linux-kernel - Re: [PATCH -next 0/4] spi: Introduce BPF based SPI mockup controller

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Date:   Tue, 30 Aug 2022 20:08:56 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Wei Yongjun <weiyongjun1@...wei.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-spi@...r.kernel.org
Subject: Re: [PATCH -next 0/4] spi: Introduce BPF based SPI mockup controller

On Fri, Aug 26, 2022 at 02:43:37PM +0000, Wei Yongjun wrote:
> This series Introduce a BPF based SPI mockup controller, which can be
> used for testing SPI drivers without real device.
> 
> The testsuite for SPI drivers will be post in other series, with also
> support for I2C and PCI bus device drivers.

I replied to some of the individual patches here with some more
detailed issues but I do have some big picture reservations with
this approach as well.  One big one is that this seems like it's
only going to be able to handle emulation of devices that are
purely synchronous - I don't see any sensible direction here for
extending to devices that need an interrupt line as well.  That
seems like a major limitation.  It's fine not to immediately
implement everything but it should be fairly clear how that would
be done when someone needs it and some of the BPF design goals I
understood seem to conflict with that.

I'm also not clear what the BPF environment is like when it comes
to extensible frameworks, as soon as you start thinking about the
problem space there are some obvious extensions with things like
more detailed register map descriptions and validating that the
operations that the driver is doing to the device are actually in
spec for the device or trying to stimulate race conditions or
error paths in the driver.

There's also the issue with getting the BPF userspace tooling
that I mentioned in reply to one of the individual patches.

Basically while this does look very simple from a kernel point of
view I worry that BPF might not be the right tool for the job and
could be a bit of a blind alley, with people rapidly running into
issues trying to do anything non-trivial.

There was a series in the past year or so from someone
implementing a similarish idea but rather than using BPF they
provided a framework for writing device emulations in qemu with
a simplified Python framework.  That seemed like a more obvious
direction here.  They were initially focusing on I2C but it's a
similar space.  Unfortunately I'm drawing a blank on who was
working on it or the name of the framework so I can't give a
useful reference here.  Their direction was towards having
something that could also be used to validate what the driver was
doing to the device from the device's perspective.  I do know it
was written up on LWN.  Hopefully someone else will remember what
I'm talking about from this vauge description.

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