Re: [PATCH 2/3] usb: dwc2: host: Giveback URB in tasklet context

[Doug Anderson <dianders@chromium.org>]

Hi,

On Wed, Nov 4, 2015 at 2:53 PM, Douglas Anderson <dianders@chromium.org> wrote:
> In commit 94dfd7edfd5c ("USB: HCD: support giveback of URB in tasklet
> context") support was added to give back the URB in tasklet context.
> Let's take advantage of this in dwc2.
>
> This speeds up the dwc2 interrupt handler considerably.
>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
>  drivers/usb/dwc2/hcd.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/drivers/usb/dwc2/hcd.c b/drivers/usb/dwc2/hcd.c
> index e79baf73c234..9e7988950c7a 100644
> --- a/drivers/usb/dwc2/hcd.c
> +++ b/drivers/usb/dwc2/hcd.c
> @@ -2273,9 +2273,7 @@ void dwc2_host_complete(struct dwc2_hsotg *hsotg, struct dwc2_qtd *qtd,
>         kfree(qtd->urb);
>         qtd->urb = NULL;
>
> -       spin_unlock(&hsotg->lock);
>         usb_hcd_giveback_urb(dwc2_hsotg_to_hcd(hsotg), urb, status);
> -       spin_lock(&hsotg->lock);
>  }
>
>  /*
> @@ -2888,7 +2886,7 @@ static struct hc_driver dwc2_hc_driver = {
>         .hcd_priv_size = sizeof(struct wrapper_priv_data),
>
>         .irq = _dwc2_hcd_irq,
> -       .flags = HCD_MEMORY | HCD_USB2,
> +       .flags = HCD_MEMORY | HCD_USB2 | HCD_BH,
>
>         .start = _dwc2_hcd_start,
>         .stop = _dwc2_hcd_stop,

In the ChromeOS gerrit
<https://chromium-review.googlesource.com/#/c/310583/> Julius Werner
points out that for EHCI it was good to take the optimization from
commit 9118f9eb4f1e ("USB: EHCI: improve interrupt qh unlink") before
this one.  I'm still trying to learn USB / dwc2 so it's unclear to me
whether we also need a similar change before landing.

I'll see if I can do some investigation about this and also some
benchmarking before and after.  Certainly profiling the interrupt
handler itself showed a huge improvement, but I'd hate to see a
regression elsewhere.

If anyone else knows better than I, please speak up!  :)

-Doug