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authorDaniel Kurtz <djkurtz@chromium.org>2016-10-07 18:55:47 +0800
committerMark Brown <broonie@kernel.org>2016-10-26 12:35:17 +0100
commit8244bd3ab405a1268223a282b32d28031f7e16fe (patch)
treee2e100f44d7c548d2bc436612751a1f0b26590e7 /drivers/spi/spi.c
parent1001354ca34179f3db924eb66672442a173147dc (diff)
downloadlinux-8244bd3ab405a1268223a282b32d28031f7e16fe.tar.bz2
spi: change post transfer udelay() to usleep_range() for long delays
The spi_transfer parameter delay_usecs allows specifying a time to wait after transferring a spi message. This wait can be quite long - some devices, such as some Chrome OS ECs, require as much as 2000 usecs after a SPI transaction, before it can respond. (cf: arch/arm64/boot/dts/nvidia/tegra132-norrin.dts: google,cros-ec-spi-msg-delay = <2000> ) Blocking a CPU for 2 msecs in a busy loop like this doesn't seem very friendly to other processes, so change the blocking delay to a sleep to allow other things to use this CPU (or so it can sleep). This should be safe to do, because: (a) A post-transaction delay like this is always specified as a minimum wait time (b) A delay here is most likely not very time sensitive, as it occurs after all data has been transferred (c) This delay occurs in a non-critical section of the spi worker thread so where it is safe to sleep. Two caveats: 1) To avoid penalizing short delays, still use udelay for delays < 10us. 2) usleep_range() very often picks the upper bound, an upper bounds 10% should be plenty. Signed-off-by: Daniel Kurtz <djkurtz@chromium.org> Signed-off-by: Mark Brown <broonie@kernel.org>
Diffstat (limited to 'drivers/spi/spi.c')
-rw-r--r--drivers/spi/spi.c10
1 files changed, 8 insertions, 2 deletions
diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 5787b723b593..42f3e1cb9212 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -1034,8 +1034,14 @@ static int spi_transfer_one_message(struct spi_master *master,
if (msg->status != -EINPROGRESS)
goto out;
- if (xfer->delay_usecs)
- udelay(xfer->delay_usecs);
+ 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->cs_change) {
if (list_is_last(&xfer->transfer_list,