On Tue, 5 Feb 2019 guido@xxxxxxxxxxxxxxxxxx wrote: > > Zitat von Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>: > > > On Mon, 4 Feb 2019, Guido Kiener wrote: > > > >> From: Guido Kiener <guido.kiener@xxxxxxxxxxxxxxxxx> > >> > >> The OUT endpoint normally blocks (NAK) subsequent packets when a > >> short packet is received and returns an incomplete queue entry to > >> the gadget driver. Thereby the gadget driver can detect > >> a short packet when reading queue entries with a length that is > >> not equal to a multiple of packet size. > >> > >> The start_queue() function enables receiving OUT packets regardless > >> of the content of the OUT FIFO. This results in a problem: > >> When receiving is enabled more OUT packets are appended to the last > >> short packet and the gadget driver cannot determine the end of a > >> short packet anymore. Furthermore the remaining data in the OUT FIFO > >> is not delivered to the gadget driver immediately and can produce > >> timeout errors. > > > > How can that happen? When the short packet is received, the endpoint > > immediately starts sending NAKs, so no more data can enter the FIFO. > > Furthermore, the incomplete transfer is returned to the gadget driver, > > which removes the short packet from the FIFO. So the FIFO has to be > > empty when start_queue() is called. > > I don't remember the exact scenario. It happens something like this: > When you have only one request (e.g. 4kB) left in your endpoint OUT > queue and a host sends 4kB + some extra bytes, then your request (4kB) > is delivered to the gadget driver while the extra bytes (short packet) > are still in the OUT FIFO. > I try to reconstruct the situation (without fix) how the driver can call > (re)start_dma. I do not see a reasonable path in the moment. Okay, I see. If there's no outstanding request when the short packet arrives, the data will remain in the FIFO. But won't this situation be detected in start_dma(), which is the only place where start_queue() is called? start_dma() has code to test specifically for a previous short OUT packet. Should the fix go there instead? Alan Stern
