linux-kernel - SDHCI: mdelay() in hot path in esdhc_pltfm_set

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Date:	Tue, 30 Jun 2015 15:43:09 +0200
From:	Holger Schurig <holgerschurig@...il.com>
To:	linux-mmc@...r.kernel.org, chris@...ntf.net,
	ulf.hansson@...aro.org, open list <linux-kernel@...r.kernel.org>
Subject: SDHCI: mdelay() in hot path in esdhc_pltfm_set_clock looses CAN (!) frames

Hi,

I noticed in a kernel 4.0.7 that I loose CAN packets when an incoming
rsync transfer changes my eMMC or SD-Card image. I used CONFIG_FRACE
to find why this is the case, and came to this trace:

# tracer: preemptoff
#
# preemptoff latency trace v1.1.5 on 4.0.7
# --------------------------------------------------------------------
# latency: 1046 us, #756/756, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
#    -----------------
#    | task: mmcqd/0-76 (uid:0 nice:0 policy:0 rt_prio:0)
#    -----------------
#  => started at: sdhci_do_set_ios
#  => ended at:   sdhci_do_set_ios
#
#
#                  _------=> CPU#
#                 / _-----=> irqs-off
#                | / _----=> need-resched
#                || / _---=> hardirq/softirq
#                ||| / _--=> preempt-depth
#                |||| /     delay
#  cmd     pid   ||||| time  |   caller
#     \   /      |||||  \    |   /
mmcqd/0-76      2d..1    1us : _raw_spin_lock_irq <-sdhci_do_set_ios
mmcqd/0-76      2d..2    2us : esdhc_writeb_le <-sdhci_do_set_ios
mmcqd/0-76      2d..2    4us : esdhc_pltfm_set_bus_width <-sdhci_do_set_ios
mmcqd/0-76      2d..2    5us : l2c210_sync <-esdhc_pltfm_set_bus_width
mmcqd/0-76      2d..2    7us : esdhc_writeb_le <-sdhci_do_set_ios
mmcqd/0-76      2d..2    9us : l2c210_sync <-esdhc_writeb_le
mmcqd/0-76      2d..2   10us : esdhc_readw_le <-sdhci_do_set_ios
mmcqd/0-76      2d..2   12us : esdhc_writew_le <-sdhci_do_set_ios
mmcqd/0-76      2d..2   14us : l2c210_sync <-esdhc_writew_le
mmcqd/0-76      2d..2   16us : l2c210_sync <-esdhc_writew_le
mmcqd/0-76      2d..2   18us : esdhc_readw_le <-sdhci_do_set_ios
mmcqd/0-76      2d..2   19us : esdhc_writew_le <-sdhci_do_set_ios
mmcqd/0-76      2d..2   21us : l2c210_sync <-esdhc_writew_le
mmcqd/0-76      2d..2   22us : esdhc_set_uhs_signaling <-sdhci_do_set_ios
mmcqd/0-76      2d..2   24us : pinctrl_select_state <-esdhc_set_uhs_signaling
mmcqd/0-76      2d..2   26us : esdhc_readl_le <-esdhc_pltfm_set_clock
mmcqd/0-76      2d..2   28us : esdhc_writel_le <-esdhc_pltfm_set_clock
mmcqd/0-76      2d..2   29us : l2c210_sync <-esdhc_writel_le
mmcqd/0-76      2d..2   31us : esdhc_readl_le <-esdhc_pltfm_set_clock
mmcqd/0-76      2d..2   33us : esdhc_writel_le <-esdhc_pltfm_set_clock
mmcqd/0-76      2d..2   34us : l2c210_sync <-esdhc_writel_le
mmcqd/0-76      2d..2   36us : l2c210_sync <-esdhc_pltfm_set_clock
mmcqd/0-76      2d..2   37us : __timer_const_udelay <-esdhc_pltfm_set_clock
mmcqd/0-76      2d..2   39us : __timer_delay <-__timer_const_udelay
mmcqd/0-76      2d..2   40us : read_current_timer <-__timer_delay
mmcqd/0-76      2d..2   41us : imx_read_current_timer <-read_current_timer
mmcqd/0-76      2d..2   43us : read_current_timer <-__timer_delay
mmcqd/0-76      2d..2   44us : imx_read_current_timer <-read_current_timer
mmcqd/0-76      2d..2   45us : read_current_timer <-__timer_delay
....

So it seems that esdhc_pltfm_set_clock() somehow waits. And really
there is an mdelay(1) there. So it waits a whopping millisecond there!

What's worse: I put a printk() into this function, just before the
mdelay(). And it get's called hundreds of times when I boot, or when I
update via rsync. And I believe that for each call the mdelay() kills
preemption.

This is bad because in the FlexCAN driver the ISR calls
napi_schedule(), which in turn clears the CAN RxFIFO. And if this
scheduling is taking too long, CAN packets are dropped. With 50 bits
per CAN packet anf 500000 bit/s this can happen quite easily (the
RxFIFO is a hardware component that can hold only 6 CAN frames before
data is lost).

The mdelay() is also bad because it makes eMMC/SD-Card access WAY
slower than they could be.

I'm not firm with SDHCI, I only see that esdhc_pltfm_set_clock() is
all the time called with 0 Hz and then with 20000000 Hz. Any tips on
how to get rid of this clock-off-clock-on dance?

Holger
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