spi: hisi-kunpeng: prevent infinite while() loop in hisi_spi_flush_fifo

[ Upstream commit 9f61daf ]

The hisi_spi_flush_fifo()'s inner while loop that lacks any timeout
mechanism. Maybe the hardware never becomes empty, the loop will spin
forever, causing the CPU to hang.

Fix this by adding a inner_limit based on loops_per_jiffy. The inner loop
now exits after approximately one jiffy if the FIFO remains non-empty, logs
a ratelimited warning, and breaks out of the outer loop. Additionally, add
a cpu_relax() inside the busy loop to improve power efficiency.

Fixes: c770d86 ("spi: Add HiSilicon SPI Controller Driver for Kunpeng SoCs")
Signed-off-by: Pei Xiao <xiaopei01@kylinos.cn>
Link: https://patch.msgid.link/d834ce28172886bfaeb9c8ca00cfd9bf1c65d5a1.1773889292.git.xiaopei01@kylinos.cn
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: Pei Xiao

drivers/spi/spi-hisi-kunpeng.c

@@ -196,8 +196,18 @@ static void hisi_spi_flush_fifo(struct hisi_spi *hs)
 	unsigned long limit = loops_per_jiffy << 1;
 
 	do {
-		while (hisi_spi_rx_not_empty(hs))
+		unsigned long inner_limit = loops_per_jiffy;
+
+		while (hisi_spi_rx_not_empty(hs) && --inner_limit) {
 			readl(hs->regs + HISI_SPI_DOUT);
+			cpu_relax();
+		}
+
+		if (!inner_limit) {
+			dev_warn_ratelimited(hs->dev, "RX FIFO flush timeout\n");
+			break;
+		}
+
 	} while (hisi_spi_busy(hs) && limit--);
 }