[PATCH] mmc: core: Throttle polling rate for CMD6

[PATCH] mmc: core: Throttle polling rate for CMD6

[Ulf Hansson]

In mmc_poll_for_busy() we loop continuously, either by sending a CMD13 or
by invoking the ->card_busy() host ops, as to detect when the card stops
signaling busy. This behaviour is problematic as it may cause CPU hogging,
especially when the busy signal time reaches beyond a few ms.

Let's fix the issue by adding a throttling mechanism, that inserts a
usleep_range() in between the polling attempts. The sleep range starts at
16-32us, but increases for each loop by a factor of 2, up until the range
reaches ~32-64ms. In this way, we are able to keep the loop fine-grained
enough for short busy signaling times, while also not hogging the CPU for
longer times.

Note that, this change is inspired by the similar throttling mechanism that
we already use for mmc_do_erase().

Reported-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
---

Hi all,

I have tried this on a ux500 platform (using the mmci mmc host driver), which
has two different eMMCs connected to the board. I manually checked that the busy
loops during boot for the worst CMD6 case, dropped from around ~50 attempts to
around 5-6 attempts, while the total loop time stayed to the similar numbers. In
general the loop times I observed for all the CMD6 sent during boot, were in the
range of 0 - 900us.

Any feedback and of course tests are greatly appreciated!

Kind regards
Ulf Hansson

---
 drivers/mmc/core/mmc_ops.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/mmc/core/mmc_ops.c b/drivers/mmc/core/mmc_ops.c
index da425ee2d9bf..446a37cc2a86 100644
--- a/drivers/mmc/core/mmc_ops.c
+++ b/drivers/mmc/core/mmc_ops.c
@@ -456,6 +456,7 @@ static int mmc_poll_for_busy(struct mmc_card *card, unsigned int timeout_ms,
 	struct mmc_host *host = card->host;
 	int err;
 	unsigned long timeout;
+	unsigned int udelay = 32, udelay_max = 32768;
 	u32 status = 0;
 	bool expired = false;
 	bool busy = false;
@@ -500,6 +501,13 @@ static int mmc_poll_for_busy(struct mmc_card *card, unsigned int timeout_ms,
 				mmc_hostname(host), __func__);
 			return -ETIMEDOUT;
 		}
+
+		/* Throttle the polling rate to avoid hogging the CPU. */
+		if (busy) {
+			usleep_range(udelay, udelay * 2);
+			if (udelay < udelay_max)
+				udelay *= 2;
+		}
 	} while (busy);
 
 	return 0;
-- 
2.17.1