The Raspberry Pi 2 Q&A thread


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by q1t1 » Thu Feb 12, 2015 7:22 pm
dom wrote:
Fidelius wrote:What MHz values do the two settings Core and SDRAM have on a Pi2 in standard mode i.e. non overlocked, compared to raspi-config's "Pi2" mode ? (The ARM7 is 900 MHz in standard mode.)

By default arm_freq=900 core_freq=250 sdram_freq=450 on Pi2.



And by default, does the Pi2 have 0 or 2 overvolt ?

Thank you
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by rahlquist » Thu Feb 12, 2015 9:06 pm
mikerr wrote:Does the Pi2 (BCM2836) have any better standby/sleep modes ?

I am interested in this too.
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by jdb » Thu Feb 12, 2015 11:06 pm
q1t1 wrote:
dom wrote:
Fidelius wrote:What MHz values do the two settings Core and SDRAM have on a Pi2 in standard mode i.e. non overlocked, compared to raspi-config's "Pi2" mode ? (The ARM7 is 900 MHz in standard mode.)

By default arm_freq=900 core_freq=250 sdram_freq=450 on Pi2.



And by default, does the Pi2 have 0 or 2 overvolt ?

Thank you


Overvolt is relative to the "default" for the SoC. For 2835, this is 1.20V. For 2836, this is 1.3125V. Each increment (or decrement) for overvolt is 25mV.
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by q1t1 » Fri Feb 13, 2015 6:30 am
jdb wrote:
Overvolt is relative to the "default" for the SoC. For 2835, this is 1.20V. For 2836, this is 1.3125V. Each increment (or decrement) for overvolt is 25mV.


I migrate from Pi1 to Pi2, so in order to apply default configuration, I chose Medium scenario in raspi-config's overlock menu.
As it put over_voltage=2 setting in config.txt , do I have to keep it or comment it ?
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by lingon » Fri Feb 13, 2015 8:20 pm
It would be nice with a complete list of changes between the BCM2835 and BCM2836. Are there other changes than the following?
    * CPU address space increased from 512 MB to 1 GB
    * 700 MHz ARMv6 core -> quad 900 MHz ARMv7 cores
    * new 512 kB L2 ARMv7 cache
    * hardware counter for the USB FIQ interrupts

What kind of L1 instruction and data caches do the ARMv7 cores have?
How many percent of the BCM2836 silicon area is used by the ARMv7 cores in total?
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by jdb » Fri Feb 13, 2015 10:52 pm
The A7 cluster has a fully cache-coherent implementation. Each core has 32KiB I- and D- L1 cache. Snooping is implemented such that an L1 miss on a local core but an L1 hit on another core (regardless of L2 state) will result in the relevant cacheline being pulled from the remote L1 into the core-local L1 cache. The latency is ~8 cycles for worst-case snooping (I think).

The A7 cluster adds a fair amount of area to BCM2836. I don't know how much exactly - apparently the die is quite rectangular because the cores were literally bolted onto the side.

The USB timer/counter was actually something that I requested - there's some Gert magic in there that means you can route the ARMCTRL FIQ to an arbitrary core and multiplex it with a very basic timer (19.2MHz/24-bit compare-counter) to get around some of the limitations in the OTG core.
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by joan » Sat Feb 14, 2015 8:40 am
http://infocenter.arm.com/help/topic/co ... DBBIH.html section 1.5 is a little ambiguous.

Is there an FPU per core in the BCM2836?

Edited to add:

/proc/cpuinfo answers the question. Identical features per core.

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Last edited by joan on Sat Feb 14, 2015 10:24 am, edited 1 time in total.
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by fruit-uk » Sat Feb 14, 2015 8:53 am
I think this is an appropriate place to ask this...

The new d/ls for RPi2 expect at least two partitions on SD, otherwise booting fails.

Is this likely to be a permanent requirement?

