[PATCH] spi: orion: yield to scheduler in transfer wait loop

[Rosen Penev <rosenp@gmail.com>]

orion_spi_wait_till_ready() busy-waits in a tight udelay(1) loop, up to
2000 iterations, and is called per byte from the polled, byte-at-a-time
transfer path. On SoCs such as the Armada 388 (e.g. SolidRun Helios4),
which also run SATA over the shared internal MBus fabric, this stalls
the CPU for the whole transfer and delays servicing of SATA interrupts.
Under sustained activity this can cause SATA timeouts and link resets
(sometimes renegotiating down to SATA II, 3 Gbps).

Add cond_resched() to the wait loop so the scheduler can run pending
IRQs between polls. This is a no-op at runtime unless the kernel is
built with CONFIG_PREEMPT enabled, where it lets other peripheral
interrupts be serviced during SPI transfers.

Built with LLVM=1 ARCH=powerpc; passes checkpatch --strict.

Assisted-by: opencode:big-pickle
Signed-off-by: Rosen Penev <rosenp@gmail.com>
---
 drivers/spi/spi-orion.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/spi/spi-orion.c b/drivers/spi/spi-orion.c
index 26a9f268b2d3..b8882229c054 100644
--- a/drivers/spi/spi-orion.c
+++ b/drivers/spi/spi-orion.c
@@ -370,7 +370,16 @@ static inline int orion_spi_wait_till_ready(struct orion_spi *orion_spi)
 		if (readl(spi_reg(orion_spi, ORION_SPI_INT_CAUSE_REG)))
 			return 1;
 
+		/*
+		 * This is a polled, byte-at-a-time transfer loop. Each
+		 * iteration busy-waits in a tight udelay() loop, which can
+		 * starve other time-sensitive peripherals (e.g. SATA) of CPU
+		 * time and interfere with them if they run on this SoC.
+		 * Yield to the scheduler between polls so pending IRQs can
+		 * be serviced.
+		 */
 		udelay(1);
+		cond_resched();
 	}
 
 	return -1;
-- 
2.55.0