linux-kernel - [RFC PATCH 20/60] hyper_dmabuf: optimized loop with less condition check

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Message-Id: <1513711816-2618-20-git-send-email-dongwon.kim@intel.com>
Date:   Tue, 19 Dec 2017 11:29:36 -0800
From:   Dongwon Kim <dongwon.kim@...el.com>
To:     linux-kernel@...r.kernel.org
Cc:     dri-devel@...ts.freedesktop.org, xen-devel@...ts.xenproject.org,
        mateuszx.potrola@...el.com, dongwon.kim@...el.com
Subject: [RFC PATCH 20/60] hyper_dmabuf: optimized loop with less condition check

Redefined nents_last, which means # of gref in the last page
of lvl2 table in any situation even if it is same as REFS_PER_PAGE.
With this, loop can be simplified with less condition check.

Signed-off-by: Dongwon Kim <dongwon.kim@...el.com>
---
 drivers/xen/hyper_dmabuf/xen/hyper_dmabuf_xen_shm.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/xen/hyper_dmabuf/xen/hyper_dmabuf_xen_shm.c b/drivers/xen/hyper_dmabuf/xen/hyper_dmabuf_xen_shm.c
index cc9860b..cb5b86f 100644
--- a/drivers/xen/hyper_dmabuf/xen/hyper_dmabuf_xen_shm.c
+++ b/drivers/xen/hyper_dmabuf/xen/hyper_dmabuf_xen_shm.c
@@ -184,8 +184,10 @@ struct page ** hyper_dmabuf_xen_map_shared_pages(int lvl3_gref, int domid, int n
 	struct gnttab_map_grant_ref *data_map_ops;
 	struct gnttab_unmap_grant_ref *data_unmap_ops;
 
-	int nents_last = nents % REFS_PER_PAGE;
-	int n_lvl2_grefs = (nents / REFS_PER_PAGE) + ((nents_last > 0) ? 1 : 0);
+	/* # of grefs in the last page of lvl2 table */
+	int nents_last = (nents - 1) % REFS_PER_PAGE + 1;
+	int n_lvl2_grefs = (nents / REFS_PER_PAGE) + ((nents_last > 0) ? 1 : 0) -
+			   (nents_last == REFS_PER_PAGE);
 	int i, j, k;
 
 	dev_dbg(hyper_dmabuf_private.device, "%s entry\n", __func__);
@@ -270,7 +272,7 @@ struct page ** hyper_dmabuf_xen_map_shared_pages(int lvl3_gref, int domid, int n
 
 	k = 0;
 
-	for (i = 0; i < (nents_last ? n_lvl2_grefs - 1 : n_lvl2_grefs); i++) {
+	for (i = 0; i < n_lvl2_grefs - 1; i++) {
 		lvl2_table = pfn_to_kaddr(page_to_pfn(lvl2_table_pages[i]));
 		for (j = 0; j < REFS_PER_PAGE; j++) {
 			gnttab_set_map_op(&data_map_ops[k],
-- 
2.7.4