I have a couple of Bs booting from 64/128MB full size SD cards - there isn't much room for a second partition on those, so I dare not update them at the moment.

Edit: I should heve added that these Pis boot to USB stick or HD, the root fs is not on SD card
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by jamesh » Sat Feb 14, 2015 10:49 am
lingon wrote:It would be nice with a complete list of changes between the BCM2835 and BCM2836. Are there other changes than the following?
    * CPU address space increased from 512 MB to 1 GB
    * 700 MHz ARMv6 core -> quad 900 MHz ARMv7 cores
    * new 512 kB L2 ARMv7 cache
    * hardware counter for the USB FIQ interrupts

What kind of L1 instruction and data caches do the ARMv7 cores have?
How many percent of the BCM2836 silicon area is used by the ARMv7 cores in total?


I put some detail on my blog a while back.

http://jamesrandominfo.blogspot.co.uk/
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by Joe Schmoe » Sat Feb 14, 2015 11:14 am
fruit-uk wrote:I think this is an appropriate place to ask this...

The new d/ls for RPi2 expect at least two partitions on SD, otherwise booting fails.

Is this likely to be a permanent requirement?

I have a couple of Bs booting from 64/128MB full size SD cards - there isn't much room for a second partition on those, so I dare not update them at the moment.

Edit: I should heve added that these Pis boot to USB stick or HD, the root fs is not on SD card


Nothing has changed. Whatever tricks you used to make this happen before - that is, taking one of the "official" downloads and converting it to a "rootfs on USB" type boot system - will work on the Pi 2 the same as they did on previous Pi models.
And some folks need to stop being fanboys and see the forest behind the trees.

(One of the best lines I've seen on this board lately)
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by fruit-uk » Sat Feb 14, 2015 11:18 am
Joe Schmoe wrote:
fruit-uk wrote:I think this is an appropriate place to ask this...

The new d/ls for RPi2 expect at least two partitions on SD, otherwise booting fails.

Is this likely to be a permanent requirement?

I have a couple of Bs booting from 64/128MB full size SD cards - there isn't much room for a second partition on those, so I dare not update them at the moment.

Edit: I should heve added that these Pis boot to USB stick or HD, the root fs is not on SD card


Nothing has changed. Whatever tricks you used to make this happen before - that is, taking one of the "official" downloads and converting it to a "rootfs on USB" type boot system - will work on the Pi 2 the same as they did on previous Pi models.


Oh, but it has. Look at a dmesg from before RPi2 release date and after, search for mmcblk.

Edit:
To suggest some kind of trickery is involved is rather a silly response.
Many Pi users are booting to either USB stick or USB HD using a standard linux kernel parameter that has worked here for many years on many Pis.
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by dom » Sat Feb 14, 2015 12:47 pm
fruit-uk wrote:Oh, but it has. Look at a dmesg from before RPi2 release date and after, search for mmcblk.

Nothing has intentionally changed. I use a single partition sdcard containing the fat partition and continue booting from an nfs mount.
If something is not working open a github issue and explain what the difference you are seeing in the dmesg log and why you think that is casing the issue.
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by fruit-uk » Sat Feb 14, 2015 1:26 pm
That is odd.

My USB partitions are all ext3fs or ext4fs. I could not boot at all until I added a second, even empty, partition to SD card.

The boot seemed not to get as far as even looking at the root given in cmdline.txt let alone mounting it.

Edit:
It would seem opening an issue on github requires me to create an account.
I really cannot cope with creating yet another account, for what will amount to just one or two posts, and all it entails at this end, especially for something others consider a non-issue, I have too many accounts set up now.

I am now aware of the issue and how it affects my machines so I am sorted. If it affects others I guess they will have to work it out for themselves.
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by pico » Sun Feb 15, 2015 5:06 am
pico wrote:Also, i've discovered that loading the peripheral modules for spi etc using the device tree is not equivalent to loading the old way... the drivers loaded using device tree seem to lead to some performance issues compared to the old drivers. Haven't investigated closely enough to figure out what is happening, but the take home is that the device_tree drivers may not perform as expected when compared to the no device-tree drivers. So in the meantime i've simply disabled the device_tree loading.


