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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: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
alistair/sunxi64-5.4-dsi
Martin Sperl 2019-02-23 08:49:48 +00:00 committed by Mark Brown
parent 154f7da56f
commit 0ff2de8bb1
No known key found for this signature in database
GPG Key ID: 24D68B725D5487D0
2 changed files with 56 additions and 10 deletions
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59
drivers/spi/spi.c
View File

@ -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);

7
include/linux/spi/spi.h
View File

@ -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;