linux-kernel - gpiolib: Questions about gpiochip_find

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Message-ID: <20101109130425.GE15927@verge.net.au>
Date:	Tue, 9 Nov 2010 22:04:25 +0900
From:	Simon Horman <horms@...ge.net.au>
To:	linux-kernel@...r.kernel.org
Cc:	Anton Vorontsov <avorontsov@...mvista.com>,
	David Brownell <dbrownell@...rs.sourceforge.net>
Subject: gpiolib: Questions about gpiochip_find_base()

Hi,

I have a few questions about gpiochip_find_base() which I hope someone can
help with.

* At the end of gpiochip_add() there is the following code:

	if (gpio_is_valid(base))
		pr_debug("%s: found new base at %d\n", __func__, base);
	return base;

  It is unclear to me why !gpio_is_valid(base) isn't an error condition
  given that the following appears in gpiochip_add(), the caller of
  gpiochip_find_base(), in the case where base isn't to be dynamic
  selected.

	if ((!gpio_is_valid(base) || !gpio_is_valid(base + chip->ngpio - 1))
			&& base >= 0) {
		status = -EINVAL;
		goto fail;
	}

* In the same vein, its unclear to me why
  the loop in gpiochip_find_base() checks values of i
  that may not be valid according to gpio_is_valid().

  Specifically I am wondering why

	for (i = ARCH_NR_GPIOS - 1; i >= 0 ; i--) {
		...
		if (!chip && !test_bit(FLAG_RESERVED, &desc->flags)) {
			...

  isn't something like

	for (i = ARCH_NR_GPIOS - 1; i >= 0 ; i--) {
		...
		if (gpio_is_valid(i) && !chip &&
		    !test_bit(FLAG_RESERVED, &desc->flags)) {
			...

* I am wondering if in gpiochip_add() the following is unnecessary
  in the case where base is dynamic as gpiochip_add() seems to
  ensure that gpio_desc[id].chip is NULL for the dynamically selected range.

	for (id = base; id < base + chip->ngpio; id++) {
		if (gpio_desc[id].chip != NULL) {
			status = -EBUSY;
			break;
		}
	}
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