[PATCH] fbtft: Improve damage_range to mark only changed rows

[Waffle0823 <csshin9928@gmail.com>]

Instead of marking the entire display as dirty, calculate
start_row and end_row based on off/len and mark only those rows.
This improves performance for partial framebuffer updates.

Signed-off-by: Waffle0823 <csshin9928@gmail.com>
---
 drivers/staging/fbtft/fbtft-core.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/staging/fbtft/fbtft-core.c b/drivers/staging/fbtft/fbtft-core.c
index 8a5ccc8ae0a1..0fbdfdaaa94d 100644
--- a/drivers/staging/fbtft/fbtft-core.c
+++ b/drivers/staging/fbtft/fbtft-core.c
@@ -415,8 +415,11 @@ static void fbtft_ops_damage_range(struct fb_info *info, off_t off, size_t len)
 {
 	struct fbtft_par *par = info->par;
 
-	/* TODO: only mark changed area update all for now */
-	par->fbtftops.mkdirty(info, -1, 0);
+	__u32 width = info->var.xres;
+	__u32 start_row = off / width;
+	__u32 end_row = (off + len - 1) / width;
+
+	par->fbtftops.mkdirty(info, start_row, end_row);
 }
 
 static void fbtft_ops_damage_area(struct fb_info *info, u32 x, u32 y, u32 width, u32 height)
-- 
2.52.0