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

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

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