On Thu, Jul 26, 2012 at 7:47 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> You can do this only by hacking up a special driver of your own.
> Since transfer completions would not be reported by IRQs in time, you
> would have to poll for transfer completions at microsecond intervals.
> This would present a rather large overhead for the computer, but it
> might work.
>
for example in such a way:
there are 2 bulk endpoints in USB device, OUT and IN:
OUT buffer format: | <ADDR> | <WRITE DATA> | <cycle type> |
IN buffer format: | <READ DATA> | <completion bit> |
Read cycle (read data by ADDR):
1. USB host sets OUT buffer to: | <ADDR> | <ignored> | 0 |
2. USB host clears <completion bit> in IN buffer: | <ignored> | 0 |
3. USB host submits URBs for OUT and IN endpoints
5. USB device gets <cycle type>, reads data by ADDR, puts data into IN
endpoint and sets <completion bit>
6. while not <completion bit> set and not timeout triggered
USB host reads IN buffer
7. USB host finishes cycle and returns data from IN buffer: | <READ DATA> | 1 |
Write cycle (write data by ADDR):
1. USB host sets OUT buffer to: | <ADDR> | <WRITE DATA> | 1 |
2. USB host clears <completion bit> in IN buffer: | <ignored> | 0 |
3. USB host submits URBs for OUT and IN endpoints
5. USB device gets <cycle type>, gets data from OUT endpoint and sets
<completion bit>
6. while not <completion bit> set and not timeout triggered
USB host reads IN buffer
7. USB host finishes cycle, IN buffer: | <ignored> | 1 |
URB completions are ignored, cycles are atomic (with a mutex), we get
latency ~1 us
can the same URB be submitted some times before completion?
if not, can different URBs be submitted with the same buffer before completion?
should it work?
--
To unsubscribe from this list: send the line "unsubscribe linux-usb" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
