swiotlb: optimize get_max_slots()

Use a simple logical shift and increment to calculate the number of slots taken by the DMA segment boundary. At least GCC-13 is not able to optimize the expression, producing this horrible assembly code on x86: cmpq $-1, %rcx je .L364 addq $2048, %rcx shrq $11, %rcx movq %rcx, %r13 .L331: // rest of the function here... // after function epilogue and return: .L364: movabsq $9007199254740992, %r13 jmp .L331 After the optimization, the code looks more reasonable: shrq $11, %r11 leaq 1(%r11), %rbx Signed-off-by: Petr Tesarik <[email protected]> Signed-off-by: Christoph Hellwig <[email protected]>
MichaIng · Aug 8, 2023 · d069ed2 · d069ed2
1 parent f94cb36
commit d069ed2
Showing 1 changed file with 1 addition and 3 deletions.
diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
@@ -903,9 +903,7 @@ static inline phys_addr_t slot_addr(phys_addr_t start, phys_addr_t idx)
  */
 static inline unsigned long get_max_slots(unsigned long boundary_mask)
 {
-	if (boundary_mask == ~0UL)
-		return 1UL << (BITS_PER_LONG - IO_TLB_SHIFT);
-	return nr_slots(boundary_mask + 1);
+	return (boundary_mask >> IO_TLB_SHIFT) + 1;
 }
 
 static unsigned int wrap_area_index(struct io_tlb_pool *mem, unsigned int index)