On Mon, 2010-02-08 at 08:59 +0100, Hans Verkuil wrote:
> On Monday 08 February 2010 08:20:14 Dmitri Belimov wrote:
> > Hi All.
> >
> > I wrote SPI bitbang master over GPIO of saa7134. Speed of writing is much
> > slow then in a Windows systems.
> > I make some tests:
> >
> > Windows, SPI bitbang 97002 bytes x 2 time of writing is around 1.2 seconds
> > Linux, SPI bitbang with call saa7134_set_gpio time of writing is 18 seconds
> > Linux, SPI bitbang without call saa7134_set_gpio time of writing is
> > 0.25seconds.
> >
> > The overhead of SPI subsystem is 0.25 seconds. Writing speed to registers
> > of the saa7134
> > tooooo slooooow.
> >
> > What you think about it?
>
> Internally the spi subsystem uses delays based on some nsecs parameter. I see
> some
> interesting code in spi_bitbang.h under #ifdef EXPAND_BITBANG_TXRX. My guess
> is
> that you use the default timings from the spi subsystem that are too high for
> this
> card.
>
> Try to discover what timings are currently in use and see if they match the
> hardware spec. If not, then you should figure out how to optimize that.
Dmitri,
When looking for timing problems in the kernel, I found the tracing
facility to be very useful.
Using tracing on the ivtv driver, I found that
ivtv-mailbox.c:ivtv_api_get_data() is a simple function being
simply inefficient. It's consuming about half the time used in
the ivtv_rq_handler() mostly doing uneeded PCI MMIO.
Here's a typical example where ivtv_api_get_data() takes over half of
the total execution time of the ivtv_irq_handler():
1) | ivtv_irq_handler() {
1) + 53.471 us | ivtv_api_get_data();
1) | stream_enc_dma_append() {
1) | ivtv_queue_move() {
1) 1.064 us | ivtv_queue_move_buf();
1) 2.943 us | }
1) 0.824 us | pci_dma_sync_single_for_device();
1) + 15.472 us | }
1) | ivtv_dma_enc_start() {
1) | ivtv_queue_move() {
1) 0.894 us | ivtv_queue_move_buf();
1) 2.730 us | }
1) | ivtv_dma_enc_start_xfer() {
1) 0.845 us | pci_dma_sync_single_for_device();
1) 7.282 us | }
1) + 13.088 us | }
1) + 91.243 us | }
And another example:
1) | ivtv_irq_handler() {
1) + 41.701 us | ivtv_api_get_data();
1) 0.884 us | pci_dma_sync_single_for_cpu();
1) | dma_post() {
1) 1.514 us | pci_dma_sync_single_for_cpu();
1) | ivtv_queue_move() {
1) 0.881 us | ivtv_queue_move_buf();
1) 2.879 us | }
1) + 12.360 us | }
1) + 64.144 us | }
Here's how I set up this sort of tracing on my Fedora 12 machine, in
case you wish to try something similar:
1. Set up access to the kernel tracer
$ su - root
# mount
# ls -al /sys/kernel/debug/
# mount -t debugfs debug /sys/kernel/debug/
# mount
# less /sys/kernel/debug/tracing/README
2. Enable dynamic function tracing
# cat /proc/sys/kernel/ftrace_enabled
# echo 1 > /proc/sys/kernel/ftrace_enabled
# cat /sys/kernel/debug/tracing/available_filter_functions
3. Perform a function_graph trace of the ivtv.ko module functions except
the I2C bus calls
# echo function_graph > /sys/kernel/debug/tracing/current_tracer
# echo 0 > /sys/kernel/debug/tracing/options/latency-format
# echo 0 > /sys/kernel/debug/tracing/options/verbose
# echo 0 > /sys/kernel/debug/tracing/tracing_max_latency
# echo > /sys/kernel/debug/tracing/set_ftrace_filter
# nm --defined-only /lib/modules/`uname
-r`/kernel/drivers/media/video/ivtv/ivtv.ko | \
grep ' [Tt] ' | grep -v trace | grep -v 'ivtv_[sg]ets[cd][al]' | \
grep -v map_single | awk '{print $3}' >
/sys/kernel/debug/tracing/set_ftrace_filter
# echo 1 > /sys/kernel/debug/tracing/tracing_enabled
(perform 'cat /dev/video0 > foo.mpg' in another window for a bit)
# echo 0 > /sys/kernel/debug/tracing/tracing_enabled
# cat /sys/kernel/debug/tracing/trace > t1.txt
# less t1.txt
4. Perform a function latency trace of the ivtv.ko module functions
except for I2C bus calls
# echo function > /sys/kernel/debug/tracing/current_tracer
# echo 1 > /sys/kernel/debug/tracing/options/latency-format
# echo 1 > /sys/kernel/debug/tracing/options/verbose
# echo 0 > /sys/kernel/debug/tracing/tracing_max_latency
# echo > /sys/kernel/debug/tracing/set_ftrace_filter
# nm --defined-only /lib/modules/`uname
-r`/kernel/drivers/media/video/ivtv/ivtv.ko | \
grep ' [Tt] ' | grep -v trace | grep -v 'ivtv_[sg]ets[cd][al]' | \
grep -v map_single | awk '{print $3}' >
/sys/kernel/debug/tracing/set_ftrace_filter
# echo 1 > /sys/kernel/debug/tracing/tracing_enabled
(perform 'cat /dev/video0 > foo.mpg' in another window for a bit)
# echo 0 > /sys/kernel/debug/tracing/tracing_enabled
# cat /sys/kernel/debug/tracing/trace > t2.txt
# less t2.txt
Hopefully that can help you set up an experiment to find your timing
delays.
Regards,
Andy
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