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

[Feng Tang]

On Tue, Jun 12, 2018 at 08:05:25PM +0800, Shawn Lin wrote:
> 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
> > > > > mounted.
> > > > >          [    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
> > > > help.
> > > 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
> > 6d796c68cd15234a33a4bd2ef7231125fea2dc6c.
> 
> 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
> details.

Thanks Shawn for your tips, we did simliar thing :) for no-sd and no-sdio, 
and even disabled SDHCI controller that has nothing connected.

- Feng