drm/shmem-helper: Simplify page offset calculation in fault handler

For a fault at address addr, the page offset is
  page_offset = (vmf->address - vma->vm_start) >> PAGE_SHIFT
              = ((addr & PAGE_MASK) - vma->vm_start) >> PAGE_SHIFT
	      = (addr - vma->vm_start) >> PAGE_SHIFT

Since the faulty logical page offset based on VMA is
  vmf->pgoff = vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT)

We can slightly simplify the calculation using
  page_offset = vmf->pgoff - vma->vm_pgoff

v11:
- remove misleading comment
- add Boris R-b

Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patch.msgid.link/20251205182231.194072-2-loic.molinari@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index dc94a27710e5f48839a5d2e9e9ea5152ff22e438..e47ab3ebb5c2206926e98d723c1e6d3a4991abb6 100644 (file)
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -577,8 +577,8 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
        struct page *page;
        pgoff_t page_offset;
 
-       /* We don't use vmf->pgoff since that has the fake offset */
-       page_offset = (vmf->address - vma->vm_start) >> PAGE_SHIFT;
+       /* Offset to faulty address in the VMA. */
+       page_offset = vmf->pgoff - vma->vm_pgoff;
 
        dma_resv_lock(shmem->base.resv, NULL);