[PATCH] swiotlb: fix default_swiotlb_limit() for non-growable default pool

[Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>]

Commit ad96ce3252db ("swiotlb: determine potential physical address
limit") changed default_swiotlb_limit() to return the maximum physical
address that could ever be used by dynamically allocated pools, so that
the value stays constant when CONFIG_SWIOTLB_DYNAMIC=y. However,
io_tlb_default_mem.phys_limit is returned even when the default pool
cannot grow, e.g. when swiotlb_init_remap() is called with a remap
callback.

This breaks Xen dom0 on x86: pci_xen_swiotlb_init() passes SWIOTLB_ANY
and xen_swiotlb_fixup(), so phys_limit is set to the top of dom0 memory,
although xen_swiotlb_fixup() makes the default pool below the 32-bit
boundary. xen_swiotlb_dma_supported() then rejects 32-bit DMA masks,
even though the bounce buffer is perfectly usable by such devices.

Return phys_limit only if the default pool can actually grow, otherwise
report the real end of the default pool, like in the
!CONFIG_SWIOTLB_DYNAMIC case.

Reported-by: Andreas Greve <andreas.greve@xxxxxxxxxx>
Reported-by: James Dingwall <james@xxxxxxxxxxxxxx>
Fixes: ad96ce3252db ("swiotlb: determine potential physical address limit")
Closes: 
https://lore.kernel.org/xen-devel/f74668db-52fd-4575-8372-7bfdf10d62ac@xxxxxxxxxx/
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: Claude Opus 5.5 (1M context) <noreply@xxxxxxxxxxxxx>
Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
---
 kernel/dma/swiotlb.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index aa2f1c4588b9..5edea93ec635 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -1844,10 +1844,10 @@ phys_addr_t default_swiotlb_base(void)
 phys_addr_t default_swiotlb_limit(void)
 {
 #ifdef CONFIG_SWIOTLB_DYNAMIC
-       return io_tlb_default_mem.phys_limit;
-#else
-       return io_tlb_default_mem.defpool.end - 1;
+       if (io_tlb_default_mem.can_grow)
+               return io_tlb_default_mem.phys_limit;
 #endif
+       return io_tlb_default_mem.defpool.end - 1;
 }
 
 #ifdef CONFIG_DEBUG_FS
-- 
2.43.0