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

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Message-ID: <20200508092236.GA9085@alpha.franken.de>
Date:   Fri, 8 May 2020 11:22:36 +0200
From:   Thomas Bogendoerfer <tsbogend@...ha.franken.de>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
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 10:13:37PM +0300, Serge Semin wrote:
> 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;								\
> +})

that looks horrible.

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

just that one ;-)

Thomas.

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