drm/panfrost: Simplify lock_region calculation

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 <[email protected]>
Reported-and-tested-by: Chris Morgan <[email protected]>
Fixes: f3ba912 ("drm/panfrost: Add initial panfrost driver")
Cc: <[email protected]>
Reviewed-by: Steven Price <[email protected]>
Reviewed-by: Rob Herring <[email protected]>
Signed-off-by: Steven Price <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: Alyssa Rosenzweig

drivers/gpu/drm/panfrost/panfrost_mmu.c

@@ -62,21 +62,12 @@ static void lock_region(struct panfrost_device *pfdev, u32 as_nr,
 {
 	u8 region_width;
 	u64 region = iova & PAGE_MASK;
-	/*
-	 * fls returns:
-	 * 1 .. 32
-	 *
-	 * 10 + fls(num_pages)
-	 * results in the range (11 .. 42)
-	 */
-
-	size = round_up(size, PAGE_SIZE);
 
-	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;
-	}
+	/* The size is encoded as ceil(log2) minus(1), which may be calculated
+	 * with fls. The size must be clamped to hardware bounds.
+	 */
+	size = max_t(u64, size, PAGE_SIZE);
+	region_width = fls64(size - 1) - 1;
 	region |= region_width;
 
 	/* Lock the region that needs to be updated */