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Date:   Thu, 2 Aug 2018 17:15:39 +0800
From:   Feng Tang <feng.tang@intel.com>
To:     Ulf Hansson <ulf.hansson@linaro.org>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "linux-mmc@vger.kernel.org" <linux-mmc@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Adrian Hunter <adrian.hunter@intel.com>, ajan@linux.intel.com,
        Alan Cox <gnomes@lxorguk.ukuu.org.uk>
Subject: Re: [PATCH] mmc: Move the mmc driver init earlier
Message-ID: <20180802091539.m5k7cdoad6hfkdfy@shbuild888>
References: <20180608095154.22413-1-feng.tang@intel.com>
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Hi Ulf,

On Tue, Jun 12, 2018 at 12:29:50PM +0200, Ulf Hansson wrote:
> On 12 June 2018 at 10:42, Feng Tang <feng.tang@intel.com> 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@intel.com> 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.

I've followed your suggestion to try 4.18-rc4 kernel, and there is around
15% reduction in the rootfs mount's waiting for eMMC storage. And our
SDHCI pci controller does support MMC_CAP_WAIT_WHILE_BUSY.

> 
> 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.
> 
> 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.

Our platform is NUC with Celeron processors, whose most IO controllers
are PCI device.

> 
> >
> >>
> >> >
> >> > Signed-off-by: Feng Tang <feng.tang@intel.com>
> >> > ---
> >> >  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-$(CONFIG_REGULATOR)               += regulator/
> >> >  # reset controllers early, since gpu drivers might rely on them to initialize
> >> >  obj-$(CONFIG_RESET_CONTROLLER) += reset/
> >> >
> >> > +# 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.

IMHO I don't think the 100ms waiting is a problem of mmc core or SDHCI
host controller :), complex HWs' initializations take time, like the
SATA disk, the GFX.

Currently the mmc folder is too late in the drivers/Makefile, and network
device, thermal, spi, usb and many other controllers get initialized 
before mmc (which is the common rootfs storage for smartphones and 
embedded devices), which triggers the not necessary "Waiting for rootfs
device" (like the 100ms extra boot time in our platform).

That's why I propose the leap the mmc subsystem earlier to avoid it.

Thanks,
Feng