linux-kernel - Re: RFC: fake IRQchip

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Date: Sat, 09 Mar 2024 10:03:40 +0000
From: Marc Zyngier <maz@...nel.org>
To: Sebastian Fricke <sebastian.fricke@...labora.com>
Cc: linux-kernel@...r.kernel.org,
	tglx@...utronix.de,
	bartosz.golaszewski@...aro.org
Subject: Re: RFC: fake IRQchip

Hi Sebastian,

On Fri, 08 Mar 2024 14:37:55 +0000,
Sebastian Fricke <sebastian.fricke@...labora.com> wrote:
> 
> Hey,
> 
> I am one of the maintainers of the media subsystem and we are currently
> reviewing a patch [1], where the author has developed a polling
> mechanism for a driver, while the hardware (Wave5 Codec) actually always
> expects an interrupt line to be present and the only reason why this
> isn't uphold is because the SoC has a defect, causing the interrupt line
> to be disabled.
> As I am a bit reluctant to litter a driver with workarounds for defective
> hardware, I suggested to the author, that he could implement fake
> IRQchip, which does polling in the background. This could first be
> implemented in the driver directory and then later possibly upstreamed
> to /drivers/irqchip.
> So, far I've got a few approving comments for that idea, but I would
> really like to know what the irqchip folks think about this.
> 
> Now my question is basically, what do you think about such a solution? Would
> you accept such a fake irqchip driver, that can be used by
> hardware without an interrupt line to fake one? Do you think there is a
> better solution or do you think that my suggestion has hidden traps?

The problem with this approach is that it cannot be a generic irqchip,
because it needs to know about the endpoint device to find out when
the interrupt has been cleared. This is specially true for level
signalling. If the device was only doing edge signalling, I could see
a vague path forward, but that's not the case here (as evidenced by
the DT bindings).

My view on this is that given that the workaround has to know quite a
few things about the generating device, it is better kept close to the
driver code.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.