[PATCH v2 3/5] drm/xe/vram: revamp CPU VRAM mapping

[Matthew Auld <matthew.auld@intel.com>]

Previously, we called devm_ioremap_wc() for the entire PCI LMEM BAR (which
can be significantly larger than usable memory, e.g., mapping 16G for a
10G card), and then simply assigned subsets of this global mapping to each
tile.

By moving the devm_ioremap_wc() call into vram_region_init() and mapping
on a per-tile basis, we restrict the virtual address space to exactly
the usable_size of each tile. The other big win is that the core kernel
will place a guard page at the end of each per-tile mapping to help
catch OOB CPU writes (e.g. into the flat CCS storage) by triggering an
immediate page fault instead of silent memory corruption.

As a consequence the global vram->mapping is now NULL. But that was
unused anyway, with CPU access already correctly routed through the per
tile mapping.

Assisted-by: Gemini:gemini-3.1-pro-preview
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
---
 drivers/gpu/drm/xe/xe_vram.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_vram.c b/drivers/gpu/drm/xe/xe_vram.c
index 56cff1e44530..04d831b101bd 100644
--- a/drivers/gpu/drm/xe/xe_vram.c
+++ b/drivers/gpu/drm/xe/xe_vram.c
@@ -55,9 +55,6 @@ static int determine_lmem_bar_size(struct xe_device *xe, struct xe_vram_region *
 	/* XXX: Need to change when xe link code is ready */
 	lmem_bar->dpa_base = 0;
 
-	/* set up a map to the total memory area. */
-	lmem_bar->mapping = devm_ioremap_wc(&pdev->dev, lmem_bar->io_start, lmem_bar->io_size);
-
 	return 0;
 }
 
@@ -196,7 +193,7 @@ static void vram_fini(void *arg)
 	struct xe_tile *tile;
 	int id;
 
-	xe->mem.vram->mapping = NULL;
+	xe_assert(xe, !xe->mem.vram->mapping);
 
 	for_each_tile(tile, xe, id) {
 		tile->mem.vram->mapping = NULL;
@@ -257,8 +254,15 @@ static int vram_region_init(struct xe_device *xe, struct xe_vram_region *vram,
 		return -ENODEV;
 	}
 
+	if (vram != xe->mem.vram) {
+		struct pci_dev *pdev = to_pci_dev(xe->drm.dev);
+
+		vram->mapping = devm_ioremap_wc(&pdev->dev, vram->io_start, vram->io_size);
+		if (!vram->mapping)
+			return -ENOMEM;
+	}
+
 	vram->dpa_base = lmem_bar->dpa_base + offset;
-	vram->mapping = lmem_bar->mapping + offset;
 	vram->usable_size = usable_size;
 
 	print_vram_region_info(xe, vram);
-- 
2.55.0