spi: axi-spi-engine: fix sleep ticks calculation

This fixes the sleep ticks calculation when generating sleep
instructions in the AXI SPI Engine driver. The previous calculation
was ignoring delays less than one microsecond and missed a microsecond
to second conversion factor.

This fixes the first issue by not rounding to microseconds. Now that
xfer->effective_speed_hz is guaranteed to be set correctly, we can use
that to simplify the calculation. This new calculation replaces the old
incorrect math.

Also add unit suffix to the delay variable for clarity while we are
touching this.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20231204-axi-spi-engine-series-2-v1-4-063672323fce@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: dlech

drivers/spi/spi-axi-spi-engine.c

@@ -168,22 +168,17 @@ static void spi_engine_gen_xfer(struct spi_engine_program *p, bool dry,
 }
 
 static void spi_engine_gen_sleep(struct spi_engine_program *p, bool dry,
-	struct spi_engine *spi_engine, unsigned int clk_div,
 	struct spi_transfer *xfer)
 {
-	unsigned int spi_clk = clk_get_rate(spi_engine->ref_clk);
 	unsigned int t;
-	int delay;
+	int delay_ns;
 
-	delay = spi_delay_to_ns(&xfer->delay, xfer);
-	if (delay < 0)
+	delay_ns = spi_delay_to_ns(&xfer->delay, xfer);
+	if (delay_ns <= 0)
 		return;
-	delay /= 1000;
 
-	if (delay == 0)
-		return;
-
-	t = DIV_ROUND_UP(delay * spi_clk, (clk_div + 1) * 2);
+	/* rounding down since executing the instruction adds a couple of ticks delay */
+	t = DIV_ROUND_DOWN_ULL((u64)delay_ns * xfer->effective_speed_hz, NSEC_PER_SEC);
 	while (t) {
 		unsigned int n = min(t, 256U);
 
@@ -224,8 +219,8 @@ static void spi_engine_precompile_message(struct spi_message *msg)
 	}
 }
 
-static void spi_engine_compile_message(struct spi_engine *spi_engine,
-	struct spi_message *msg, bool dry, struct spi_engine_program *p)
+static void spi_engine_compile_message(struct spi_message *msg, bool dry,
+				       struct spi_engine_program *p)
 {
 	struct spi_device *spi = msg->spi;
 	struct spi_controller *host = spi->controller;
@@ -261,7 +256,7 @@ static void spi_engine_compile_message(struct spi_engine *spi_engine,
 		}
 
 		spi_engine_gen_xfer(p, dry, xfer);
-		spi_engine_gen_sleep(p, dry, spi_engine, clk_div - 1, xfer);
+		spi_engine_gen_sleep(p, dry, xfer);
 
 		if (xfer->cs_change) {
 			if (list_is_last(&xfer->transfer_list, &msg->transfers)) {
@@ -515,7 +510,7 @@ static int spi_engine_prepare_message(struct spi_controller *host,
 	spi_engine_precompile_message(msg);
 
 	p_dry.length = 0;
-	spi_engine_compile_message(spi_engine, msg, true, &p_dry);
+	spi_engine_compile_message(msg, true, &p_dry);
 
 	size = sizeof(*p->instructions) * (p_dry.length + 1);
 	p = kzalloc(sizeof(*p) + size, GFP_KERNEL);
@@ -533,7 +528,7 @@ static int spi_engine_prepare_message(struct spi_controller *host,
 
 	st->sync_id = ret;
 
-	spi_engine_compile_message(spi_engine, msg, false, p);
+	spi_engine_compile_message(msg, false, p);
 
 	spi_engine_program_add_cmd(p, false, SPI_ENGINE_CMD_SYNC(st->sync_id));