linux-kernel - [PATCH v9 5/5] bitmap: extend comment to bitmap_print_bitmask/list_to

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Date:   Fri, 6 Aug 2021 23:02:51 +1200
From:   Barry Song <song.bao.hua@...ilicon.com>
To:     <andriy.shevchenko@...ux.intel.com>, <yury.norov@...il.com>,
        <gregkh@...uxfoundation.org>, <linux-kernel@...r.kernel.org>
CC:     <akpm@...ux-foundation.org>, <dave.hansen@...el.com>,
        <linux@...musvillemoes.dk>, <rafael@...nel.org>,
        <rdunlap@...radead.org>, <agordeev@...ux.ibm.com>,
        <sbrivio@...hat.com>, <jianpeng.ma@...el.com>,
        <valentin.schneider@....com>, <peterz@...radead.org>,
        <bristot@...hat.com>, <guodong.xu@...aro.org>,
        <tangchengchang@...wei.com>, <prime.zeng@...ilicon.com>,
        <yangyicong@...wei.com>, <tim.c.chen@...ux.intel.com>,
        <linuxarm@...wei.com>, Barry Song <song.bao.hua@...ilicon.com>
Subject: [PATCH v9 5/5] bitmap: extend comment to bitmap_print_bitmask/list_to_buf

From: Yury Norov <yury.norov@...il.com>

Extend comment to new function to warn potential users about caveats.

Signed-off-by: Yury Norov <yury.norov@...il.com>
Signed-off-by: Barry Song <song.bao.hua@...ilicon.com>
---
 lib/bitmap.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/lib/bitmap.c b/lib/bitmap.c
index 73746d96af81..663dd81967d4 100644
--- a/lib/bitmap.c
+++ b/lib/bitmap.c
@@ -568,6 +568,24 @@ static int bitmap_print_to_buf(bool list, char *buf, const unsigned long *maskp,
  * and it can break the size limit of converted decimal list and hexadecimal
  * bitmask.
  *
+ * WARNING!
+ *
+ * This function is not a replacement for sprintf() or bitmap_print_to_pagebuf().
+ * It is intended to workaround sysfs limitations discussed above and should be
+ * used carefully in general case for the following reasons:
+ *  - Time complexity is O(nbits^2/count), comparing to O(nbits) for snprintf().
+ *  - Memory complexity is O(nbits), comparing to O(1) for snprintf().
+ *  - @off and @count are NOT offset and number of bits to print.
+ *  - If printing part of bitmap as list, the resulting string is not a correct
+ *    list representation of bitmap. Particularly, some bits within or out of
+ *    related interval may be erroneously set or unset. The format of the string
+ *    may be broken, so bitmap_parselist-like parser may fail parsing it.
+ *  - If printing the whole bitmap as list by parts, user must ensure the order
+ *    of calls of the function such that the offset is incremented linearly.
+ *  - If printing the whole bitmap as list by parts, user must keep bitmap
+ *    unchanged between the very first and very last call. Otherwise concatenated
+ *    result may be incorrect, and format may be broken.
+ *
  * Returns the number of characters actually printed to @buf
  */
 int bitmap_print_bitmask_to_buf(char *buf, const unsigned long *maskp,
-- 
2.25.1