoure Architecture Rev. 6.02
> ADDR spans from bit 12 to bit 55. So your patch fits only for P5600.

> Does the wider mask cause any problems ?

No, it won't. Bits written to the [40:62] range will be just ignored,
while reading from there should return zeros. Setting GENMASK_ULL(55, 12)
would also work. Though this solution is a bit workarounding because
MIPS_MAAR_ADDR wouldn't reflect the real mask of the ADDR field. Something
like the next macro would work better:

+#define MIPS_MAAR_ADDR							\
+({									\
+	u64 __mask;							\
+									\
+	if (cpu_has_lpa && read_c0_pagegrain() & PG_ELPA) {		\
+		__mask = GENMASK_ULL(55, 12);				\
+	else								\
+		__mask = GENMASK_ULL(31, 12);				\
+									\
+	__mask;								\
+})

What do you think? What is better: the macro above or setting
GENMASK_ULL(55, 12)?

BTW I've just figured out, that since XPA is currently only supported by
kernels with CPU_MIPS32x config enabled, then only MIPS32 may have extended
physical addressing of 2^60 bytes if CONFIG_XPA is enabled. Generic MIPS64
doesn't support the extended phys addressing so only 2^36 bytes are available
on such platforms. (Loongson64 doesn't count, the platform code sets the
PG_ELPA bit manually in kernel-entry-init.h)

-Sergey

> 
> Thomas.
> 
> -- 
> Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
> good idea.                                                [ RFC1925, 2.3 ]