drm/i915/gtt: allow overriding the pt alignment

On some platforms we have alignment restrictions when accessing LMEM
from the GTT. In the next few patches we need to be able to modify the
page-tables directly via the GTT itself.
Suggested-by: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220218184752.7524-10-ramalingam.c@intel.com

drivers/gpu/drm/i915/gt/intel_gtt.h

@@ -200,6 +200,14 @@ void *__px_vaddr(struct drm_i915_gem_object *p);
 struct i915_vm_pt_stash {
 	/* preallocated chains of page tables/directories */
 	struct i915_page_table *pt[2];
+	/*
+	 * Optionally override the alignment/size of the physical page that
+	 * contains each PT. If not set defaults back to the usual
+	 * I915_GTT_PAGE_SIZE_4K. This does not influence the other paging
+	 * structures. MUST be a power-of-two. ONLY applicable on discrete
+	 * platforms.
+	 */
+	int pt_sz;
 };
 
 struct i915_vma_ops {
@@ -595,7 +603,7 @@ void free_scratch(struct i915_address_space *vm);
 
 struct drm_i915_gem_object *alloc_pt_dma(struct i915_address_space *vm, int sz);
 struct drm_i915_gem_object *alloc_pt_lmem(struct i915_address_space *vm, int sz);
-struct i915_page_table *alloc_pt(struct i915_address_space *vm);
+struct i915_page_table *alloc_pt(struct i915_address_space *vm, int sz);
 struct i915_page_directory *alloc_pd(struct i915_address_space *vm);
 struct i915_page_directory *__alloc_pd(int npde);
 

drivers/gpu/drm/i915/gt/intel_ppgtt.c

@@ -12,7 +12,7 @@
 #include "gen6_ppgtt.h"
 #include "gen8_ppgtt.h"
 
-struct i915_page_table *alloc_pt(struct i915_address_space *vm)
+struct i915_page_table *alloc_pt(struct i915_address_space *vm, int sz)
 {
 	struct i915_page_table *pt;
 
@@ -20,7 +20,7 @@ struct i915_page_table *alloc_pt(struct i915_address_space *vm)
 	if (unlikely(!pt))
 		return ERR_PTR(-ENOMEM);
 
-	pt->base = vm->alloc_pt_dma(vm, I915_GTT_PAGE_SIZE_4K);
+	pt->base = vm->alloc_pt_dma(vm, sz);
 	if (IS_ERR(pt->base)) {
 		kfree(pt);
 		return ERR_PTR(-ENOMEM);
@@ -221,17 +221,25 @@ int i915_vm_alloc_pt_stash(struct i915_address_space *vm,
 			   u64 size)
 {
 	unsigned long count;
-	int shift, n;
+	int shift, n, pt_sz;
 
 	shift = vm->pd_shift;
 	if (!shift)
 		return 0;
 
+	pt_sz = stash->pt_sz;
+	if (!pt_sz)
+		pt_sz = I915_GTT_PAGE_SIZE_4K;
+	else
+		GEM_BUG_ON(!IS_DGFX(vm->i915));
+
+	GEM_BUG_ON(!is_power_of_2(pt_sz));
+
 	count = pd_count(size, shift);
 	while (count--) {
 		struct i915_page_table *pt;
 
-		pt = alloc_pt(vm);
+		pt = alloc_pt(vm, pt_sz);
 		if (IS_ERR(pt)) {
 			i915_vm_free_pt_stash(vm, stash);
 			return PTR_ERR(pt);