linux-kernel - Re: [PATCH 2/2] MIPS: make kgdb depend on FPU support

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Message-ID: <20210210113830.xeechzpctz5repv5@maple.lan>
Date:   Wed, 10 Feb 2021 11:38:30 +0000
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     "Maciej W. Rozycki" <macro@...am.me.uk>
Cc:     Arnd Bergmann <arnd@...nel.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Arnd Bergmann <arnd@...db.de>,
        kernel test robot <lkp@...el.com>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        Paul Cercueil <paul@...pouillou.net>,
        Paul Burton <paulburton@...nel.org>,
        linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] MIPS: make kgdb depend on FPU support

On Mon, Feb 08, 2021 at 06:03:08PM +0100, Maciej W. Rozycki wrote:
> On Fri, 22 Jan 2021, Arnd Bergmann wrote:
> 
> > From: Arnd Bergmann <arnd@...db.de>
> > 
> > kgdb fails to build when the FPU support is disabled:
> > 
> > arch/mips/kernel/kgdb.c: In function 'dbg_set_reg':
> > arch/mips/kernel/kgdb.c:147:35: error: 'struct thread_struct' has no member named 'fpu'
> >   147 |    memcpy((void *)&current->thread.fpu.fcr31, mem,
> >       |                                   ^
> > arch/mips/kernel/kgdb.c:155:34: error: 'struct thread_struct' has no member named 'fpu'
> >   155 |   memcpy((void *)&current->thread.fpu.fpr[fp_reg], mem,
> > 
> > This is only relevant for CONFIG_EXPERT=y, so disallowing it
> > in Kconfig is an easier workaround than fixing it properly.
> 
>  Wrapping the relevant parts of this file into #ifdef MIPS_FP_SUPPORT 
> would be as easy though and would qualify as a proper fix given that we 
> have no XML description support for the MIPS target (so we need to supply 
> the inexistent registers in the protocol; or maybe we can return NULL in 
> `dbg_get_reg' to get them padded out in the RSP packet, I haven't checked 
> if generic KGDB code supports this feature).

Returning NULL should be fine.

The generic code will cope OK. The values in the f.p. registers may
act a little odd if gdb uses a 'G' packet to set them to non-zero values
(since kgdb will cache the values gdb sent it) but the developer
operating the debugger will probably figure out what is going on without
too much pain.


Daniel.