Just following up on my own post on this: I've tested SPI performance using both the device_tree SPI drivers and the older non-device_tree drivers on a Pi1 Model B now as well on a Pi2, and the results are that the performance differences only show up on the Pi2, but not on the Model B.

Booting from same image on both Model B and Pi2 (same sd card, actually, using an adapator).


For the tests, I'm running this config.txt to test device_tree drivers:

Code: Select all
#uncomment to overclock the arm. 700 MHz is the default.
arm_freq=840

# for more options see http://elinux.org/RPi_config.txt
gpu_mem=16
core_freq=250
sdram_freq=420
over_voltage=0
force_turbo=1

dtparam=i2c=on
dtparam=spi=on


and this config.txt to test older, non device_tree drivers:

Code: Select all
#uncomment to overclock the arm. 700 MHz is the default.
arm_freq=840

# for more options see http://elinux.org/RPi_config.txt
gpu_mem=16
core_freq=250
sdram_freq=420
over_voltage=0
force_turbo=1

device_tree=


No discernable performance difference using these two config.txt variations on the Model B, but there is significant difference in SPI performance on Pi2. So it does seem to be a PI2 specific issue at this stage.

Thoughts?
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by shuckle » Sun Feb 15, 2015 6:51 am
Looking the new 2B+, the power circuits looks different than in B+ and I have learned that it uses a bit more power. Is it now safe to feed for example 6 V to the microUSB or is it still limited to 5V only?
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by rpdom » Sun Feb 15, 2015 8:04 am
fruit-uk wrote:
Joe Schmoe wrote:Nothing has changed. Whatever tricks you used to make this happen before - that is, taking one of the "official" downloads and converting it to a "rootfs on USB" type boot system - will work on the Pi 2 the same as they did on previous Pi models.


Oh, but it has. Look at a dmesg from before RPi2 release date and after, search for mmcblk.

I don't have an a dmesg from before the release date, but I can say that I have not noticed the problem that you appear to be having. From the fact that no one else seems to have been having this problem, might I suggest that something may be misconfigured on your system?

Just for the record, from one of my fully updated (via apt-get) systems:
Code: Select all
rpdom@raspi:~$ dmesg | grep mmc
[    0.000000] Kernel command line: dma.dmachans=0x7f35 bcm2708_fb.fbwidth=1856 bcm2708_fb.fbheight=1048 bcm2708.boardrev=0x2 bcm2708.serial=0xe77070eb smsc95xx.macaddr=B8:27:EB:70:70:EB bcm2708_fb.fbswap=1 sdhci-bcm2708.emmc_clock_freq=250000000 vc_mem.mem_base=0xec00000 vc_mem.mem_size=0x10000000  dwc_otg.lpm_enable=0 console=tty1 root=/dev/sda1 rootfstype=ext4 elevator=deadline rootwait
[    1.593004] mmc0: new MMC card at address 0001
[    1.608538] mmcblk0: mmc0:0001 MMC    122 MiB
[    1.620835]  mmcblk0: p1
rpdom@raspi:~$ dmesg | grep root
[    0.000000] Kernel command line: dma.dmachans=0x7f35 bcm2708_fb.fbwidth=1856 bcm2708_fb.fbheight=1048 bcm2708.boardrev=0x2 bcm2708.serial=0xe77070eb smsc95xx.macaddr=B8:27:EB:70:70:EB bcm2708_fb.fbswap=1 sdhci-bcm2708.emmc_clock_freq=250000000 vc_mem.mem_base=0xec00000 vc_mem.mem_size=0x10000000  dwc_otg.lpm_enable=0 console=tty1 root=/dev/sda1 rootfstype=ext4 elevator=deadline rootwait
[    1.587431] Waiting for root device /dev/sda1...
[    5.049776] VFS: Mounted root (ext4 filesystem) readonly on device 8:1.
rpdom@raspi:~$ uname -a
Linux raspi 3.18.5+ #744 PREEMPT Fri Jan 30 18:19:07 GMT 2015 armv6l GNU/Linux

