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From: Jordan Niethe <jniethe5@gmail.com>
Date: Tue, 24 Mar 2020 14:21:01 +1100
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Subject: Re: [PATCH v4 03/16] powerpc: Use a datatype for instructions
To: Michael Ellerman <mpe@ellerman.id.au>
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Cc: Alistair Popple <alistair@popple.id.au>,
 Nicholas Piggin <npiggin@gmail.com>, Balamuruhan S <bala24@linux.ibm.com>,
 linuxppc-dev@lists.ozlabs.org, Daniel Axtens <dja@axtens.net>
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On Tue, Mar 24, 2020 at 1:58 PM Michael Ellerman <mpe@ellerman.id.au> wrote:
>
> Nicholas Piggin <npiggin@gmail.com> writes:
> > Jordan Niethe's on March 23, 2020 7:28 pm:
> >> On Mon, Mar 23, 2020 at 5:27 PM Nicholas Piggin <npiggin@gmail.com> wrote:
> >>> Jordan Niethe's on March 20, 2020 3:17 pm:
> >>> > Currently unsigned ints are used to represent instructions on powerpc.
> >>> > This has worked well as instructions have always been 4 byte words.
> >>> > However, a future ISA version will introduce some changes to
> >>> > instructions that mean this scheme will no longer work as well. This
> >>> > change is Prefixed Instructions. A prefixed instruction is made up of a
> >>> > word prefix followed by a word suffix to make an 8 byte double word
> >>> > instruction. No matter the endianess of the system the prefix always
> >>> > comes first. Prefixed instructions are only planned for powerpc64.
> >>> >
> >>> > Introduce a ppc_inst type to represent both prefixed and word
> >>> > instructions on powerpc64 while keeping it possible to exclusively have
> >>> > word instructions on powerpc32, A latter patch will expand the type to
> >>> > include prefixed instructions but for now just typedef it to a u32.
> >>> >
> >>> > Later patches will introduce helper functions and macros for
> >>> > manipulating the instructions so that powerpc64 and powerpc32 might
> >>> > maintain separate type definitions.
> >>>
> >>> ppc_inst_t I would slightly prefer for a typedef like this.
> >> Are _t types meant to be reserved?
> >
> > No, just convention that structs are not normally typedefed unless
> > they are a pervasive interface that gets passed around a lot but
> > does not get accessed without accessor functions much. When you do
> > typedef them, add a _t (or less frequently _s/_u/etc). pte_t,
> > cpumask_t, atomic_t.
>
> Ideally we wouldn't use a typedef, we'd just have:
>
> struct ppc_inst {
>         u32 val;
> #ifdef CONFIG_PPC64
>         u32 suffix;
> #endif
> };
>
> That may make the conversion harder though, because you more or less
> have to update all usages at once.
Okay I will give that a try.
>
> cheers