On Fri, Apr 09, 2010 at 10:32:19AM -0400, Alan Stern wrote: > On Thu, 8 Apr 2010, Sarah Sharp wrote: > > > Recently Ramya tried to enqueue a very large transfer to the xHCI driver > > and filled the endpoint ring for that bulk endpoint. Because of that > > I've been looking at doing dynamic ring resizing. I've run into a > > slight problem. > > > > In xhci-pci.c, David Vrabel set hcd->self.sg_tablesize to > > (TRBS_PER_SEGMENT - 1), or 63 entries. The USB storage driver currently > > sets the Scsi_Host->sg_tablesize based on the hcd's table size, which > > limits the number of scatter-gather entries. So even if I add more > > segments to the rings, the driver will never enqueue a transfer with > > more than 63 sglist entries. > > > > If I want to dynamically expand the rings to include multiple segments > > and thus support more sglist entries, what should I set > > hcd->self.sg_tablesize to? > > It is supposed to be the maximum acceptable size. If you dynamically > expand the ring then there is no maximum, so set it to the largest > value that will fit. There really isn't a maximum number of segments that an endpoint ring can hold, so I guess ~0 will do. I think there is a limit on the number of event ring segments, but that really maps to a limit on the number of URBs in flight for all devices on the system, not to a limit on sglist entries. > > If I set it to something large (like EHCI > > sets it to ~0), can I reject transfers if I run out of memory while > > allocating new ring segments? Should I use -E2BIG or -ENOMEM? > > -ENOMEM. > > However there's nothing wrong with capping things at some reasonable > size. If someone tries to submit a request that violates the driver's > specified limit, it's their own fault. Was Ramya's very large transfer > a single bulk URB? Did it use scatter-gather? Yes, Ramya's transfer was one URB with a scatter-gather list of ~120 entries. He was working on a UASP driver that sets the max_sectors field in scsi_host_template to 960 or 1024, which makes some of the sglists very large. Sarah Sharp -- 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