Perhaps you could give examples of what you are seeing?
To suggest some kind of trickery is involved is rather a silly response.
Many Pi users are booting to either USB stick or USB HD using a standard linux kernel parameter that has worked here for many years on many Pis.

and is still working, from what we can see.
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by fruit-uk » Sun Feb 15, 2015 8:43 am
My equivalent from today on a B+ is..
Code: Select all
root@rpi:/home/fruit#  dmesg | grep mmc
[    0.000000] Kernel command line: dma.dmachans=0x7f35 bcm2708_fb.fbwidth=720 bcm2708_fb.fbheight=576 bcm2708.boardrev=0x10 bcm2708.serial=0x9356eca4 smsc95xx.macaddr=B8:27:EB:56:EC:A4 bcm2708_fb.fbswap=1 bcm2708.disk_led_gpio=47 bcm2708.disk_led_active_low=0 sdhci-bcm2708.emmc_clock_freq=250000000 vc_mem.mem_base=0x1ec00000 vc_mem.mem_size=0x20000000  dwc_otg.lpm_enable=0 console=tty1 root=/dev/sda3 rootfstype=ext4 elevator=deadline rootwait
[    1.525784] mmc0: host does not support reading read-only switch, assuming write-enable
[    1.530518] mmc0: new high speed SDHC card at address 59b4
[    1.533629] mmcblk0: mmc0:59b4 00000 7.35 GiB
[    1.537600]  mmcblk0: p1 p2
root@rpi:/home/fruit#  dmesg | grep root
[    0.000000] Kernel command line: dma.dmachans=0x7f35 bcm2708_fb.fbwidth=720 bcm2708_fb.fbheight=576 bcm2708.boardrev=0x10 bcm2708.serial=0x9356eca4 smsc95xx.macaddr=B8:27:EB:56:EC:A4 bcm2708_fb.fbswap=1 bcm2708.disk_led_gpio=47 bcm2708.disk_led_active_low=0 sdhci-bcm2708.emmc_clock_freq=250000000 vc_mem.mem_base=0x1ec00000 vc_mem.mem_size=0x20000000  dwc_otg.lpm_enable=0 console=tty1 root=/dev/sda3 rootfstype=ext4 elevator=deadline rootwait
[    1.510251] Waiting for root device /dev/sda3...
[    3.779486] VFS: Mounted root (ext4 filesystem) readonly on device 8:3.
root@rpi:/home/fruit#  uname -a
Linux rpi 3.18.7+ #755 PREEMPT Thu Feb 12 17:14:31 GMT 2015 armv6l GNU/Linux

so very similar except for the first part and mmcblk.
Differences in command line are
mine..
Code: Select all
Kernel command line:  bcm2708_fb.fbwidth=720 bcm2708_fb.fbheight=576 bcm2708.boardrev=0x10 bcm2708.serial=0x9356eca4 smsc95xx.macaddr=B8:27:EB:56:EC:A4  bcm2708.disk_led_gpio=47 bcm2708.disk_led_active_low=0  vc_mem.mem_base=0x1ec00000 vc_mem.mem_size=0x20000000    root=/dev/sda3 rootfstype=ext4 
yours..
Code: Select all
Kernel command line:  bcm2708_fb.fbwidth=1856 bcm2708_fb.fbheight=1048 bcm2708.boardrev=0x2 bcm2708.serial=0xe77070eb smsc95xx.macaddr=B8:27:EB:70:70:EB   vc_mem.mem_base=0xec00000 vc_mem.mem_size=0x10000000    root=/dev/sda1 rootfstype=ext4 


There are differences in the vc_mem.mem_ entries, could that be the issue (this is a B+)?
There is nothing else that looks to be causing those differences in output to me but perhaps I am overlooking something.

