Re: [PATCH v5 0/4] Per-parent domains for realtek-rtl IRQ driver

From: Sander Vanheule <sander@svanheule.net>
To: Marc Zyngier <maz@kernel.org>
Cc: Rob Herring <robh+dt@kernel.org>,
	devicetree@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
	Birger Koblitz <mail@birger-koblitz.de>,
	Bert Vermeulen <bert@biot.com>, John Crispin <john@phrozen.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 0/4] Per-parent domains for realtek-rtl IRQ driver
Date: Mon, 28 Feb 2022 13:57:13 +0100	[thread overview]
Message-ID: <4a8dac0dc296920c6e58584d48d6eaf6644b0322.camel@svanheule.net> (raw)
In-Reply-To: <87czjo49ht.wl-maz@kernel.org>

Hi Marc,

On Tue, 2022-02-15 at 12:09 +0000, Marc Zyngier wrote:
> On Mon, 14 Feb 2022 18:56:57 +0000,
> Sander Vanheule <sander@svanheule.net> wrote:
> > 
> > The original implementation for this interrupt controller/router used
> > an interrupt-map parser to determine which parent interrupts were
> > present. However, this controller is not transparent, so a list of
> > parent interrupts seems more appropriate, while also getting rid of the
> > assumed routing to parent interrupts.
> > 
> > Additionally, N real cascaded interrupts are implemented, instead of
> > handling all input interrupts with one cascaded interrupt. Otherwise it
> > is possible that the priority of the parent interrupts is not respected.
> 
> My original question[1] still stands. An old kernel breaks with a new
> DT. I am not convinced that this is an acceptable outcome.
> 
>         M.
> 
> [1] https://lore.kernel.org/all/874k585efy.wl-maz@kernel.org

My apologies for the delay in replying, although I suppose the lack of response
from others perhaps indicates that there is little interest maintaining old
kernel/new DT compatibility for this hardware. John has previously argued in
favour of breaking compatibility [2].

Chances of someone running a vanilla kernel build on this hardware are close to
zero at this moment. The most important part, the internal ethernet switch, is
only supported with out-of-tree patches. If patches can be included on an old
(LTS) kernel to provide networking support, then patches can be included to be
compatible with a new DT specification for the interrupts as well. OpenWrt does
exactly this: use an old (5.10) kernel with new upstream features backported.

The binding could be adjusted to allow (but deprecate) interrupt-map for the new
two-part compatibles. This would require a new DT to both specify two-cell
interrupt specifiers, and an equivalent interrupt-map definition to ensure
perfect two-way compatibility. This duplicated info would need to be maintained
for years however, as LTS kernels stay around for a long time. In my opinion,
breaking compatibility with old kernels would allow us to move forward with a
cleaner driver and binding.

Best,
Sander

[2]
https://lore.kernel.org/all/9c169aad-3c7b-2ffb-90a2-1ca791a3f411@phrozen.org/