drm/panfrost: Simplify lock_region calculation

stable inclusion
from stable-5.10.67
commit 95251e6833fa38199b3189d5621bc19b0c027e4b
bugzilla: 182619 https://gitee.com/openeuler/kernel/issues/I4EWO7

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=95251e6833fa38199b3189d5621bc19b0c027e4b



--------------------------------

commit b5fab345 upstream.

In lock_region, simplify the calculation of the region_width parameter.
This field is the size, but encoded as ceil(log2(size)) - 1.
ceil(log2(size)) may be computed directly as fls(size - 1). However, we
want to use the 64-bit versions as the amount to lock can exceed
32-bits.

This avoids undefined (and completely wrong) behaviour when locking all
memory (size ~0). In this case, the old code would "round up" ~0 to the
nearest page, overflowing to 0. Since fls(0) == 0, this would calculate
a region width of 10 + 0 = 10. But then the code would shift by
(region_width - 11) = -1. As shifting by a negative number is undefined,
UBSAN flags the bug. Of course, even if it were defined the behaviour is
wrong, instead of locking all memory almost none would get locked.

The new form of the calculation corrects this special case and avoids
the undefined behaviour.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reported-and-tested-by: Chris Morgan <macromorgan@hotmail.com>
Fixes: f3ba9122 ("drm/panfrost: Add initial panfrost driver")
Cc: <stable@vger.kernel.org>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210824173028.7528-2-alyssa.rosenzweig@collabora.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Acked-by: Weilong Chen <chenweilong@huawei.com>

Signed-off-by: Chen Jun <chenjun102@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>