Re: [PATCH] arm64: fpsimd: Added API to manage fpsimd state inside kernel

From: Catalin Marinas <catalin.marinas@arm.com>
To: Wooyeon Kim <wooy88.kim@samsung.com>
Cc: 'Mark Rutland' <mark.rutland@arm.com>,
	'Bhupesh Sharma' <bhsharma@redhat.com>,
	'Julien Grall' <julien.grall@arm.com>,
	'Vincenzo Frascino' <vincenzo.frascino@arm.com>,
	'Will Deacon' <will@kernel.org>,
	yhwan.joo@samsung.com, 'Anisse Astier' <aastier@freebox.fr>,
	'Marc Zyngier' <maz@kernel.org>,
	'Allison Randal' <allison@lohutok.net>,
	'Sanghoon Lee' <shoon114.lee@samsung.com>,
	'Wooki Min' <wooki.min@samsung.com>,
	'Dave Martin' <Dave.Martin@arm.com>,
	'Kees Cook' <keescook@chromium.org>,
	'Suzuki K Poulose' <suzuki.poulose@arm.com>,
	jihun.kim@samsung.com,
	'Kristina Martsenko' <kristina.martsenko@arm.com>,
	'Jeongtae Park' <jtp.park@samsung.com>,
	'Thomas Gleixner' <tglx@linutronix.de>,
	linux-arm-kernel@lists.infradead.org,
	'Steve Capper' <steve.capper@arm.com>,
	'Greg Kroah-Hartman' <gregkh@linuxfoundation.org>,
	linux-kernel@vger.kernel.org, 'James Morse' <james.morse@arm.com>,
	'Sudeep Holla' <sudeep.holla@arm.com>,
	dh.han@samsung.com
Subject: Re: [PATCH] arm64: fpsimd: Added API to manage fpsimd state inside kernel
Date: Thu, 11 Jun 2020 15:11:02 +0100	[thread overview]
Message-ID: <20200611141101.GA31408@gaia> (raw)
In-Reply-To: <001401d63fd4$95646690$c02d33b0$@samsung.com>

On Thu, Jun 11, 2020 at 06:42:12PM +0900, Wooyeon Kim wrote:
> I am in charge of camera driver development in Samsung S.LSI division.
> 
> In order to guarantee real time processing such as Camera 3A algorithm in
> current or ongoing projects, prebuilt binary is loaded and used in kernel
> space, rather than user space.

Thanks for the additional details.

If you do such intensive processing in an IRQ context you'd probably
introduce additional IRQ latency. Wouldn't offloading such work to a
real-time (user) thread help? In a non-preempt-rt kernel, I don't think
you can get much in terms of (soft) guarantees for IRQ latency anyway.

> Because the binary is built with other standard library which could use
> FPSIMD register, kernel API should keep the original FPSIMD state for other
> user tasks.

Can you not recompile those libraries not to use FP?

As Mark said, for a kernel API we require at least an in-kernel,
upstreamed, user of that functionality.

> In the case of the kernel_neon_begin / kernel_neon_end that you mentioned,
> there is a limitation that cannot be used in hardirq context.
> Also, if another kernel task switching occurs while kernel API is being
> used, fpsimd register corruption may occur.

kernel_neon_begin/end disable preemption, so you can't have a task
switch (you can have interrupts though but we don't allow FPSIMD in IRQ
context).

-- 
Catalin

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