On Mon, Jun 17, 2013 at 3:37 PM, Adrian Burns <[email protected]>wrote:
> hi,
> a newbie here,
>
Hi!
> using flyswatter2 and olimex arm debuggers
> working with openocd 6.1 codebase
>
> in order to read back some registers i have to issue a number of ir/dr
> scans to my tap chain and its working fine but im looking at ways to speed
> it up if possible
>
> so im calling jtag_add_ir_scan() and jtag_add_dr_scan() a number of times
> in sequence
>
> is this the most efficient way to do a number of ir/dr scans in a row?
>
If you follow the hints below, the generated commands to the FTDI chip in
the adapter *should* be close to optimal. Then of course it depends on how
high you can boost the frequency.
And as always, profiling is the key to optimization.
>
> code looks something like this...
>
>
It would be easier to help if you showed more than an incomplete extract
from the code.
...
> ...
> /* setup IR with correct opcode */
> struct scan_field *fields;
> fields = malloc(sizeof(struct scan_field));
> memset(fields, 0, sizeof(struct scan_field));
>
You can avoid the (probably insignificant) malloc overhead if you keep the
fields on the stack. You only need to maintain the fields array and the
out_value until after the corresponding jtag_add_*_scan(). It's only the
in_value storage you need to keep available until the queue is executed.
fields->num_bits = ir_len;
> fields->out_value = in;
> fields->in_value = out;
>
(Reversed in/out? Not that out_value is scanned IN to the TAP and in_value
is scanned OUT. But you had this working already.)
I guess this is a made-up example anyway, but it will be slightly more
efficient if you keep in_value = NULL for the IR scans and out_value = NULL
for DR scans (assuming data scanned into DR is irrelevant).
jtag_add_ir_scan(t->tap, fields, TAP_IDLE);
> ...
> ...
> /* then do drscan */
> /* then do next irscan */
> /* then do next drscan */
> etc
>
Since you're not showing the complete code I can't tell if you're doing it
already, but it's important that you queue up all scans first and then
execute the queue once.
I.e.
jtag_add_ir_scan();
jtag_add_dr_scan();
jtag_add_ir_scan();
jtag_add_dr_scan();
...
jtag_execute_queue();
not
jtag_add_ir_scan();
jtag_add_dr_scan();
jtag_execute_queue();
jtag_add_ir_scan();
jtag_add_dr_scan();
jtag_execute_queue();
...
Each execute will incur a significant delay due to USB roundtrip. So don't
call jtag_execute_queue() until you really need the data.
Also you'll want to use the new ftdi adapter driver instead of the ft2232
one because it's much faster on some workloads.
/Andreas
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