linux-kernel - Re: [PATCH v3 00/10] gpiolib: Handle immutable irq

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Date:   Thu, 12 May 2022 20:08:28 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        Thierry Reding <thierry.reding@...il.com>,
        Joey Gouly <joey.gouly@....com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Hector Martin <marcan@...can.st>,
        Sven Peter <sven@...npeter.dev>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Andy Gross <agross@...nel.org>,
        Jeffrey Hugo <jeffrey.l.hugo@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Basavaraj Natikar <Basavaraj.Natikar@....com>,
        Shyam Sundar S K <Shyam-sundar.S-k@....com>,
        linux-gpio@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-arm-msm@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v3 00/10] gpiolib: Handle immutable irq_chip structures

On Tue, Apr 19, 2022 at 03:18:36PM +0100, Marc Zyngier wrote:
> This is a followup from [2].
> 
> I recently realised that the gpiolib play ugly tricks on the
> unsuspecting irq_chip structures by patching the callbacks.
> 
> Not only this breaks when an irq_chip structure is made const (which
> really should be the default case), but it also forces this structure
> to be copied at nauseam for each instance of the GPIO block, which is
> a waste of memory.

Is this brings us to the issue with IRQ chip name?

The use case in my mind is the following:
1) we have two or more GPIO chips that supports IRQ;
2) the user registers two IRQs of the same (by number) pin on different chips;
3) cat /proc/interrupt will show 'my_gpio_chip XX', where XX is the number.

So, do I understand correct current state of affairs?

If so, we have to fix this to have any kind of ID added to the chip name that
we can map /proc/interrupts output correctly.

> My current approach is to add a new irq_chip flag (IRQCHIP_IMMUTABLE)
> which does what it says on the tin: don't you dare writing to them.
> Gpiolib is further updated not to install its own callbacks, and it
> becomes the responsibility of the driver to call into the gpiolib when
> required. This is similar to what we do for other subsystems such as
> PCI-MSI.
> 
> 5 drivers are updated to this new model: M1, QC, Tegra, pl061 and AMD
> (as I actively use them) keeping a single irq_chip structure, marking
> it const, and exposing the new flag.
> 
> Nothing breaks, the volume of change is small, the memory usage goes
> down and we have fewer callbacks that can be used as attack vectors.
> What's not to love?
> 
> Since there wasn't any objection in the previous round of review, I'm
> going to take this series into -next to see if anything breaks at
> scale.


-- 
With Best Regards,
Andy Shevchenko