spi: Helper for deriving timeout values

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

I recently came across an issue with the Atmel spi controller driver
which would stop my transfers after a too small timeout when performing
big transfers (reading a 4MiB flash in one transfer). My initial idea
was to derive a the maximum amount of time a transfer would take
depending on its size and use that as value to avoid erroring-out when
not relevant. Mark wanted to go further by creating a core helper doing
that, based on the heuristics from the sun6i driver.

Here is a small series of 3 patches doing exactly that.

1 files changed, 17 insertions, 0 deletions

diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index cfe42f8cd7a4..32c94eae8926 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -1261,6 +1261,23 @@ static inline bool spi_is_bpw_supported(struct spi_device *spi, u32 bpw)
 	return false;
 }
 
+/**
+ * spi_controller_xfer_timeout - Compute a suitable timeout value
+ * @ctlr: SPI device
+ * @xfer: Transfer descriptor
+ *
+ * Compute a relevant timeout value for the given transfer. We derive the time
+ * that it would take on a single data line and take twice this amount of time
+ * with a minimum of 500ms to avoid false positives on loaded systems.
+ *
+ * Returns: Transfer timeout value in milliseconds.
+ */
+static inline unsigned int spi_controller_xfer_timeout(struct spi_controller *ctlr,
+						       struct spi_transfer *xfer)
+{
+	return max(xfer->len * 8 * 2 / (xfer->speed_hz / 1000), 500U);
+}
+
 /*---------------------------------------------------------------------------*/
 
 /* SPI transfer replacement methods which make use of spi_res */