My pre-RPi2 mmcblk entries looked very similar to the ones you've posted from memory but I can check if needed.
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by rpdom » Sun Feb 15, 2015 9:04 am
fruit-uk wrote:There are differences in the vc_mem.mem_ entries, could that be the issue (this is a B+)?

That particular Pi is a early Rev1.0 B with 256MB (boardrev=0x2), so that would explain the memory map differences.

I do have a B+ I can test it on. I just need to copy the boot partition to a microSD card.
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by fruit-uk » Sun Feb 15, 2015 9:16 am
Just looked at another B+, same as first one so same revision perhaps, though there was some time difference between the two purchases - must have been at the bottom of the pile I guess ;)

Are you going to try with a single SD partition? That would be helpful to me.
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by rpdom » Sun Feb 15, 2015 10:19 am
fruit-uk wrote:Are you going to try with a single SD partition? That would be helpful to me.

There is only one SD partition on that Pi.
Here's the results for a B+ (copied the data from SD to microSD and used the same USB HD).
Code: Select all
rpdom@raspi:~$ dmesg | grep mmc
[    0.000000] Kernel command line: dma.dmachans=0x7f35 bcm2708_fb.fbwidth=1856 bcm2708_fb.fbheight=1048 bcm2708.boardrev=0x10 bcm2708.serial=0x818332f7 smsc95xx.macaddr=B8:27:EB:83:32:F7 bcm2708_fb.fbswap=1 bcm2708.disk_led_gpio=47 bcm2708.disk_led_active_low=0 sdhci-bcm2708.emmc_clock_freq=250000000 vc_mem.mem_base=0x1ec00000 vc_mem.mem_size=0x20000000  dwc_otg.lpm_enable=0 console=tty1 root=/dev/sda1 rootfstype=ext4 elevator=deadline rootwait
[    1.589765] mmc0: host does not support reading read-only switch, assuming write-enable
[    1.595460] mmc0: new SD card at address c0c1
[    1.608416] mmcblk0: mmc0:c0c1 SU02G 1.89 GiB
[    1.628841]  mmcblk0: p1
rpdom@raspi:~$ dmesg | grep root
[    0.000000] Kernel command line: dma.dmachans=0x7f35 bcm2708_fb.fbwidth=1856 bcm2708_fb.fbheight=1048 bcm2708.boardrev=0x10 bcm2708.serial=0x818332f7 smsc95xx.macaddr=B8:27:EB:83:32:F7 bcm2708_fb.fbswap=1 bcm2708.disk_led_gpio=47 bcm2708.disk_led_active_low=0 sdhci-bcm2708.emmc_clock_freq=250000000 vc_mem.mem_base=0x1ec00000 vc_mem.mem_size=0x20000000  dwc_otg.lpm_enable=0 console=tty1 root=/dev/sda1 rootfstype=ext4 elevator=deadline rootwait
[    1.578128] Waiting for root device /dev/sda1...
[   14.256235] VFS: Mounted root (ext4 filesystem) readonly on device 8:1.
rpdom@raspi:~$ uname -a
Linux raspi 3.18.5+ #744 PREEMPT Fri Jan 30 18:19:07 GMT 2015 armv6l GNU/Linux
rpdom@raspi:~$ cat /boot/cmdline.txt
dwc_otg.lpm_enable=0 console=tty1 root=/dev/sda1 rootfstype=ext4 elevator=deadline rootwait

Also, fdisk of the microSD card
Code: Select all
rpdom@raspi:~$ /sbin/fdisk -l /dev/mmcblk0

Disk /dev/mmcblk0: 2032 MB, 2032664576 bytes
3 heads, 28 sectors/track, 47262 cylinders, total 3970048 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

