1 files changed, 50 insertions, 0 deletions
diff --git a/target/linux/bcm27xx/patches-5.10/950-0448-spi-bcm2835-Workaround-fix-for-zero-length-transfers.patch b/target/linux/bcm27xx/patches-5.10/950-0448-spi-bcm2835-Workaround-fix-for-zero-length-transfers.patch
new file mode 100644
index 0000000000..2b9d30ff18
--- /dev/null
+++ b/target/linux/bcm27xx/patches-5.10/950-0448-spi-bcm2835-Workaround-fix-for-zero-length-transfers.patch
@@ -0,0 +1,50 @@
+From 54545710d40fd00e3c727603a446500b1b9b3d88 Mon Sep 17 00:00:00 2001
+From: Phil Elwell <phil@raspberrypi.com>
+Date: Thu, 28 Jan 2021 11:30:04 +0000
+Subject: [PATCH] spi: bcm2835: Workaround/fix for zero-length
+ transfers
+
+A relatively recent commit ([1]) contained optimisation for the PIO
+SPI FIFO-filling functions. The commit message includes the phrase
+"[t]he blind and counted loops are always called with nonzero count".
+This is technically true, but it is still possible for count to become
+zero before the loop is entered - if tfr->len is zero. Moving the loop
+exit condition to the end of the loop saves a few cycles, but results
+in a near-infinite loop should the revised count be zero on entry.
+
+Strangely, zero-lengthed transfers aren't filtered by the SPI framework
+and, even more strangely, the Python3 spidev library is triggering them
+for no obvious reason.
+
+Avoid the problem completely by bailing out of the main transfer
+function early if trf->len is zero, although there may be a case for
+moving the mitigation into the framework.
+
+See: https://github.com/raspberrypi/linux/issues/4100
+
+Signed-off-by: Phil Elwell <phil@raspberrypi.com>
+
+[1] 26751de25d25 ("spi: bcm2835: Micro-optimise FIFO loops")
+---
+ drivers/spi/spi-bcm2835.c | 10 ++++++++++
+ 1 file changed, 10 insertions(+)
+
+--- a/drivers/spi/spi-bcm2835.c
++++ b/drivers/spi/spi-bcm2835.c
+@@ -1092,6 +1092,16 @@ static int bcm2835_spi_transfer_one(stru
+ 	unsigned long hz_per_byte, byte_limit;
+ 	u32 cs = bs->prepare_cs[spi->chip_select];
+ 
++	if (unlikely(!tfr->len)) {
++		static int warned;
++
++		if (!warned)
++			dev_warn(&spi->dev,
++				 "zero-length SPI transfer ignored\n");
++		warned = 1;
++		return 0;
++	}
++
+ 	/* set clock */
+ 	spi_hz = tfr->speed_hz;
+