gpu: ipu-v3: pre: don't trigger update if buffer address doesn't change

On a NOP double buffer update where current buffer address is the same
as the next buffer address, the SDW_UPDATE bit clears too late. As we
are now using this bit to determine when it is safe to signal flip
completion to userspace this will delay completion of atomic commits
where one plane doesn't change the buffer by a whole frame period.

Fix this by remembering the last buffer address and just skip the
double buffer update if it would not change the buffer address.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
[p.zabel@pengutronix.de: initialize last_bufaddr in ipu_pre_configure]
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

drivers/gpu/ipu-v3/ipu-pre.c

@@ -106,6 +106,7 @@ struct ipu_pre {
 	void			*buffer_virt;
 	bool			in_use;
 	unsigned int		safe_window_end;
+	unsigned int		last_bufaddr;
 };
 
 static DEFINE_MUTEX(ipu_pre_list_mutex);
@@ -185,6 +186,7 @@ void ipu_pre_configure(struct ipu_pre *pre, unsigned int width,
 
 	writel(bufaddr, pre->regs + IPU_PRE_CUR_BUF);
 	writel(bufaddr, pre->regs + IPU_PRE_NEXT_BUF);
+	pre->last_bufaddr = bufaddr;
 
 	val = IPU_PRE_PREF_ENG_CTRL_INPUT_PIXEL_FORMAT(0) |
 	      IPU_PRE_PREF_ENG_CTRL_INPUT_ACTIVE_BPP(active_bpp) |
@@ -242,7 +244,11 @@ void ipu_pre_update(struct ipu_pre *pre, unsigned int bufaddr)
 	unsigned short current_yblock;
 	u32 val;
 
+	if (bufaddr == pre->last_bufaddr)
+		return;
+
 	writel(bufaddr, pre->regs + IPU_PRE_NEXT_BUF);
+	pre->last_bufaddr = bufaddr;
 
 	do {
 		if (time_after(jiffies, timeout)) {