Support mmc DDR52 timing in the mmc driver for eMMC modules

[t-linden]

According to page 18 in https://elinux.org/images/9/91/Clement-sd-mmc-high-speed-support-in-linux-kernel_0.pdf
the DDR mode for eMMC was introduced with MMC 4.4 in 2009. It can run at up to 52 MHz and used at 3 V.
This is useful from the point of view of eMMC modules attached with a passive SD-card adapter to a Raspberry Pi as this would allow the available bandwidth to double without lowering the voltage to 1.8 V. On Raspberrry Pi models preceeding the Raspberry Pi Model 4 B this is important, because they don't support 1.8 V at all. The current mmc driver on the Raspberry Pi uses the timing "mmc high-speed", but using the timing "mmc DDR52" would be needed to achieve the double performance.

To reproduce
Insert an eMMC module on a SD-adapter into a Raspberry Pi. The maximum read performance is about 20 MB/s.

Expected behaviour
Insert an eMMC module on a SD-adapter into a Raspberry Pi. The expected maximum read performance would be about 40 MB/s.

Actual behaviour
Even if a eMMC module supports the timing "mmc DDR52" the mmc driver does not enable this mode.

System
See some details in this link at https://www.raspberrypi.org/forums/viewtopic.php?f=63&t=282639

[pelwell]

If you can point out the change which needs to be made we'll consider it, but without an DDR52-capable EMMC and an SD adaptor our options are limited.

[t-linden]

AFAIK your colleague timg236 has two units of the RasPiKey which is an integrated eMMC-uSD module which is DDR52 capable according to the manufacturer. If you can get one of those then that could be helpful.
raspberrypi/firmware#1235
http://www.uugear.com/product/raspikey-plug-and-play-emmc-module-for-raspberry-pi/

The needed driver patch should check for the capabilities of the eMMC module and do some checks on those.
On the Raspberry Pi 4 B the hardware can do 1.8 V so to save power one should run at that voltage when switching to DDR52.
On the previous Raspberry Pi models one would need to run at 3.3 V with DDR52.

There is already a patch that apparently does this, please see:
https://src.fedoraproject.org/rpms/kernel/blob/606a7e904bc8f49a3dcac2c59e3006a55711662b/f/arm-sdhci-esdhc-imx-fixes.patch

[timg236]

This looks like something which should be added to the upstream sdhci-iproc driver and maybe backported.

CM4 has EMMC support so that would be the best test

[lategoodbye]

Back in the days i experiment with this, but it wasn't reliable:
lategoodbye/rpi-zero@e011a84

Usually this also requires modifications to the pinctrl settings because of the high frequency signals.

[t-linden]

On which Raspberry Pi models did you find this patch unreliable?
What kind of eMMC module and eMMC-uSD adapter did you use?
What kind of error messages did you see?

On the Raspberry Pi 4 B this should work without any problems because it is already running at DDR50 speed with a SD-card.

[pelwell]

@t-linden At the moment this issue comes across as a wish for us to do an experiment on your behalf rather than a request to make something you've tested yourself available to others. Words like "should" and "AFAIK" are give-aways.

If this isn't the case, please re-open, providing a tested patch for us to incorporate.

[lategoodbye]

On which Raspberry Pi models did you find this patch unreliable?

This patch like the branch suggests was only tested on CM1.

What kind of eMMC module and eMMC-uSD adapter did you use?

Only the eMMC onboard.

What kind of error messages did you see?

Sorry this is 2 years ago. AFAIR there were random errors from the MMC subsystem.

On the Raspberry Pi 4 B this should work without any problems because it is already running at DDR50 speed with a SD-card.

Yes, but this shouldn't require any changes to the sdhci-iproc since the BCM2711 has a much better eMMC IP.

[t-linden]

The eMMC module that I have does not run att DDR52 with the kernel on a Raspberry Pi 4 B

Linux raspberrypi 5.4.51-v7l+ #1333 SMP Mon Aug 10 16:51:40 BST 2020 armv7l GNU/Linux

sudo cat /sys/kernel/debug/mmc0/ios
clock: 52000000 Hz
actual clock: 50000000 Hz
vdd: 21 (3.3 ~ 3.4 V)
bus mode: 2 (push-pull)
chip select: 0 (don't care)
power mode: 2 (on)
bus width: 2 (4 bits)
timing spec: 1 (mmc high-speed)
signal voltage: 0 (3.30 V)
driver type: 0 (driver type B)

so a change is required to get it to DDR52 speed with this kind of module.

[lategoodbye]

Are there any error messages in the kernel log like "switch to bus width 4 ddr failed"?

[t-linden]

There is no line like that seen with dmesg nor in /var/log

Based on these lines from dmesg I would assume that the driver used is the mmc-bcm2835

[ 1.332709] mmc-bcm2835 fe300000.mmcnr: could not get clk, deferring probe
[ 2.674279] mmc-bcm2835 fe300000.mmcnr: mmc_debug:0 mmc_debug2:0
[ 2.683181] mmc-bcm2835 fe300000.mmcnr: DMA channel allocated

[pelwell]

That's the SD1 interface, used for SDIO to the WiFi controller.

[lategoodbye]

@t-linden In case you are able to rebuild the Raspberry Pi 4 DTB, please try this patch:

https://gist.github.com/lategoodbye/583ef0d7cd74a39d6c8f7d7f3d2c3613

[t-linden]

Which driver is then the default one in use on the BCM2711?

[lategoodbye]

Which driver is then the default one in use on the BCM2711?

sdhci-iproc for emmc2 interface