linux-kernel - Re: [PATCH 1/6] powerpc/time: Rename mftbl() to mftb()

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Message-ID: <20201001204417.GP28786@gate.crashing.org>
Date:   Thu, 1 Oct 2020 15:44:17 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] powerpc/time: Rename mftbl() to mftb()

On Thu, Oct 01, 2020 at 12:42:39PM +0000, Christophe Leroy wrote:
> On PPC64, we have mftb().
> On PPC32, we have mftbl() and an #define mftb() mftbl().
> 
> mftb() and mftbl() are equivalent, their purpose is to read the
> content of SPRN_TRBL, as returned by 'mftb' simplified instruction.
> 
> binutils seems to define 'mftbl' instruction as an equivalent
> of 'mftb'.
> 
> However in both 32 bits and 64 bits documentation, only 'mftb' is
> defined, and when performing a disassembly with objdump, the displayed
> instruction is 'mftb'
> 
> No need to have two ways to do the same thing with different
> names, rename mftbl() to have only mftb().

There are mttbl and mttbu insns (and no mttb insn); they write a 32-bit
half for the time base.  There is an mftb, and an mftbu.  mftbu reads
the upper half, while mftb reads the *whole* register.  SPR 269 is the
TBU register, while SPR 268 is called both TB and TBL.  Yes, it is
confusing :-)

The "mftb" name is much clearer than "mftbl" (on 64-bit), because it
reads the whole 64-bit register.  On 32-bit mftbl is clearer (but not
defined in the architecture, not officially an insn or even an extended
mnemonic).

Segher