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drm/panfrost: Simplify lock_region calculation

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commit b5fab34565 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: f3ba91228e ("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>
This commit is contained in:
Alyssa Rosenzweig
2021-08-24 13:30:25 -04:00
committed by Greg Kroah-Hartman
parent b80a99e048
commit 95251e6833
1 changed files with 5 additions and 14 deletions
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19
drivers/gpu/drm/panfrost/panfrost_mmu.c
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@ -59,21 +59,12 @@ static void lock_region(struct panfrost_device *pfdev, u32 as_nr,
{ {
u8 region_width; u8 region_width;
u64 region = iova & PAGE_MASK; u64 region = iova & PAGE_MASK;
/*
* fls returns: /* The size is encoded as ceil(log2) minus(1), which may be calculated
* 1 .. 32 * with fls. The size must be clamped to hardware bounds.
*
* 10 + fls(num_pages)
* results in the range (11 .. 42)
*/ */
size = max_t(u64, size, PAGE_SIZE);
size = round_up(size, PAGE_SIZE); region_width = fls64(size - 1) - 1;
region_width = 10 + fls(size >> PAGE_SHIFT);
if ((size >> PAGE_SHIFT) != (1ul << (region_width - 11))) {
/* not pow2, so must go up to the next pow2 */
region_width += 1;
}
region |= region_width; region |= region_width;
/* Lock the region that needs to be updated */ /* Lock the region that needs to be updated */