Consider the placement of waitfor statements

The boot script contains multiple calls to waitfor, which ensure that a resource manager is loaded before any of the programs that might use it. This is a very good practice, since the programs that follow may fail if they don't find the resource they require. However, in the default boot script, these waitfor statements are grouped to make sense, rather than to ensure maximum performance. For example, consider the following (simplified) sample code:

            
...

# I2C driver
display_msg starting I2C driver...

# I2C0 interface
i2c-omap35xx-j5 -i 70 -p0x48028000 --u0
waitfor /dev/i2c0

# I2C1 interface
i2c-omap35xx-j5 -i 71 -p0x4802A000 --u1
waitfor /dev/i2c1

# I2C2 interface
i2c-omap35xx-j5 -i 30 -p0x4819C000 --u2
waitfor /dev/i2c2

# I2C3 interface
i2c-omap35xx-j5 -i 31 -p0x4819E000 --u3
waitfor /dev/i2c3

# USB OTG Host Controller driver
io-usb-otg -vvv -d dm816x-mg ioport=0x47401400,irq=18
waitfor /dev/io-usb/io-usb 4
devb-umass cam pnp

# AUDIO Driver - I2C must be running
display_msg Starting Audio driver...
# MCASP2
io-audio -vv -d mcasp-j5_aic3106 mcasp=2
waitfor /dev/snd/pcmC0D0p

# SPI driver
display_msg starting SPI driver...
# SPI 0
spi-master -u0 -d dm816x base=0x48030100,irq=65,somi=0,edma=1,edmairq=529,edmachannel=17

# PCIe server
display_msg Starting PCI server...
pci-dm814x
waitfor /dev/pci 4

 ...

This script does the reasonable thing of starting each driver, then waiting for it to finish loading before continuing. Some of these drivers require hardware initialization. If a driver is waiting on the hardware, then waitfor can prevent the next program from loading prematurely.

The behavior of waitfor is very simple: it polls the device, and if the device isn't found, it sleeps for 100 milliseconds and tries again. It terminates when either the device is found or the timeout is reached, whichever happens first. As a result, each waitfor might do nothing except poll and hold up the rest of the show. You want the CPU 100% utilized during the boot—any idle time adds to the total boot duration. Ideally, then, each waitfor would do a single device check that succeeds and then continues. An ordering that breaks the logical grouping can minimize unwanted sleeps by using other program loads to introduce any required delay.

For instance, let's say that you need to start an IDE driver in your boot process. That driver must wait for the hardware to initialize, an operation that always takes 100 milliseconds. That's what waitfor does: it waits until your driver has the hardware initialized before proceeding. But why waste those 100 milliseconds? After starting the IDE driver, start your USB driver (or any other software) that can effectively utilize that time. If your USB driver takes 100 milliseconds to prepare the hardware, you've gotten some extra time "for free." Then, when you actually need the IDE device, the waitfor test will succeed immediately. And you've managed to shorten the total boot time.

See the following code for an example of modifying the script in this way:

...

# I2C driver
# We won't wait for any of these, since nothing needs them yet
display_msg starting I2C driver...

# I2C0 interface
i2c-omap35xx-j5 -i 70 -p0x48028000 --u0

# I2C1 interface
i2c-omap35xx-j5 -i 71 -p0x4802A000 --u1

# I2C2 interface
i2c-omap35xx-j5 -i 30 -p0x4819C000 --u2

# I2C3 interface
i2c-omap35xx-j5 -i 31 -p0x4819E000 --u3

# USB OTG Host Controller driver
display_msg Starting USB OTG Host driver...
io-usb-otg -vvv -d dm816x-mg ioport=0x47401400,irq=18

# Start the SPI driver before checking on USB, since SPI doesn't rely on io-usb-otg
    
# SPI driver
display_msg starting SPI driver...
# SPI 0
spi-master -u0 -d dm816x base=0x48030100,irq=65,somi=0,edma=1,edmairq=529,edmachannel=17

# Check on USB relocated from above
waitfor /dev/io-usb/io-usb 4
devb-umass cam pnp

# PCIe server
display_msg Starting PCI server...
pci-dm814x
waitfor /dev/pci 4

# I2C driver should be up by now, and we need it for audio
waitfor /dev/i2c0
waitfor /dev/i2c1
waitfor /dev/i2c2
waitfor /dev/i2c3

# The audio driver requires I2C, so we've moved it later in the boot script
# (after SPI and PCIe), to allow more time for the I2C drivers to initialize

# AUDIO Driver - I2C must be running
# McASP2
io-audio -vv -d mcasp-j5_aic3106 mcasp=2
waitfor /dev/snd/pcmC0D0p

...

These examples illustrate the benefits of optimized waitfor placement. This technique has a potential drawback, however: the driver might not be waiting on the hardware, but rather using the processor to do real work. In that case, the reordering will cause all the drivers to load at once, which will make the task scheduler continually switch between all the active threads. This can be less efficient than the first method.

To determine whether reordering improves boot performance, use tracelogger to capture a system profiler snapshot during boot. If the snapshot shows blocks of time where the CPU is idle after a driver load and indicates that calls are being made into the kernel every 100 milliseconds, then that driver is a reasonable target for this technique.