linux-kernel - Re: [PATCH v4 2/4] input: apple_z2: Add a driver for Apple Z2 touchscreens

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Message-ID: <CAMT+MTSTtwsoWMPuF9cHt1oHE8geWYZr4t7EU44wq5m3zbSyew@mail.gmail.com>
Date: Thu, 16 Jan 2025 19:41:58 +0100
From: Sasha Finkelstein <fnkl.kernel@...il.com>
To: Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc: devnull+fnkl.kernel.gmail.com@...nel.org, alyssa@...enzweig.io, 
	asahi@...ts.linux.dev, conor+dt@...nel.org, devicetree@...r.kernel.org, 
	dmitry.torokhov@...il.com, j@...nau.net, krzk+dt@...nel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-input@...r.kernel.org, 
	linux-kernel@...r.kernel.org, marcan@...can.st, neal@...pa.dev, 
	robh@...nel.org, rydberg@...math.org, sven@...npeter.dev
Subject: Re: [PATCH v4 2/4] input: apple_z2: Add a driver for Apple Z2 touchscreens

On Thu, 16 Jan 2025 at 19:29, Christophe JAILLET
<christophe.jaillet@...adoo.fr> wrote:
>
> >> +    z2->tx_buf = devm_kzalloc(dev, sizeof(struct
> >> apple_z2_read_interrupt_cmd), GFP_KERNEL);
> >> +    z2->rx_buf = devm_kzalloc(dev, 4096, GFP_KERNEL);
> >
> > This will allocate 8192 bytes because of the way the allocator works.
> > It needs around 40 bytes for the devm stuff + 4096 requested. So
> > rounding rules will allocate 8192 bytes.
> >
> > So either you could allocate "for free" much more space, or you could
> > allocate (and document...)
> >      z2->rx_buf = devm_kzalloc(dev, 4096 - sizeof(struct devres),
> > GFP_KERNEL);
> >
> > or have an explicit devm_add_action_or_reset() that would require less
> > memory, but would add some LoC.
> >
>
> or leave it as-is, of course ;-)
>
> >
> > See https://elixir.bootlin.com/linux/v6.13-rc3/source/drivers/base/
> > devres.c#L97
> >

The precise size does not matter in this case, 4096 was picked semi-arbitrarily
as a "nice-looking" number that is definitely bigger than the maximum
message size.