AM4377: Linux SPI performance

I agree that mcspi_wait_for_reg_bit() could be at play here. Looking at the difference in commits there is a commit that stands out 13d515c796ad ("spi: omap2-mcspi: Switch to readl_poll_timeout()") which is likely suspect. Try reverting that commit (git revert 13d515c796ad) on top of v5.7.1 to see if this fixes things. If it does we can get this fixed in the upstream Kernel.

$ gitl v4.19.45...v5.7.1 drivers/spi/spi-omap2-mcspi.c
32f2fc5dc399 spi: spi-omap2-mcspi: Support probe deferral for DMA channels
e4e8276a4f65 spi: spi-omap2-mcspi: Handle DMA size restriction on AM65x
8d8584912a43 spi: omap2-mcspi: Remove redundant checks
c942fddf8793 treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 157
baf8b9f8d260 spi: omap2-mcspi: Fix DMA and FIFO event trigger size mismatch
842aeeac335e spi: omap2-mcspi: Add missing suspend and resume calls
91b9deefedf4 spi: omap2-mcspi: Add missing suspend and resume calls
89e8b9cb8465 spi: omap2-mcspi: Add slave mode support
b682cffa3ac6 spi: omap2-mcspi: Set FIFO DMA trigger level to word length
13d515c796ad spi: omap2-mcspi: Switch to readl_poll_timeout()
$ git show 13d515c796ad
commit 13d515c796adc49a49b0cd2212ccd7f43a37fc5a
Author: Vignesh R <vigneshr@ti.com>
Date:   Mon Oct 15 12:08:27 2018 +0530

    spi: omap2-mcspi: Switch to readl_poll_timeout()

    Use standard readl_poll_timeout() macro for polling on status bits.

    Signed-off-by: Vignesh R <vigneshr@ti.com>
    Signed-off-by: Mark Brown <broonie@kernel.org>

diff --git a/drivers/spi/spi-omap2-mcspi.c b/drivers/spi/spi-omap2-mcspi.c
index 508c61c669e7..985f00d8a964 100644
--- a/drivers/spi/spi-omap2-mcspi.c
+++ b/drivers/spi/spi-omap2-mcspi.c
@@ -33,6 +33,7 @@
 #include <linux/of.h>
 #include <linux/of_device.h>
 #include <linux/gcd.h>
+#include <linux/iopoll.h>

 #include <linux/spi/spi.h>
 #include <linux/gpio.h>
@@ -353,19 +354,9 @@ static void omap2_mcspi_set_fifo(const struct spi_device *spi,

 static int mcspi_wait_for_reg_bit(void __iomem *reg, unsigned long bit)
 {
-       unsigned long timeout;
-
-       timeout = jiffies + msecs_to_jiffies(1000);
-       while (!(readl_relaxed(reg) & bit)) {
-               if (time_after(jiffies, timeout)) {
-                       if (!(readl_relaxed(reg) & bit))
-                               return -ETIMEDOUT;
-                       else
-                               return 0;
-               }
-               cpu_relax();
-       }
-       return 0;
+       u32 val;
+
+       return readl_poll_timeout(reg, val, val & bit, 1, MSEC_PER_SEC);
 }

 static void omap2_mcspi_rx_callback(void *data)

Regards, Andreas