linux-kernel - Re: [PATCH] genirq: allow selection of number of sparse irqs

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Date:   Tue, 2 Aug 2022 15:37:47 -0700
From:   Daniel Walker <danielwa@...co.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        George Cherian <george.cherian@...vell.com>,
        sgoutham@...vell.com, "BOBBY Liu (bobbliu)" <bobbliu@...co.com>,
        xe-linux-external@...co.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] genirq: allow selection of number of sparse irqs

On Sat, Jul 30, 2022 at 10:59:05AM +0100, Marc Zyngier wrote:
> > 
> > Marvell submitted a similar change, but non-selectable, about a
> > month ago.
> 
> Which wasn't really acceptable either.
> 
> > 
> > The limitation prevents Cisco and Marvell hardware from
> > functioning. I don't think we're well versed enough on the generic
> > irq system to implement what your suggesting, even if we did Thomas
> > would not likely accept it.
> 
> I don't think you can speak for Thomas here. In my experience of
> working with him, he's in general much more inclined to look at a
> scalable, long term solution than at a point hack. Specially given
> that we already use xarrays for MSIs.
 
Your welcome make the attempt yourself, if you believe in it.

> > Your suggestion is more of a long term solution vs. our short term
> > solution.
> 
> Exactly. Experience shows that short term hacks are almost always a
> bad idea and result in something that isn't maintainable.

Thomas introduced the "hack" in c1ee626 in 2011.

It's more of a question of if someone has the time an and/or inclination to make the changes
your requesting.

Marvell and Cisco only require to increase the size and keep the status quo, and
nothing is wrong with that.

> > I'm not wedded to any solution, we just need to relieve
> > the limitation so our hardware starts working. I would imagine other
> > companies have this issue, but I don't know which ones currently.
> 
> This architecture has been in the wild for the best part of 10 years,
> in Linux for 8 years, and nobody so far screamed because of this
> perceived limitation. It would help if you described exactly what
> breaks in your system, because just saying "give me more" is not
> exactly helping (there are other limitations in the GICv3 ITS driver
> that may bite you anyway).

We need more irq lines because we have a lot of devices.. I suppose it's
possible there's some defect in the kernel which eats up or wastes irq lines,
but I don't think so. We have devices which use a lot of irq lines.

> > I would rather to use an upstream solution verses holding the
> > patches privately.  I would suggest if this limitation would not be
> > overcome for 3-4 releases the short term solution should be
> > acceptable over that time frame to be replaced by something else
> > after that.
> 
> If you want to have an impact on the features being merged in the
> upstream kernel, a good start would be to take feedback on board.

We did that.. I updated the patch from Marvell's original to allow it to be
selectable, this was requested by someone on this list.


Daniel