Re: [PATCH] swiotlb: ensure io_tlb_default_mem spinlock always initialised

[Robin Murphy]

On 2022-07-11 08:26, Ben Dooks wrote:
> On 08/07/2022 21:32, Robin Murphy wrote:
> > On 2022-07-08 18:08, Ben Dooks wrote:
> > > If the io_tlb_default_mem is used by a device that then gets used
> > > by the swiotlb code, the spinlock warning is triggered causing a
> > > lot of stack-trace.
> >
> > Hang on, how have we got as far as trying to allocate out of an 
> > uninitialised SWIOTLB at all? That seems like either something's gone 
> > more fundamentally wrong or we're missing a proper check somewhere. I 
> > don't think papering over it like this is right.
> >
> > Thanks,
> > Robin
> We have a system where we have no DMA capable memory in the 32bit
> window, which means even if we initialise swiotlb we don't have
> any dma capable memory. The code says go use an IOMMU but we don't
> have one of those either (and all peripherals are capable of DMA
> from any of the memory, so there shouldn't be the need for one)
>
> Is there any other way of fixing this DMA allocation issue?

If none of your peripherals should need SWIOTLB, then the fact that
you're ending up in swiotlb_map() at all is a clear sign that
something's wrong. Most likely someone's forgotten to set their DMA
masks correctly.

However, by inspection it seems we do have a bug here as well, for which
the correct fix should be as below. The fireworks you're *supposed* to
get in that situation are considerably louder and more obvious than a
DEBUG_SPINLOCK complaint ;)

Thanks,
Robin.

----->8-----
Subject: [PATCH] swiotlb: Fail map correctly with failed io_tlb_default_mem

In the failure case of trying to use a buffer which we'd previously
failed to allocate, the "!mem" condition is no longer sufficient since
io_tlb_default_mem became static and assigned by default. Update the
condition to work as intended per the rest of that conversion.

Fixes: 463e862ac63e ("swiotlb: Convert io_default_tlb_mem to static allocation")
Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
---
 kernel/dma/swiotlb.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index cb50f8d38360..5830dce6081b 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -580,7 +580,7 @@ phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
 	int index;
 	phys_addr_t tlb_addr;
 
-	if (!mem)
+	if (!mem || !mem->nslabs)
 		panic("Can not allocate SWIOTLB buffer earlier and can't now provide you with the DMA bounce buffer");
 
 	if (cc_platform_has(CC_ATTR_MEM_ENCRYPT))
--
2.36.1.dirty