linux-kernel - [PATCH 1/1] powerpc/kernel/iommu: Align size for IOMMU_PAGE

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Date:   Thu, 18 Mar 2021 14:44:14 -0300
From:   Leonardo Bras <leobras.c@...il.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Leonardo Bras <leobras.c@...il.com>,
        Alexey Kardashevskiy <aik@...abs.ru>,
        Nicolin Chen <nicoleotsuka@...il.com>,
        Niklas Schnelle <schnelle@...ux.ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: [PATCH 1/1] powerpc/kernel/iommu: Align size for IOMMU_PAGE_SIZE() to save TCEs

Currently both iommu_alloc_coherent() and iommu_free_coherent() align the
desired allocation size to PAGE_SIZE, and gets system pages and IOMMU
mappings (TCEs) for that value.

When IOMMU_PAGE_SIZE < PAGE_SIZE, this behavior may cause unnecessary
TCEs to be created for mapping the whole system page.

Example:
- PAGE_SIZE = 64k, IOMMU_PAGE_SIZE() = 4k
- iommu_alloc_coherent() is called for 128 bytes
- 1 system page (64k) is allocated
- 16 IOMMU pages (16 x 4k) are allocated (16 TCEs used)

It would be enough to use a single TCE for this, so 15 TCEs are
wasted in the process.

Update iommu_*_coherent() to make sure the size alignment happens only
for IOMMU_PAGE_SIZE() before calling iommu_alloc() and iommu_free().

Also, on iommu_range_alloc(), replace ALIGN(n, 1 << tbl->it_page_shift)
with IOMMU_PAGE_ALIGN(n, tbl), which is easier to read and does the
same.

Signed-off-by: Leonardo Bras <leobras.c@...il.com>
Reviewed-by: Alexey Kardashevskiy <aik@...abs.ru>
---
 arch/powerpc/kernel/iommu.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/kernel/iommu.c b/arch/powerpc/kernel/iommu.c
index 5b69a6a72a0e..3329ef045805 100644
--- a/arch/powerpc/kernel/iommu.c
+++ b/arch/powerpc/kernel/iommu.c
@@ -851,6 +851,7 @@ void *iommu_alloc_coherent(struct device *dev, struct iommu_table *tbl,
 	unsigned int order;
 	unsigned int nio_pages, io_order;
 	struct page *page;
+	size_t size_io = size;
 
 	size = PAGE_ALIGN(size);
 	order = get_order(size);
@@ -877,8 +878,9 @@ 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;
-	io_order = get_iommu_order(size, tbl);
+	size_io = IOMMU_PAGE_ALIGN(size_io, tbl);
+	nio_pages = size_io >> tbl->it_page_shift;
+	io_order = get_iommu_order(size_io, tbl);
 	mapping = iommu_alloc(dev, tbl, ret, nio_pages, DMA_BIDIRECTIONAL,
 			      mask >> tbl->it_page_shift, io_order, 0);
 	if (mapping == DMA_MAPPING_ERROR) {
@@ -893,10 +895,9 @@ void iommu_free_coherent(struct iommu_table *tbl, size_t size,
 			 void *vaddr, dma_addr_t dma_handle)
 {
 	if (tbl) {
-		unsigned int nio_pages;
+		size_t size_io = IOMMU_PAGE_ALIGN(size, tbl);
+		unsigned int nio_pages = size_io >> tbl->it_page_shift;
 
-		size = PAGE_ALIGN(size);
-		nio_pages = size >> tbl->it_page_shift;
 		iommu_free(tbl, dma_handle, nio_pages);
 		size = PAGE_ALIGN(size);
 		free_pages((unsigned long)vaddr, get_order(size));
-- 
2.29.2