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Date:   Wed, 10 Feb 2021 11:38:30 +0000
From:   Daniel Thompson <daniel.thompson@linaro.org>
To:     "Maciej W. Rozycki" <macro@orcam.me.uk>
Cc:     Arnd Bergmann <arnd@kernel.org>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Arnd Bergmann <arnd@arndb.de>,
        kernel test robot <lkp@intel.com>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        Paul Cercueil <paul@crapouillou.net>,
        Paul Burton <paulburton@kernel.org>,
        linux-mips@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] MIPS: make kgdb depend on FPU support
Message-ID: <20210210113830.xeechzpctz5repv5@maple.lan>
References: <20210122110307.934543-1-arnd@kernel.org>
 <20210122110307.934543-2-arnd@kernel.org>
 <alpine.DEB.2.21.2102081748280.35623@angie.orcam.me.uk>
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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@arndb.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.