On Thu, 4 Apr 2019, Suwan Kim wrote: > Hi Alan, > > On Mon, Apr 01, 2019 at 10:43:24AM -0400, Alan Stern wrote: > > On Mon, 1 Apr 2019, Suwan Kim wrote: > > > > > Patch "USB: HCD: support giveback of URB in tasklet context"[1] > > > introduced giveback of urb in tasklet context. [1] This patch was > > > applied to ehci but not xhci. [2] This patch significantly reduces > > > the hard irq time of xhci. Especially for uvc driver, the hard irq > > > including the uvc completion function runs quite long but applying > > > this patch reduces the hard irq time of xhci. > > > > Please read the kerneldoc for usb_submit_urb() and usb_kill_urb(), in > > particular, the parts that describe how isochronous URBs are treated. > > Can you guarantee that with this patch applied, xhci-hcd will continue > > to work as the kerneldoc describes? > > > > I read the description of usb_submit_urb() and usb_kill_urb() and i > think that xhci-hcd with which this patch is applied works as the > description of usb_submit_urb() and usb_kill_urb(). > > In the case of usb_submit_urb(), xhci spec 4.10.3.1 "Ring Overrun and > Underrun" describes the behavior of xhci when an isochronous ring is > empty due to the late URB submission in driver. (In this patch, empty > isochronous ring can happen due to tasklet scheduling delay in URB > complete function which calls the next usb_submit_urb()) > > According to the xhci spec, xHC deals with a late isochronous URB > according to the SIA(Start Isoch ASAP) flag of TRB and SIA flag is > set according to URB_ISO_ASAP flag in xhci_queue_isoc_tx(). > > If the SIA flag is set, xHC will schedule the late isochronous URB in > the next "Endpoint Service Interval Time" (next available frame) and > transmits ischronous URB in that frame. > > If the SIA flag is cleared (URB_ISO_ASAP flag is cleared), xHC generates > "Missed Service Error" event and skips the late isochronous URB and > doen't transmit it. When the interrupt handler (xhci_irq) receives > "Missed Service Error" event, it returns the late isochronous URB to > the driver calling usb_hcd_giveback_urb() with -EXDEV error code in > usb_iso_packet_descriptor->status at skip_isoc_td(). So xhci behavior > about the late isochronous URB in spec and implementation is same > with the description of usb_submit_urb(). > > In the case of usb_kill_urb(), description says that it waits until > the URB complete function finishes and the important point is that > whether the USB complete function is called early or late doesn't > affect the behavior of usb_kill_urb() because __usb_hcd_giveback_urb() > wakes up usb_kill_urb() after calling URB complete function. > So, pending a URB complete function in tasklet doesn't affect the > behavior of xhci in usb_kill_urb(). Okay, good. I just wanted to make sure you were aware of the issues and had checked that using tasklets wouldn't cause any problems. Alan Stern
