linux-kernel - Re: [PATCH] irqchip/xilinx: Expose Kconfig option

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Date:   Fri, 16 Apr 2021 18:53:33 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Robert Hancock <robert.hancock@...ian.com>
Cc:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "michal.simek@...inx.com" <michal.simek@...inx.com>
Subject: Re: [PATCH] irqchip/xilinx: Expose Kconfig option

On Fri, 16 Apr 2021 17:05:49 +0100,
Robert Hancock <robert.hancock@...ian.com> wrote:
> 
> On Fri, 2021-04-16 at 14:41 +0100, Marc Zyngier wrote:
> > On Fri, 16 Apr 2021 00:32:50 +0100,
> > Robert Hancock <robert.hancock@...ian.com> wrote:
> > > Previously the XILINX_INTC config option was hidden and only
> > > auto-selected on the MicroBlaze platform. However, this IP can also be
> > > used on other platforms. Allow this option to be user-enabled.
> > > 
> > > Signed-off-by: Robert Hancock <robert.hancock@...ian.com>
> > 
> > I don't think this is a good idea. In general, people have no idea
> > which interrupt controller they need to select. So you either end-up
> > with a missing interrupt controller, or a bunch you really don't need.
> > 
> > This is essentially a platform constraint, and this should directly be
> > selected by the platform if you have this IP in your system.
> > 
> > Thanks,
> > 
> > 	M.
> 
> The problem is essentially that at the platform level, we don't know, other
> than in the MicroBlaze case where we know it will be used as the platform's
> primary interrupt controller. In our case, we are using this IP core on the
> ZynqMP platform as a secondary cascaded interrupt controller in the FPGA
> portion of the device.
> But many ZynqMP configurations wouldn't have this device present, it
> depends on what the user instantiates in the programmable logic.
> Also, this core could just as easily be instantiated in standalone
> Xilinx FPGAs which could be connected to many different platforms
> over a PCIe, AXI, etc.  bus.

Not compiling it for some users is great if you happen to *know* what
you have to select, which is probably a single digit percentage of the
people that build their own kernel. At least having it to depend on
ZYNQMP (or some other FPGA platform) would narrow it down.

And if you have some other HW in your FPGA, you can make the config
fragment for this HW select the right interrupt controller. But I'm
definitely not keen on making this a universally user-selectable
driver.

> So I don't think having this as a platform constraint makes sense.

I don't think imposing this on *everyone*, across all supported
architectures and platforms makes any sense. Surely, people who build
their own HW (because that's what we are talking about here) can be
bothered to add the small Kconfig fragment that is required to their
kernel build.

	M.

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