Re: [PATCH 1/1] powerpc/kernel/iommu: Align size for IOMMU_PAGE_SIZE() to save TCEs

From: Leonardo Bras
Date: Thu Apr 22 2021 - 02:53:27 EST


Hello,

This patch was also reviewed when it was part of another patchset:
http://patchwork.ozlabs.org/project/linuxppc-dev/patch/20200911170738.82818-4-leobras.c@xxxxxxxxx/

On Thu, 2021-03-18 at 14:44 -0300, Leonardo Bras wrote:
> 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@xxxxxxxxx>
> Reviewed-by: Alexey Kardashevskiy <aik@xxxxxxxxx>
> ---
>  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));