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Understanding Boot Time Variability with the Zynq ZC702

One of the challenges of boot time reduction is understanding why the boot time of a device may vary with each reboot, this is important because we strive not only for minimal boot times but for consistently minimal boot times. This post uses a Xilinx Zynq platform to demonstrate how we can measure, understand and find the causes of boot time variability. We’ll also provide an insight into how we use automation at Embedded Bits to improve the process.

To explore boot time variability we’ll be using Xilinx’s Zynq-7000 based ZC702 evaluation kit. The Zynq range of SoC’s cleverly combine a dual-core Cortex A9 MPCore with programmable logic (Artix-7 FPGA). The ZC702 is provided with a ‘Base Targeted Reference Design (TRD)‘ (a Linux distribution on an SD card) – we’ll use this to perform our investigation against.

By its very nature, the only way to measure variability is to measure the boot time over and over again during successive runs (time consuming!). At Embedded Bits where possible we install development boards into our board farm – along with providing benefits such as board sharing and collaborative working, it crucially provides automation – we’ll take advantage of this as we explore boot time variability. We’ll start by instructing the farm to repeatedly reboot the board whilst capturing boot logs.

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One Second Cold Linux Boot with OpenCV

As a means to demonstrate our boot time reduction skills, last November we put together a demo which shows an Embedded Linux device cold-booting in less than a second.

The demo consists of an ARM Cortex-A9 based device connected to a camera, 7-segment display and HDMI display. The device uses the camera along with OpenCV based software to count the number of yellow balls present on the table beneath and display the count on the 7-segment display. The device also outputs the camera image and ball detection illustration on the LCD display.

The device is able to do all this within one second of software reset, here is the video:

We’ve put lots of information on how we achieved this on our Boot Time Demos page which can be found here.

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Scripted Modification of Kernel Configs

Whilst recently making changes to an embedded Linux distribution I came across a modest but very powerful feature of the kernel’s tried and test Kconfig system. What I discovered was a script that allows for scripted modification of kernel configuration files, i.e. .config and *_defconfig files.

The script, which first appeared in the 2.6.29 kernel can be found in the kernel’s scripts/ directory and its usage is very straight forward. Here are some examples:

# Enable timing information on printk's
./scripts/config --enable CONFIG_PRINTK_TIME

# Change the path of the initramfs
./scripts/config --set-str CONFIG_INITRAMFS_SOURCE ../fs

# Increase the size of the kernel log buffer
./scripts/config --set-val CONFIG_LOG_BUF_SHIFT 14

# Enable LZO compression for SquashFS in ~/test_defconfig
./scripts/config --file ~/test_defconfig --enable CONFIG_SQUASHFS
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Using ‘delay_use’ to speed up USB Enumeration

Whilst investigating ways to improve the cold boot time of embedded Linux I came across a little known control parameter of the USB stack known as ‘delay_use‘. It’s a parameter that describes the amount of time given to Mass Storage Devices to allow them to ‘settle down’ before being used. This article examines ‘delay_use’ and identifies how it may be used to reduce boot time and improve responsiveness.

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Understanding “vmalloc region overlap”

I recently came across the following disconcerting message in my kernel’s boot output:

Truncating RAM at 40000000-5fffffff to -57ffffff (vmalloc region overlap).
...
Kernel command line: console=ttySC0,115200 mem=512M
...
Memory: 384MB = 384MB total

Which is the kernel’s way of saying “I understand there may be some RAM here – but I’m not going to use it all”. So what is the cause of this warning? And what do we need to do to reclaim that lost RAM?

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A nasty string initialization bug in C

Today I encountered a bug that was quite difficult to find regarding strings. In order for strings to work they must be null-terminated, and this implies that an array of characters can contain a string with a length equal to the array size minus one, because there must be space for the null character. I found out that, when initializing array of chars with strings, the compiler does not complain if just the null character doesn’t fit.

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Writing an MLO for a BeagleBoard XM

Before I started playing with the BeagleBoard XM I’ve had never booted a board directly from an MMC card and I didn’t have a clue what an ‘MLO’ file was. After some research on the internet it seemed apparent that it was used in place of the traditional first stage boot loader: XLoader. In fact it in most cases it is XLoader – a quick invocation of my toolchain’s string implementation seemed to correlate with this:

$ arm-none-linux-gnueabi-strings /media/boot/MLO | grep X-Loader
()*+,-./0123456789:;< =>?Texas Instruments X-Loader 1.5.0 (Mar 27 2011 - 17:37:56)
X-Loader hangs

I was curious as to why it was named MLO, how the board boots into this Image and how I can create my own MLO with some bare metal code. This post answers these questions and results in a very simple executable MLO file. It will probably satisfy those readers who like to understand and write all the code that runs on a board. It’s very easy to use a boot loader like UBoot to obtain and execute an image from an MMC card – but it adds boot time, duplication and complexity. Besides it’s fun to get close to the metal…

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The Right Approach to Minimal Boot Times [ELCE Video]

Last year at the ELCE 2010 conference in Cambridge I performed a talk about reducing boot times of embedded Linux devices. The video of this talk has now been posted on-line:


ELCE 2010 – The Right Approach to Minimal Boot Times ELCE Video (Best viewed in Chrome)

The accompanying slides for this presentation can be found here

If you wish to see the accompanying YouTube video – please see my earlier post. [© 2011 embedded-bits.co.uk]

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cat /proc/meminfo : MemTotal

Linux manages it’s physical memory in clever and often efficient ways – as a result it’s not uncommon to only think about how the memory in your system is being used when we run into performance issues. And this is where the frustration can begin – without fully understanding how memory is managed, it can be very difficult to answer some seemingly straight-forward questions like ‘How much free memory do I have?‘ or ‘How much memory is this process taking?‘. There are a lot of complications and as a result performance monitoring can be a challenge.

I was determined to fully understand precisely what the various memory figures report by the kernel mean and understand – on a practical level – the implications of Linux’s memory management on our performance sensitive applications. In this multi-part post we’ll attempt to debunk many of the mysteries of Linux’s memory.

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validate_sb: bad superblock, error 8

I’ve recently left the dark ages and entered the world of UBI and UBIFS – it’s generally been a pleasant experience. However whilst getting to grips with it, I occasionally recieved the following error upon mount:

# mount -t ubifs /dev/ubi0_1  /tmp/
UBIFS error (pid 1096): validate_sb: bad superblock, error 8
mount: mounting /dev/ubi0_1 on /tmp/ failed: Invalid argument

The usual approach of googling my way out of problems didn’t give me much luck so I did some exploring. The cause of this failure, despite having been successful in flashing the UBIFS image – was that the UBI volume I was using was too small. The solution is therefore to increase the size of the volume.

Unfortunately though, my current project required me to create a UBI volume of only the minimum size to support a given UBIFS image – yet it would seem that determining the minimum size of a UBI volume for a given UBIFS image was not trivial. I had to do some more digging to find the answer…

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