On 8/25/2016 12:38 AM, Felipe Balbi wrote: > We need to decrement in both cases (enq > deq and > enq < deq) > > Signed-off-by: Felipe Balbi <felipe.balbi@xxxxxxxxxxxxxxx> > --- > drivers/usb/dwc3/gadget.c | 5 +---- > 1 file changed, 1 insertion(+), 4 deletions(-) > > diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c > index 594e82595634..e4ef9ed1a143 100644 > --- a/drivers/usb/dwc3/gadget.c > +++ b/drivers/usb/dwc3/gadget.c > @@ -884,12 +884,9 @@ static u32 dwc3_calc_trbs_left(struct dwc3_ep *dep) > return DWC3_TRB_NUM - 1; > } > > - trbs_left = dep->trb_dequeue - dep->trb_enqueue; > + trbs_left = dep->trb_dequeue - dep->trb_enqueue - 1; > trbs_left &= (DWC3_TRB_NUM - 1); > > - if (dep->trb_dequeue < dep->trb_enqueue) > - trbs_left--; > - > return trbs_left; > } > > Hi Felipe, I don't think that's right. The decrement is accounting for the fact that the link TRB is taking up one of the free spots. That is only the case when the deq < enq. See the examples below to illustrate this Example 1 (DWC3_TRB_NUM = 8): After the initial state of the ring, queue on TRB. 0: TRB <- deq 1: empty <- enq 2: empty 3: empty 4: empty 5: empty 6: empty 7: LINK TRB enq=1, deq=0 num_free => (deq - enq) % DWC3_TRB_NUM => (0 - 1) % 8 => 0xffffffff % 8 => 7 slots free One of the free slots is take up by link trb so decrement by 1: 7 - 1 => 6 slots free Example 2 (DWC3_TRB_NUM = 8): The enqueue pointer has wrapped around and started enqueue from the beginning of the ring again, but the dequeue pointer hasn't wrapped yet. 0: TRB 1: empty <- enq 2: empty 3: empty 4: TRB <- deq 5: TRB 6: TRB 7: LINK TRB enq = 1, deq = 4 num_free => (deq - enq) % DWC3_TRB_NUM => (4 - 1) % 8 => 3 slots free No need to decrement by 1. Regards, John -- 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
