[PATCH v3] usb: gadget: aspeed: improve vhub port irq handling
rentao.bupt at gmail.com
Wed Apr 8 15:40:23 AEST 2020
On Wed, Apr 08, 2020 at 09:36:16AM +1000, Benjamin Herrenschmidt wrote:
> On Mon, 2020-04-06 at 23:02 -0700, Tao Ren wrote:
> > I ran some testing on my ast2400 and ast2500 BMC and looks like the
> > for() loop runs faster than for_each_set_bit_from() loop in my
> > environment. I'm not sure if something needs to be revised in my test
> > code, but please kindly share your suggestions:
> > I use get_cycles() to calculate execution time of 2 different loops, and
> > ast_vhub_dev_irq() is replaced with barrier() to avoid "noise"; below
> > are the results:
> > - when downstream port number is 5 and only 1 irq bit is set, it takes
> > ~30 cycles to finish for_each_set_bit() loop, and 20-25 cycles to
> > finish the for() loop.
> > - if downstream port number is 5 and all 5 bits are set, then
> > for_each_set_bit() loop takes ~50 cycles and for() loop takes ~25
> > cycles.
> > - when I increase downsteam port number to 16 and set 1 irq bit, the
> > for_each_set_bit() loop takes ~30 cycles and for() loop takes 25
> > cycles. It's a little surprise to me because I thought for() loop
> > would cost 60+ cycles (3 times of the value when port number is 5).
> > - if downstream port number is 16 and all irq status bits are set,
> > then for_each_set_bit() loop takes 60-70 cycles and for() loop takes
> > 30+ cycles.
> I suspect the CPU doesn't have an efficient find-zero-bit primitive,
> check the generated asm. In that case I would go back to the simple for
_find_next_bit_le() function is defined in arch/arm/lib/findbit.S. I'm
looking at the code: will run more tests and send out patch v4 with
simple for loop later.
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