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Date: Fri, 2 May 2025 10:04:27 +0200
From: Lorenzo Pieralisi <lpieralisi@kernel.org>
To: Marc Zyngier <maz@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>, Rob Herring <robh@kernel.org>,
	Krzysztof Kozlowski <krzk+dt@kernel.org>,
	Conor Dooley <conor+dt@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	Sascha Bischoff <sascha.bischoff@arm.com>,
	Timothy Hayes <timothy.hayes@arm.com>,
	"Liam R. Howlett" <Liam.Howlett@oracle.com>,
	Mark Rutland <mark.rutland@arm.com>,
	linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
	devicetree@vger.kernel.org
Subject: Re: [PATCH v2 21/22] irqchip/gic-v5: Add GICv5 IWB support
Message-ID: <aBR8i56xO6uNCt/G@lpieralisi>
References: <20250424-gicv5-host-v2-0-545edcaf012b@kernel.org>
 <20250424-gicv5-host-v2-21-545edcaf012b@kernel.org>
 <867c31j20i.wl-maz@kernel.org>
 <aBJO9GEyb+0t6W6u@lpieralisi>
 <86tt64h0x9.wl-maz@kernel.org>
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In-Reply-To: <86tt64h0x9.wl-maz@kernel.org>

On Thu, May 01, 2025 at 03:15:46PM +0100, Marc Zyngier wrote:

[...]

> > If we are, we use the msi_alloc_info_t->hwirq to define the LPI eventid,
> > basically the IWB wire, if not we just allocate an eventid available from
> > the device bitmap.
> > 
> > Other than that (and being forced to provide an IWB irqchip.irq_write_msi_msg()
> > pointer even if the IWB can't write anything otherwise we dereference
> > NULL) this works.
> 
> Not even MBIGEN allows you to change the event. If you really want to
> ensure things are even tighter, invent a MSI_FLAG_HARDCODED_MSG flag,
> and pass that down the prepare path.

I tried to set a new alloc flag in the IWB msi_domain_template.ops.set_desc()
callback and it works.

It can be set in the IWB driver (and does not change anything else),
it works so happy days.

> > Is there a better way to implement this ? I would post this code with
> > v3 but instead of waiting I thought I could inline it here, feel free
> > to ignore it (or flame me if it is a solved problem I failed to spot,
> > we need to find a way for the IWB driver to pass the "fixed event" info
> > to the ITS - IWB eventIDs are hardwired it is not like the MBIgen where
> > the irq_write_msi_msg() callback programs the wire-to-eventid
> > translation in HW).
> 
> It's *exactly* the same. And see above for a potential explicit
> solution. The empty irq_write_msi_msg() is not a problem. It's
> actually pretty clean, given how the whole thing works.
> 
> Please fold this into your v3, and we'll take it from there.

I will with the new alloc flag above, thanks.

Lorenzo