spi: Add spi_mem_calc_op_duration() helper

Merge series from Miquel Raynal <miquel.raynal@bootlin.com>:

Add a spi_mem_calc_op_duration() helper

drivers/spi/spi-mem.c

@@ -562,6 +562,36 @@ void spi_mem_adjust_op_freq(struct spi_mem *mem, struct spi_mem_op *op)
 }
 EXPORT_SYMBOL_GPL(spi_mem_adjust_op_freq);
 
+/**
+ * spi_mem_calc_op_duration() - Derives the theoretical length (in ns) of an
+ *			        operation. This helps finding the best variant
+ *			        among a list of possible choices.
+ * @op: the operation to benchmark
+ *
+ * Some chips have per-op frequency limitations, PCBs usually have their own
+ * limitations as well, and controllers can support dual, quad or even octal
+ * modes, sometimes in DTR. All these combinations make it impossible to
+ * statically list the best combination for all situations. If we want something
+ * accurate, all these combinations should be rated (eg. with a time estimate)
+ * and the best pick should be taken based on these calculations.
+ *
+ * Returns a ns estimate for the time this op would take.
+ */
+u64 spi_mem_calc_op_duration(struct spi_mem_op *op)
+{
+	u64 ncycles = 0;
+	u32 ns_per_cycles;
+
+	ns_per_cycles = 1000000000 / op->max_freq;
+	ncycles += ((op->cmd.nbytes * 8) / op->cmd.buswidth) / (op->cmd.dtr ? 2 : 1);
+	ncycles += ((op->addr.nbytes * 8) / op->addr.buswidth) / (op->addr.dtr ? 2 : 1);
+	ncycles += ((op->dummy.nbytes * 8) / op->dummy.buswidth) / (op->dummy.dtr ? 2 : 1);
+	ncycles += ((op->data.nbytes * 8) / op->data.buswidth) / (op->data.dtr ? 2 : 1);
+
+	return ncycles * ns_per_cycles;
+}
+EXPORT_SYMBOL_GPL(spi_mem_calc_op_duration);
+
 static ssize_t spi_mem_no_dirmap_read(struct spi_mem_dirmap_desc *desc,
 				      u64 offs, size_t len, void *buf)
 {

include/linux/spi/spi-mem.h

@@ -424,6 +424,7 @@ bool spi_mem_default_supports_op(struct spi_mem *mem,
 
 int spi_mem_adjust_op_size(struct spi_mem *mem, struct spi_mem_op *op);
 void spi_mem_adjust_op_freq(struct spi_mem *mem, struct spi_mem_op *op);
+u64 spi_mem_calc_op_duration(struct spi_mem_op *op);
 
 bool spi_mem_supports_op(struct spi_mem *mem,
 			 const struct spi_mem_op *op);