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From:   Alexander Stein <alexander.stein@ew.tq-group.com>
To:     Linus Walleij <linus.walleij@linaro.org>
Cc:     Matthias Schiffer <matthias.schiffer@ew.tq-group.com>,
        linux-arm-kernel@lists.infradead.org,
        Mark Brown <broonie@kernel.org>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Conor Dooley <conor+dt@kernel.org>, linux-spi@vger.kernel.org,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux@ew.tq-group.com
Subject: Re: [PATCH 1/2] spi: dt-bindings: introduce linux,use-rt-queue flag
Date:   Fri, 09 Jun 2023 11:13:39 +0200
Message-ID: <4784636.LvFx2qVVIh@steina-w>
Organization: TQ-Systems GmbH
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References: <20230602115201.415718-1-matthias.schiffer@ew.tq-group.com> <3241150.44csPzL39Z@steina-w> <CACRpkda2yQefuUMn6Ot8Ns6nNgfC25i9nwmzwxOy5fUg=A44hQ@mail.gmail.com>
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Am Freitag, 9. Juni 2023, 10:42:04 CEST schrieb Linus Walleij:
> On Fri, Jun 9, 2023 at 10:15=E2=80=AFAM Alexander Stein
>=20
> <alexander.stein@ew.tq-group.com> wrote:
> > > A agree with Mark that a command line option isn't that bad. It's
> > > something
> > > that pertains to just the Linux kernel after all? And you can put that
> > > command line option in the default device tree, in chosen, if you wan=
t.
> >=20
> > I don't like the idea of a command line enabling realtime scheduling for
> > all instances of the SPI controller driver or even all SPI controllers.
> > Actually this might be worse if a non-rt SPI bus is considered for RT
> > scheduling. IMHO this should be configurable per SPI controller,
>=20
> OK that's a fair point.
>=20
> I don't think command line arguments are necessarily global by
> nature, AFAIK it's fine to invent something like pl022.4.rt_sched=3D1
> where 4 is the instance number. Parsing it is just code.

Now we are touching the topic of non-deterministic device names/numbers. Th=
is=20
gets worse if your SPI controller is attached to some other device, althoug=
h=20
RT capabilities are rather limited in that case anyway.

> > e.g. a sysfs attribute.
>=20
> But it needs to be set before userspace is up :/

Does it? IMHO a realtime system is allowed to use blocking mechanism (e.g.=
=20
dynamic memory allocatin and so on) during startup/configuration phase,=20
ignoring any deadlines.
Once it starts operating this is a no-go.
This seems rather similar to configure scheduling priority for IRQ threads =
on=20
RT preempt systems. IIRC according to RT folks, this is considered an=20
administration task, thus the responsibility of userspace.

> I fully sympathize with this problem, because I have faced
> similar problems myself.

You mean RT-scheduling before userspace is up? Can you elaborate the issues=
=20
you see?

> My fallback solution for this driver would be to keep using the
> old DT property (which was merged when reviewing was
> not as strict) if that works, or use undocumented DT properties,
> it's not the end of the world but does leave the bad taste of
> a work not finished.

I was surprised to see the driver specific property for configuring RT sche=
d=20
as well. I assume the intention of this series is to support this feature f=
or=20
other SPI controller drivers as well. So some kind of feature has to be add=
ed=20
anyway.

Best regards,
Alexander

> Yours,
> Linus Walleij


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