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From: Kent Gibson <warthog618@gmail.com>
To: linux-kernel@vger.kernel.org,
	linux-gpio@vger.kernel.org,
	brgl@bgdev.pl,
	linus.walleij@linaro.org,
	andy@kernel.org
Cc: Kent Gibson <warthog618@gmail.com>
Subject: [PATCH v3 5/5] gpiolib: cdev: improve documentation of get/set values
Date: Fri, 15 Dec 2023 10:38:05 +0800
Message-Id: <20231215023805.63289-6-warthog618@gmail.com>
In-Reply-To: <20231215023805.63289-1-warthog618@gmail.com>
References: <20231215023805.63289-1-warthog618@gmail.com>
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Add documentation of the algorithm used to perform scatter/gather
of the requested lines and values in linereq_get_values() and
linereq_set_values_unlocked() to improve maintainability.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
---
 drivers/gpio/gpiolib-cdev.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
index b956664f8649..37794de691f3 100644
--- a/drivers/gpio/gpiolib-cdev.c
+++ b/drivers/gpio/gpiolib-cdev.c
@@ -1383,9 +1383,18 @@ static long linereq_get_values(struct linereq *lr, void __user *ip)
 	if (copy_from_user(&lv, ip, sizeof(lv)))
 		return -EFAULT;
 
+	/*
+	 * gpiod_get_array_value_complex() requires compacted desc and val
+	 * arrays, rather than the sparse ones in lv.
+	 * Calculation of num_get and construction of the desc array is
+	 * optimized to avoid allocation for the desc array for the common
+	 * num_get == 1 case.
+	 */
+	/* scan requested lines to calculate the subset to get */
 	for (num_get = 0, i = 0; i < lr->num_lines; i++) {
 		if (lv.mask & BIT_ULL(i)) {
 			num_get++;
+			/* capture desc for the num_get == 1 case */
 			descs = &lr->lines[i].desc;
 		}
 	}
@@ -1394,6 +1403,7 @@ static long linereq_get_values(struct linereq *lr, void __user *ip)
 		return -EINVAL;
 
 	if (num_get != 1) {
+		/* build compacted desc array */
 		descs = kmalloc_array(num_get, sizeof(*descs), GFP_KERNEL);
 		if (!descs)
 			return -ENOMEM;
@@ -1414,6 +1424,7 @@ static long linereq_get_values(struct linereq *lr, void __user *ip)
 
 	lv.bits = 0;
 	for (didx = 0, i = 0; i < lr->num_lines; i++) {
+		/* unpack compacted vals for the response */
 		if (lv.mask & BIT_ULL(i)) {
 			if (lr->lines[i].sw_debounced)
 				val = debounced_value(&lr->lines[i]);
@@ -1439,14 +1450,25 @@ static long linereq_set_values_unlocked(struct linereq *lr,
 	unsigned int i, didx, num_set;
 	int ret;
 
+	/*
+	 * gpiod_set_array_value_complex() requires compacted desc and val
+	 * arrays, rather than the sparse ones in lv.
+	 * Calculation of num_set and construction of the descs and vals arrays
+	 * is optimized to minimize scanning the lv->mask, and to avoid
+	 * allocation for the desc array for the common num_set == 1 case.
+	 */
 	bitmap_zero(vals, GPIO_V2_LINES_MAX);
+	/* scan requested lines to determine the subset to be set */
 	for (num_set = 0, i = 0; i < lr->num_lines; i++) {
 		if (lv->mask & BIT_ULL(i)) {
+			/* setting inputs is not allowed */
 			if (!test_bit(FLAG_IS_OUT, &lr->lines[i].desc->flags))
 				return -EPERM;
+			/* add to compacted values */
 			if (lv->bits & BIT_ULL(i))
 				__set_bit(num_set, vals);
 			num_set++;
+			/* capture desc for the num_set == 1 case */
 			descs = &lr->lines[i].desc;
 		}
 	}
@@ -1454,7 +1476,7 @@ static long linereq_set_values_unlocked(struct linereq *lr,
 		return -EINVAL;
 
 	if (num_set != 1) {
-		/* build compacted desc array and values */
+		/* build compacted desc array */
 		descs = kmalloc_array(num_set, sizeof(*descs), GFP_KERNEL);
 		if (!descs)
 			return -ENOMEM;
-- 
2.39.2