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Date:   Mon, 16 Oct 2023 14:04:55 +0300
From:   Matti Vaittinen <mazziesaccount@gmail.com>
To:     Matti Vaittinen <mazziesaccount@gmail.com>,
        Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Cc:     Jonathan Cameron <jic23@kernel.org>,
        Lars-Peter Clausen <lars@metafoo.de>,
        linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH v2 2/2] iio: buffer: document known issue
Message-ID: <ef61c2c1e9a1c5e9f713f656871fdcb1652afdc2.1697452986.git.mazziesaccount@gmail.com>
References: <cover.1697452986.git.mazziesaccount@gmail.com>
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Add documentation explaining why the code which scans all available scan
masks is checking only a single long worth of bits even though the code
was intended to be supporting masks wider than single long.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
---
 drivers/iio/industrialio-buffer.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-b=
uffer.c
index 176d31d9f9d8..09c41e9ccf87 100644
--- a/drivers/iio/industrialio-buffer.c
+++ b/drivers/iio/industrialio-buffer.c
@@ -413,6 +413,22 @@ static const unsigned long *iio_scan_mask_match(const =
unsigned long *av_masks,
 {
 	if (bitmap_empty(mask, masklength))
 		return NULL;
+	/*
+	 * The condition here do not handle multi-long masks correctly.
+	 * It only checks the first long to be zero, and will use such mask
+	 * as a terminator even if there was bits set after the first long.
+	 *
+	 * Correct check would require using:
+	 * while (!bitmap_empty(av_masks, masklength))
+	 * instead. This is potentially hazardous because the
+	 * avaliable_scan_masks is a zero terminated array of longs - and
+	 * using the proper bitmap_empty() check for multi-long wide masks
+	 * would require the array to be terminated with multiple zero longs -
+	 * which is not such an usual pattern.
+	 *
+	 * As writing of this no multi-long wide masks were found in-tree, so
+	 * the simple while (*av_masks) check is working.
+	 */
 	while (*av_masks) {
 		if (strict) {
 			if (bitmap_equal(mask, av_masks, masklength))
--=20
2.41.0


--=20
Matti Vaittinen, Linux device drivers
ROHM Semiconductors, Finland SWDC
Kiviharjunlenkki 1E
90220 OULU
FINLAND

~~~ "I don't think so," said Rene Descartes. Just then he vanished ~~~
Simon says - in Latin please.
~~~ "non cogito me" dixit Rene Descarte, deinde evanescavit ~~~
Thanks to Simon Glass for the translation =3D]=20

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