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Subject[PATCH 10/16] staging: gasket: page table: remove extraneous memory barriers
Date
From: Todd Poynor <toddpoynor@google.com>

Some explicit memory barriers in the page table code are not necessary,
either because:

(a) The barrier follows a non-relaxed MMIO access that already performs
a read or write memory barrier.

(b) The barrier follows DMA API calls for which the device-visible
effects of IOMMU programming are guaranteed to be flushed to the IOMMU
prior to the call returning, and doesn't need to sync with normal memory
access.

Signed-off-by: Todd Poynor <toddpoynor@google.com>
---
drivers/staging/gasket/gasket_page_table.c | 15 +++++----------
1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/drivers/staging/gasket/gasket_page_table.c b/drivers/staging/gasket/gasket_page_table.c
index 4d2499269499b..53492f4fad6aa 100644
--- a/drivers/staging/gasket/gasket_page_table.c
+++ b/drivers/staging/gasket/gasket_page_table.c
@@ -317,8 +317,6 @@ static void gasket_free_extended_subtable(struct gasket_page_table *pg_tbl,

/* Release the page table from the device */
writeq(0, slot);
- /* Force sync around the address release. */
- mb();

if (pte->dma_addr)
dma_unmap_page(pg_tbl->device, pte->dma_addr, PAGE_SIZE,
@@ -504,8 +502,6 @@ static int gasket_perform_mapping(struct gasket_page_table *pg_tbl,
(void *)page_to_phys(page));
return -1;
}
- /* Wait until the page is mapped. */
- mb();
}

/* Make the DMA-space address available to the device. */
@@ -604,12 +600,13 @@ static void gasket_perform_unmapping(struct gasket_page_table *pg_tbl,
*/
for (i = 0; i < num_pages; i++) {
/* release the address from the device, */
- if (is_simple_mapping || ptes[i].status == PTE_INUSE)
+ if (is_simple_mapping || ptes[i].status == PTE_INUSE) {
writeq(0, &slots[i]);
- else
+ } else {
((u64 __force *)slots)[i] = 0;
- /* Force sync around the address release. */
- mb();
+ /* sync above PTE update before updating mappings */
+ wmb();
+ }

/* release the address from the driver, */
if (ptes[i].status == PTE_INUSE) {
@@ -898,8 +895,6 @@ static int gasket_alloc_extended_subtable(struct gasket_page_table *pg_tbl,
/* Map the page into DMA space. */
pte->dma_addr = dma_map_page(pg_tbl->device, pte->page, 0, PAGE_SIZE,
DMA_BIDIRECTIONAL);
- /* Wait until the page is mapped. */
- mb();

/* make the addresses available to the device */
dma_addr = (pte->dma_addr + pte->offset) | GASKET_VALID_SLOT_FLAG;
--
2.18.0.597.ga71716f1ad-goog
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