[PATCH v4 5/5] gpiolib: cdev: improve documentation of get/set values

Kent Gibson
2023-12-16T00:17:58Z

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>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
---
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 68fa9714e643..d4953787f361 100644
--- a/drivers/gpio/gpiolib-cdev.c
+++ b/drivers/gpio/gpiolib-cdev.c
@@ -1385,9 +1385,18 @@ 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;
}
}
@@ -1396,6 +1405,7 @@ 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;
@@ -1416,6 +1426,7 @@ 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]);
@@ -1441,14 +1452,25 @@ 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;
}
}
@@ -1456,7 +1478,7 @@ 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