swiotlb: add a flag whether SWIOTLB is allowed to grow

Add a config option (CONFIG_SWIOTLB_DYNAMIC) to enable or disable dynamic
allocation of additional bounce buffers.

If this option is set, mark the default SWIOTLB as able to grow and
restricted DMA pools as unable.

However, if the address of the default memory pool is explicitly queried,
make the default SWIOTLB also unable to grow. This is currently used to set
up PCI BAR movable regions on some Octeon MIPS boards which may not be able
to use a SWIOTLB pool elsewhere in physical memory. See octeon_pci_setup()
for more details.

If a remap function is specified, it must be also called on any dynamically
allocated pools, but there are some issues:

- The remap function may block, so it should not be called from an atomic
  context.
- There is no corresponding unremap() function if the memory pool is
  freed.
- The only in-tree implementation (xen_swiotlb_fixup) requires that the
  number of slots in the memory pool is a multiple of SWIOTLB_SEGSIZE.

Keep it simple for now and disable growing the SWIOTLB if a remap function
was specified.

Signed-off-by: Petr Tesarik <petr.tesarik.ext@huawei.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

include/linux/swiotlb.h

@@ -100,6 +100,7 @@ struct io_tlb_pool {
  * @debugfs:	The dentry to debugfs.
  * @force_bounce: %true if swiotlb bouncing is forced
  * @for_alloc:  %true if the pool is used for memory allocation
+ * @can_grow:	%true if more pools can be allocated dynamically.
  * @total_used:	The total number of slots in the pool that are currently used
  *		across all areas. Used only for calculating used_hiwater in
  *		debugfs.
@@ -112,6 +113,9 @@ struct io_tlb_mem {
 	struct dentry *debugfs;
 	bool force_bounce;
 	bool for_alloc;
+#ifdef CONFIG_SWIOTLB_DYNAMIC
+	bool can_grow;
+#endif
 #ifdef CONFIG_DEBUG_FS
 	atomic_long_t total_used;
 	atomic_long_t used_hiwater;