Re: [PATCH v2] powerpc/iommu: DMA address offset is incorrectly calculated with 2MB TCEs

[Gaurav Batra]

Hello Alexey,

I recently joined IOMMU team. There was a bug reported by test team 
where Mellanox driver was timing out during configuration. I proposed a 
fix for the same, which is below in the email.

You suggested a fix for Srikar's reported problem. Basically, both these 
fixes will resolve Srikar and Mellanox driver issues. The problem is 
with 2MB DDW.

Since you have extensive knowledge of IOMMU design and code, in your 
opinion, which patch should we adopt?

Thanks a lot

Gaurav

On 4/20/23 2:45 PM, Gaurav Batra wrote:
Hello Michael,

I was looking into the Bug: 199106 
(https://bugzilla.linux.ibm.com/show_bug.cgi?id=199106).

In the Bug, Mellanox driver was timing out when enabling SRIOV device.

I tested, Alexey's patch and it fixes the issue with Mellanox driver. 
The down side

to Alexey's fix is that even a small memory request by the driver will 
be aligned up

to 2MB. In my test, the Mellanox driver is issuing multiple requests 
of 64K size.

All these will get aligned up to 2MB, which is quite a waste of 
resources.


In any case, both the patches work. Let me know which approach you 
prefer. In case

we decide to go with my patch, I just realized that I need to fix 
nio_pages in

iommu_free_coherent() as well.


Thanks,

Gaurav

On 4/20/23 10:21 AM, Michael Ellerman wrote:
Gaurav Batra <gba...@linux.vnet.ibm.com> writes:
When DMA window is backed by 2MB TCEs, the DMA address for the mapped
page should be the offset of the page relative to the 2MB TCE. The code
was incorrectly setting the DMA address to the beginning of the TCE
range.

Mellanox driver is reporting timeout trying to ENABLE_HCA for an SR-IOV
ethernet port, when DMA window is backed by 2MB TCEs.
I assume this is similar or related to the bug Srikar reported?

https://lore.kernel.org/linuxppc-dev/20230323095333.gi1005...@linux.vnet.ibm.com/

In that thread Alexey suggested a patch, have you tried his patch? He
suggested rounding up the allocation size, rather than adjusting the
dma_handle.

Fixes: 3872731187141d5d0a5c4fb30007b8b9ec36a44d
That's not the right syntax, it's described in the documentation how to
generate it.

It should be:

   Fixes: 387273118714 ("powerps/pseries/dma: Add support for 2M 
IOMMU page size")

cheers

diff --git a/arch/powerpc/kernel/iommu.c b/arch/powerpc/kernel/iommu.c
index ee95937bdaf1..ca57526ce47a 100644
--- a/arch/powerpc/kernel/iommu.c
+++ b/arch/powerpc/kernel/iommu.c
@@ -517,7 +517,7 @@ int ppc_iommu_map_sg(struct device *dev, struct 
iommu_table *tbl,
          /* Convert entry to a dma_addr_t */
          entry += tbl->it_offset;
          dma_addr = entry << tbl->it_page_shift;
-        dma_addr |= (s->offset & ~IOMMU_PAGE_MASK(tbl));
+        dma_addr |= (vaddr & ~IOMMU_PAGE_MASK(tbl));
            DBG("  - %lu pages, entry: %lx, dma_addr: %lx\n",
                  npages, entry, dma_addr);
@@ -904,6 +904,7 @@ void *iommu_alloc_coherent(struct device *dev, 
struct iommu_table *tbl,
      unsigned int order;
      unsigned int nio_pages, io_order;
      struct page *page;
+    int tcesize = (1 << tbl->it_page_shift);
        size = PAGE_ALIGN(size);
      order = get_order(size);
@@ -930,7 +931,8 @@ void *iommu_alloc_coherent(struct device *dev, 
struct iommu_table *tbl,
      memset(ret, 0, size);
        /* Set up tces to cover the allocated range */
-    nio_pages = size >> tbl->it_page_shift;
+    nio_pages = IOMMU_PAGE_ALIGN(size, tbl) >> tbl->it_page_shift;
+
      io_order = get_iommu_order(size, tbl);
      mapping = iommu_alloc(dev, tbl, ret, nio_pages, 
DMA_BIDIRECTIONAL,
                    mask >> tbl->it_page_shift, io_order, 0);
@@ -938,7 +940,8 @@ void *iommu_alloc_coherent(struct device *dev, 
struct iommu_table *tbl,
          free_pages((unsigned long)ret, order);
          return NULL;
      }
-    *dma_handle = mapping;
+
+    *dma_handle = mapping | ((u64)ret & (tcesize - 1));
      return ret;
  }
  --