Unverified Commit 0ff2de8b authored by Martin Sperl's avatar Martin Sperl Committed by Mark Brown
Browse files

spi: core: allow defining time that cs is deasserted 



For some SPI devices that support speed_hz > 1MHz the default 10 us delay
when cs_change = 1 is typically way to long and may result in poor spi bus
utilization.

This patch makes it possible to control the delay at micro or nano second
resolution on a per spi_transfer basis. It even allows an "as fast as
possible" mode with:
    xfer.cs_change_delay_unit = SPI_DELAY_UNIT_NSECS;
    xfer.cs_change_delay = 0;

The delay code is shared between delay_usecs and cs_change_delay for
consistency and reuse, so in the future this change_delay_unit could also
apply to delay_usec as well.

Note that on slower SOCs/CPU actually reaching ns deasserts on cs is not
realistic as the gpio overhead alone (without any delays added ) may
already leave cs deasserted for more than 1us - at least on a raspberry pi.
But at the very least this way we can keep it as short as possible.

Signed-off-by: default avatarMartin Sperl <kernel@martin.sperl.org>
Signed-off-by: default avatarMark Brown <broonie@kernel.org>
parent 154f7da5

drivers/spi/spi.c

+49 −10
Original line number Diff line number Diff line
@@ -1090,6 +1090,52 @@ static int spi_transfer_wait(struct spi_controller *ctlr, 
	return 0;

}



static void _spi_transfer_delay_ns(u32 ns)

{

	if (!ns)

		return;

	if (ns <= 1000) {

		ndelay(ns);

	} else {

		u32 us = DIV_ROUND_UP(ns, 1000);



		if (us <= 10)

			udelay(us);

		else

			usleep_range(us, us + DIV_ROUND_UP(us, 10));

	}

}



static void _spi_transfer_cs_change_delay(struct spi_message *msg,

					  struct spi_transfer *xfer)

{

	u32 delay = xfer->cs_change_delay;

	u32 unit = xfer->cs_change_delay_unit;



	/* return early on "fast" mode - for everything but USECS */

	if (!delay && unit != SPI_DELAY_UNIT_USECS)

		return;



	switch (unit) {

	case SPI_DELAY_UNIT_USECS:

		/* for compatibility use default of 10us */

		if (!delay)

			delay = 10000;

		else

			delay *= 1000;

		break;

	case SPI_DELAY_UNIT_NSECS: /* nothing to do here */

		break;

	default:

		dev_err_once(&msg->spi->dev,

			     "Use of unsupported delay unit %i, using default of 10us\n",

			     xfer->cs_change_delay_unit);

		delay = 10000;

	}

	/* now sleep for the requested amount of time */

	_spi_transfer_delay_ns(delay);

}



/*

 * spi_transfer_one_message - Default implementation of transfer_one_message()

 *
@@ -1148,14 +1194,8 @@ static int spi_transfer_one_message(struct spi_controller *ctlr, 
		if (msg->status != -EINPROGRESS)

			goto out;



		if (xfer->delay_usecs) {

			u16 us = xfer->delay_usecs;



			if (us <= 10)

				udelay(us);

			else

				usleep_range(us, us + DIV_ROUND_UP(us, 10));

		}

		if (xfer->delay_usecs)

			_spi_transfer_delay_ns(xfer->delay_usecs * 1000);



		if (xfer->cs_change) {

			if (list_is_last(&xfer->transfer_list,
@@ -1163,7 +1203,7 @@ static int spi_transfer_one_message(struct spi_controller *ctlr, 
				keep_cs = true;

			} else {

				spi_set_cs(msg->spi, false);

				udelay(10);

				_spi_transfer_cs_change_delay(msg, xfer);

				spi_set_cs(msg->spi, true);

			}

		}
@@ -3757,4 +3797,3 @@ err0: 
 * include needing to have boardinfo data structures be much more public.

 */

postcore_initcall(spi_init);

include/linux/spi/spi.h

+7 −0
Original line number Diff line number Diff line
@@ -735,6 +735,9 @@ extern void spi_res_release(struct spi_controller *ctlr, 
 * @bits_per_word: select a bits_per_word other than the device default

 *      for this transfer. If 0 the default (from @spi_device) is used.

 * @cs_change: affects chipselect after this transfer completes

 * @cs_change_delay: delay between cs deassert and assert when

 *      @cs_change is set and @spi_transfer is not the last in @spi_message

 * @cs_change_delay_unit: unit of cs_change_delay

 * @delay_usecs: microseconds to delay after this transfer before

 *	(optionally) changing the chipselect status, then starting

 *	the next transfer or completing this @spi_message.
@@ -824,6 +827,10 @@ struct spi_transfer { 
	u8		bits_per_word;

	u8		word_delay_usecs;

	u16		delay_usecs;

	u16		cs_change_delay;

	u8		cs_change_delay_unit;

#define SPI_DELAY_UNIT_USECS	0

#define SPI_DELAY_UNIT_NSECS	1

	u32		speed_hz;

	u16		word_delay;

CRA Git | Maintained and supported by SUSTech CRA and CCSE