linux-kernel - Re: [PATCH 20/24] irqchip/gic-v5: Add GICv5 LPI/IPI support

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Message-ID: <Z/4T7lpXSFgomIai@lpieralisi>
Date: Tue, 15 Apr 2025 10:08:14 +0200
From: Lorenzo Pieralisi <lpieralisi@...nel.org>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Marc Zyngier <maz@...nel.org>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Sascha Bischoff <sascha.bischoff@....com>,
	Timothy Hayes <timothy.hayes@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org, ast@...nel.org
Subject: Re: [PATCH 20/24] irqchip/gic-v5: Add GICv5 LPI/IPI support

On Mon, Apr 14, 2025 at 10:37:57AM -0400, Liam R. Howlett wrote:

[...]

> > I can go for an IDA unless someone see a point in pursuing the current
> > approach - that I would update according to feedback, at least with
> > this thread you get the full picture.
> 
> Eventually, we plan to swap out the backing of the IDA to use the maple
> tree.  I think we could add range support as part of this change.
> 
> If you were to add an interface to contiguous allocate a number of IDAs
> by putting your loop in the IDA interface, we'd be able to switch that
> to a range store during the conversion and keep the use case visible
> during the planning stages.
> 
> Having that interface would make it obvious the change is necessary and
> wouldn't be missed.

Yep understood but on the other hand it would force me to allocate a set
of contiguous IDs, which is a bit cumbersome with the current IDA
(alloc them one by one - if any fails to be contiguous restart, rince,
repeat - it is a double whammy).

I allocate a range because I know the maple tree handle them efficiently
and to be honest, a range allocator is all I need, there isn't any in
the core kernel (there are plenty - on purpose - range allocators, BPF
range tree and GICv3 ITS LPIs allocator are good examples, I could reuse
them instead of reinventing the wheel).

FWIW, when Alexei implemented the BPF arena he vetted other examples.

https://lore.kernel.org/bpf/20241105212001.38980-1-alexei.starovoitov@gmail.com/

I don't think the code I wrote is that complex either and it minimizes
entry storage waste - probably you don't want to see it in the kernel
because it is not a MT usage you'd expect - storage wise, I need to
measure it but I think it definitely requires fewer bytes than a
preallocated bitmap - when the IDA range API is in I would be glad
to swap to it.

> Unfortunately, I don't really have a timeline on changing the IDA to the
> maple tree.

I understand, it is a chicken and egg situation, see above.

> Please keep me Cc'ed on whatever you decide.

I'd need something promptly, so I will go for a simple solution (bitmap
or IDA 1-by-1) if the current MT one has to be scrapped.

Thank you,
Lorenzo