        Device Boot      Start         End      Blocks   Id  System
/dev/mmcblk0p1              28      250879      125426    c  W95 FAT32 (LBA)

The SD card in the B is a 128MB MMC card, with one FAT32 partition taking up the whole card. The uSD I used in the B+ is a 2GB card and I just dd'd the whole card to the uSD for testing.
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by rurwin » Sun Feb 15, 2015 10:44 am
I can't find it now, but there was a question about running with USB drives and the answer mentioned that the boot SD needed the files to be in the first few blocks of the device. It seems to me that having two partitions is the only way to ensure that on a large device. However on a 128MB device it's a given.
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by fruit-uk » Sun Feb 15, 2015 10:54 am
Mm... that is puzzling.

I see your cmdline.txt differs from mine, which has evolved over time and been copied from Pi to Pi..
Code: Select all
dwc_otg.lpm_enable=0 dwc_otg.speed=1 dwc_otg.fiq_fix_enable=1 kgdboc=ttyAMA0,115200 console=tty1 root=/dev/sda3 rootfstype=ext3 rootwait
However it is still working on the Pis that now have two partitions.

I've just noticed rurwins post as I write this... that is a possiblity and perhaps the most likely explanation. The SD on the B+ is 8GB.

I suppose I had better try one of the older Bs with 64 or 128MB cards and see what happens. I've been reluctant up to now as I thought I'd see the same issue. I'll post whether a success or otherwise shortly.
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by rpdom » Sun Feb 15, 2015 11:11 am
fruit-uk wrote:I've just noticed rurwins post as I write this... that is a possiblity and perhaps the most likely explanation. The SD on the B+ is 8GB.

That's easy to test. I've got an 8GB card here. I'll create a single large partition on it and see what happens.
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by fruit-uk » Sun Feb 15, 2015 11:36 am
I have just updated a B with a 128MB SD card, single partition and all is well, interestingly..
Code: Select all
root@nightcam:/home/fruit#  dmesg | grep mmc
[    0.000000] Kernel command line: dma.dmachans=0x7f35 bcm2708_fb.fbwidth=656 bcm2708_fb.fbheight=416 bcm2708.boardrev=0xe bcm2708.serial=0x86a67a0 smsc95xx.macaddr=B8:27:EB:6A:67:A0 bcm2708_fb.fbswap=1 sdhci-bcm2708.emmc_clock_freq=250000000 vc_mem.mem_base=0x1ec00000 vc_mem.mem_size=0x20000000  dwc_otg.lpm_enable=0 console=ttyAMA0,115200 kgdboc=ttyAMA0,115200 console=tty1 root=/dev/sda3 rootfstype=ext3 rootwait rootdelay=5
[    3.050871] mmc0: host does not support reading read-only switch, assuming write-enable
[    3.062266] mmc0: new SD card at address ec05
[    3.069099] mmcblk0: mmc0:ec05 S128B 122 MiB
[    3.077561]  mmcblk0: p1
A difference in that last line with p1 but no p2 but where is it coming from?
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by rpdom » Sun Feb 15, 2015 12:59 pm
I had no problems with a 8GB card with the partition taking up the whole card. Still booted my B+ to the USB disk.

fruit-uk wrote:A difference in that last line with p1 but no p2 but where is it coming from?

The messages at that point are from the kernel when it detects the card. It has read the partition table and found those partitions. It doesn't check the contents of the partitions at that point. If you see p1 and p2 then you have two partitions on the card.

rurwin wrote:I can't find it now, but there was a question about running with USB drives and the answer mentioned that the boot SD needed the files to be in the first few blocks of the device. It seems to me that having two partitions is the only way to ensure that on a large device.

That doesn't seem to be the problem that fruit-uk is having. It is getting past the boot loaded and has started the Linux kernel. The kernel has no such limitations concerning file locatations (and they've already been read by that point anyway).

It is puzzling, but I can't reproduce the problem.
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