linux-kernel - Re: [PATCH RFC v2 1/3] tty: serial core: provide method to search uart by phandle

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Date:	Tue, 30 Jun 2015 11:09:10 +0300
From:	Sergei Zviagintsev <sergei@...v.net>
To:	"Dr. H. Nikolaus Schaller" <hns@...delico.com>
Cc:	Marek Belisko <marek@...delico.com>, Jiri Slaby <jslaby@...e.cz>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	Peter Hurley <peter@...leysoftware.com>,
	Mark Rutland <mark.rutland@....com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Sebastian Reichel <sre@...nel.org>,
	NeilBrown <neil@...wn.name>,
	Grant Likely <grant.likely@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-serial@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>
Subject: Re: [PATCH RFC v2 1/3] tty: serial core: provide method to search
 uart by phandle

Hi,

On Mon, Jun 29, 2015 at 06:44:23PM +0200, Dr. H. Nikolaus Schaller wrote:
[...]
> >> +	list_for_each_entry(uart, &uart_list, head) {
> >> +		if (node != uart->dev->of_node)
> >> +			continue;
> >> +
> >> +		return uart;
> > 
> > We can easily save three lines here :)
> 
> Hm. We have copied from here:
> 
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/usb/phy/phy.c?id=refs/tags/v4.1#n65
> 
> So please let us know how you want to save 3 lines.

Sorry for not being specific, I just meant that it could be written as

if (node == uart->dev->of_node)
	return uart;

> >> +/**
> >> + * devm_serial_get_uart_by_phandle - find the uart by phandle
> >> + * @dev - device that requests this uart
> >> + * @phandle - name of the property holding the uart phandle value
> >> + * @index - the index of the uart
> >> + *
> >> + * Returns the uart_port associated with the given phandle value,
> >> + * after getting a refcount to it, -ENODEV if there is no such uart or
> >> + * -EPROBE_DEFER if there is a phandle to the uart, but the device is
> >> + * not yet loaded. While at that, it also associates the device with
> >> + * the uart using devres. On driver detach, release function is invoked
> >> + * on the devres data, then, devres data is freed.
> > 
> > Add -ENOMEM and -EINVAL, remove -EPROBE_DEFER?
> 
> Well, if the device is not loaded it means the caller must return -EPROBE_DEFER
> anyways since it can’t complete it’s probe function.

That was my mistake, from the first sight I hadn't found where
-EPROBE_DEFER is actually returned from the code and thus decided that
kernel-doc has an error. But now I see it, thanks.

> >> +
> >> +	*ptr = uart;
> >> +	devres_add(dev, ptr);
> > 
> > What is the point of assigning value to *ptr?
> 
> Good question. I think it is necessary to store a copy of the found uart/phy instead of a reference.
> Therefore the assignment to *ptr copies into the new memory area allocated above by 
> 
> ptr = devres_alloc(devm_serial_uart_release, sizeof(*ptr), GFP_KERNEL);
> 
> This makes the dev the owner of the data - instead of unknown ownership before.
> 
> It is the same as here (but it might be wrong/unnecessary there as well):
> 
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/usb/phy/phy.c?id=refs/tags/v4.1#n209
> 
> Maybe it has something to do with the unfinished devm_serial_uart_release().

Indeed. I haven't noticed this through the first quick look into the
code. Thank you for explanation.
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