spi: Reintroduce spi_set_cs_timing()

commit 4ccf359 ("spi: remove spi_set_cs_timing()"), removed the
method as noboby used it. Nobody used it probably because some SPI
controllers use some default large cs-setup time that covers the usual
cs-setup time required by the spi devices. There are though SPI controllers
that have a smaller granularity for the cs-setup time and their default
value can't fulfill the spi device requirements. That's the case for the
at91 QSPI IPs where the default cs-setup time is half of the QSPI clock
period. This was observed when using an sst26vf064b SPI NOR flash which
needs a spi-cs-setup-ns = <7>; in order to be operated close to its maximum
104 MHz frequency.

Call spi_set_cs_timing() in spi_setup() just before calling spi_set_cs(),
as the latter needs the CS timings already set.
If spi->controller->set_cs_timing is not set, the method will return 0.
There's no functional impact expected for the existing drivers. Even if the
spi-mt65xx.c and spi-tegra114.c drivers set the set_cs_timing method,
there's no user for them as of now. The only tested user of this support
will be a SPI NOR flash that comunicates with the Atmel QSPI controller for
which the support follows in the next patches.

One will notice that this support is a bit different from the one that was
removed in commit 4ccf359 ("spi: remove spi_set_cs_timing()"),
because this patch adapts to the changes done after the removal: the move
of the cs delays to the spi device, the retirement of the lelgacy GPIO
handling. The mutex handling was removed from spi_set_cs_timing() because
we now always call spi_set_cs_timing() in spi_setup(), which already
handles the spi->controller->io_mutex, so use the mutex handling from
spi_setup().

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20221117105249.115649-4-tudor.ambarus@microchip.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: ambarus

drivers/spi/spi.c

@@ -3621,6 +3621,37 @@ static int __spi_validate_bits_per_word(struct spi_controller *ctlr,
 	return 0;
 }
 
+/**
+ * spi_set_cs_timing - configure CS setup, hold, and inactive delays
+ * @spi: the device that requires specific CS timing configuration
+ *
+ * Return: zero on success, else a negative error code.
+ */
+static int spi_set_cs_timing(struct spi_device *spi)
+{
+	struct device *parent = spi->controller->dev.parent;
+	int status = 0;
+
+	if (spi->controller->set_cs_timing && !spi->cs_gpiod) {
+		if (spi->controller->auto_runtime_pm) {
+			status = pm_runtime_get_sync(parent);
+			if (status < 0) {
+				pm_runtime_put_noidle(parent);
+				dev_err(&spi->controller->dev, "Failed to power device: %d\n",
+					status);
+				return status;
+			}
+
+			status = spi->controller->set_cs_timing(spi);
+			pm_runtime_mark_last_busy(parent);
+			pm_runtime_put_autosuspend(parent);
+		} else {
+			status = spi->controller->set_cs_timing(spi);
+		}
+	}
+	return status;
+}
+
 /**
  * spi_setup - setup SPI mode and clock rate
  * @spi: the device whose settings are being modified
@@ -3717,6 +3748,12 @@ int spi_setup(struct spi_device *spi)
 		}
 	}
 
+	status = spi_set_cs_timing(spi);
+	if (status) {
+		mutex_unlock(&spi->controller->io_mutex);
+		return status;
+	}
+
 	if (spi->controller->auto_runtime_pm && spi->controller->set_cs) {
 		status = pm_runtime_resume_and_get(spi->controller->dev.parent);
 		if (status < 0) {