Re: [PATCH] mmc: Move the mmc driver init earlier

From: Shawn Lin
Date: Tue Jun 12 2018 - 08:06:39 EST

On 2018/6/12 18:29, Ulf Hansson wrote:
On 12 June 2018 at 10:42, Feng Tang <feng.tang@xxxxxxxxx> wrote:
Hi Ulf,

Thanks for the review.

On Tue, Jun 12, 2018 at 08:25:44AM +0200, Ulf Hansson wrote:
On 8 June 2018 at 11:51, Feng Tang <feng.tang@xxxxxxxxx> wrote:
When doing some boot time optimization for an eMMC rootfs NUCs,
we found the rootfs may spend around 100 microseconds waiting
for eMMC card to be initialized, then the rootfs could be
[ 1.216561] Waiting for root device /dev/mmcblk1p1...
[ 1.289262] mmc1: new HS400 MMC card at address 0001
[ 1.289667] mmcblk1: mmc1:0001 R1J56L 14.7 GiB
[ 1.289772] mmcblk1boot0: mmc1:0001 R1J56L partition 1 8.00 MiB
[ 1.289869] mmcblk1boot1: mmc1:0001 R1J56L partition 2 8.00 MiB
[ 1.289967] mmcblk1rpmb: mmc1:0001 R1J56L partition 3 4.00 MiB
[ 1.292798] mmcblk1: p1 p2 p3
[ 1.300576] EXT4-fs (mmcblk1p1): couldn't mount as ext3 due to feature incompatibilities
[ 1.300912] EXT4-fs (mmcblk1p1): couldn't mount as ext2 due to feature incompatibilities

And this is a common problem for smartphones, tablets, embedded
devices and automotive products. This patch will make the eMMC/SD
card start initializing earlier, by changing its order in drivers/Makefile.

On our platform, the waiting for eMMC card is almost eliminated with the patch,
which is critical to boot time.

I am wondering what kernel version you are running here. There have
been some changes to the mmc initialization path, which perhaps can
These logs in commit msg are based on kernel 4.14, and the patch is generated
against kernel 4.17.

Right. So it's quite recent, even if lot's of changes have been made
to the mmc core since then.

A few things (old/new) that is important.
1) Check if your mmc host driver support MMC_CAP_WAIT_WHILE_BUSY. That
should have some effect, avoiding unnecessary polling.

2) Since 4.18 rc1, you will be able to configure an over estimated
"power on" delay (via DT as well). Look at commit

Sorry to chime in. We could also reduce the "power on" delay via hosts'
->set_ios() callback.

3) If you use a DT based platform, I think what people do is to
re-organize the order of device nodes, such that as many as possible
-EPROBE_DEFER is avoided to be returned by drivers. This is also not a
good solution, but the best we have at this moment.

Also DT based platform is allowed to add no-sd and no-sdio for the eMMC
node, which could slighly help reduce the boot time.
You could refer to Documentation/devicetree/bindings/mmc/mmc.txt for

Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
drivers/Makefile | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/Makefile b/drivers/Makefile
index 24cd47014657..c473afd3c688 100644
--- a/drivers/Makefile
+++ b/drivers/Makefile
@@ -50,6 +50,9 @@ obj-$(CONFIG_REGULATOR) += regulator/
# reset controllers early, since gpu drivers might rely on them to initialize

+# put mmc early as many morden devices use emm/sd card as rootfs storage
+obj-y += mmc/

Your suggested approach isn't really a solution, as it may work for
your particular case but not for everybody else.

Do you mean the patch may break some platforms? Yes, I only tested on
some IA based NUCs, and I did think about other architectures, things
that may affect MMC are gpio/clk/pinctrl, and those are still earlier
than mmc after change.

I don't know if it breaks things, potentially it could, if drivers
don't implement support for -EPROBE_DEFER properly.

However, more importantly, it's not real fix to the problem, just
something that seems to work for you.

Kind regards
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Best Regards
Shawn Lin