netdev - Re: [PATCH net-next V2] net/mlx5: Improve write-combining test reliability for ARM64 Grace CPUs

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Message-ID: <20250915224810.GM1024672@nvidia.com>
Date: Mon, 15 Sep 2025 19:48:10 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Tariq Toukan <tariqt@...dia.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Saeed Mahameed <saeedm@...dia.com>,
	Leon Romanovsky <leon@...nel.org>, Mark Bloch <mbloch@...dia.com>,
	Sabrina Dubroca <sd@...asysnail.net>, netdev@...r.kernel.org,
	linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
	Gal Pressman <gal@...dia.com>, Leon Romanovsky <leonro@...dia.com>,
	Michael Guralnik <michaelgur@...dia.com>,
	Moshe Shemesh <moshe@...dia.com>, Will Deacon <will@...nel.org>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
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	Dave Hansen <dave.hansen@...ux.intel.com>,
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	Justin Stitt <justinstitt@...gle.com>, linux-s390@...r.kernel.org,
	llvm@...ts.linux.dev, Ingo Molnar <mingo@...hat.com>,
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	Yisen Zhuang <yisen.zhuang@...wei.com>,
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	Leon Romanovsky <leonro@...lanox.com>, linux-arch@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Mark Rutland <mark.rutland@....com>,
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	Niklas Schnelle <schnelle@...ux.ibm.com>,
	Jijie Shao <shaojijie@...wei.com>,
	Patrisious Haddad <phaddad@...dia.com>
Subject: Re: [PATCH net-next V2] net/mlx5: Improve write-combining test
 reliability for ARM64 Grace CPUs

On Mon, Sep 15, 2025 at 03:27:58PM -0700, Nathan Chancellor wrote:
> On Mon, Sep 15, 2025 at 03:18:59PM -0700, Nathan Chancellor wrote:
> > On Mon, Sep 15, 2025 at 11:35:08AM +0300, Tariq Toukan wrote:
> > ...
> > > diff --git a/drivers/net/ethernet/mellanox/mlx5/core/Makefile b/drivers/net/ethernet/mellanox/mlx5/core/Makefile
> > > index d77696f46eb5..06d0eb190816 100644
> > > --- a/drivers/net/ethernet/mellanox/mlx5/core/Makefile
> > > +++ b/drivers/net/ethernet/mellanox/mlx5/core/Makefile
> > > @@ -176,3 +176,9 @@ mlx5_core-$(CONFIG_PCIE_TPH) += lib/st.o
> > >  
> > >  obj-$(CONFIG_MLX5_DPLL) += mlx5_dpll.o
> > >  mlx5_dpll-y :=	dpll.o
> > > +
> > > +#
> > > +# NEON WC specific for mlx5
> > > +#
> > > +mlx5_core-$(CONFIG_KERNEL_MODE_NEON) += lib/wc_neon_iowrite64_copy.o
> > > +FLAGS_lib/wc_neon_iowrite64_copy.o += $(CC_FLAGS_FPU)
> > 
> > Does this work as is? I think this needs to be CFLAGS instead of FLAGS
> > but I did not test to verify.
> 
> Also, Documentation/core-api/floating-point.rst states that code should
> also use CFLAGS_REMOVE_ for CC_FLAGS_NO_FPU as well as adding
> CC_FLAGS_FPU.
> 
>   CFLAGS_REMOVE_lib/wc_neon_iowrite64_copy.o += $(CC_FLAGS_NO_FPU)

I wondered if you needed the seperate compilation unit at all since it
it all done with inline assembly.. Since the makefile seems to have a
typo, it suggests you don't need the compilation unit and it could
just be a little inline protected by CONFIG_KERNEL_MODE_NEON.

Jason