Re: [PATCH v3 00/10] gpiolib: Handle immutable irq_chip structures

From: Marc Zyngier <maz@kernel.org>
To: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: linux-kernel@vger.kernel.org,
	Linus Walleij <linus.walleij@linaro.org>,
	Bartosz Golaszewski <brgl@bgdev.pl>,
	Thierry Reding <thierry.reding@gmail.com>,
	Joey Gouly <joey.gouly@arm.com>,
	Jonathan Hunter <jonathanh@nvidia.com>,
	Hector Martin <marcan@marcan.st>, Sven Peter <sven@svenpeter.dev>,
	Alyssa Rosenzweig <alyssa@rosenzweig.io>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	Andy Gross <agross@kernel.org>,
	Jeffrey Hugo <jeffrey.l.hugo@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Basavaraj Natikar <Basavaraj.Natikar@amd.com>,
	Shyam Sundar S K <Shyam-sundar.S-k@amd.com>,
	linux-gpio@vger.kernel.org, linux-tegra@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-arm-msm@vger.kernel.org, kernel-team@android.com
Subject: Re: [PATCH v3 00/10] gpiolib: Handle immutable irq_chip structures
Date: Thu, 12 May 2022 23:15:11 +0100	[thread overview]
Message-ID: <87v8uamm8g.wl-maz@kernel.org> (raw)
In-Reply-To: <Yn0/DIl3+i/heRH6@smile.fi.intel.com>

On Thu, 12 May 2022 18:08:28 +0100,
Andy Shevchenko <andy.shevchenko@gmail.com> wrote:
> 
> 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.

/proc/interrupts isn't a dumping ground for debug information. Yes,
some irqchips do that, and they have been fixed by providing the
irq_print_chip callback, thus ensuring that the irq_chip structure is
never written to. I would have loved to simply get rid of the variable
string, but this is obviously ABI, and we can't break that.

> 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.

This is already done.

That's not an excuse to add more of those though. We already have the
required infrastructure in debugfs, and that's where this sort of
thing should happen.

	M.

-- 
Without deviation from the norm, progress is not possible.