Hi, "Du, Changbin" <changbin.du@xxxxxxxxx> writes: >> > If it happen, we can keep the excess data for next i/o, or >> > report an error. But we cannot silently drop data, because >> > USB layer should ensure the data integrality it has transferred, >> > otherwise applications may get corrupt data if it doesn't >> > detect this case. >> >> and when has this actually happened ? Host should not send more data in >> this case, if it does, it's an error on the host side. Also, returning >> -EOVERFLOW is not exactly correct here, because you'd violate POSIX >> specification of read(), right ? >> > This can happen if the host side app force kill-restart, not taking care of this > special condition(and we are not documented), or even it is a bug. Usually APPs > may has a protocol to control the packet size, but protocol mismatch can happen > if either side encounter an error. > > Anyway, this is real. If kernel return success and drop data, the error may > explosion later, or its totally hided (but why some data lost in kernel? > Kernel cannot tell userspace we cannot be trusted sometimes, right?). > so IMO, if this is an error, we need report an error or fix it, not hide it. > > The POSIX didn't say read cannot return "-EOVERFLOW", it says: > " Other errors may occur, depending on the object connected to fd." > > If "-EOVERFLOW" is not suitable, EFAULT, or any suggestions? > >> > Here, we simply report an error to userspace to let userspace >> > proccess. Actually, userspace applications should negotiate >> >> no, this violates POSIX. Care to explain what problem are you actually >> facing ? >> > Why this violates POSIX? Could you give more details? read(5) should return at mode 5 bytes. If there are more, than 5 bytes, we don't error out, we just return the requested 5 bytes and wait for a further read. What I'm more concerned, however, is why we received more than expected data. What's on the extra bytes ? Can you capture dwc3 traces ? Perhaps add a few traces doing a hexdump (using __print_hex()) of the data in req->buf. > The problem is device side app sometimes received incorrect data caused > by the dropping. Most times the error can be detected by APP itself, but why ? app did e.g. read(5), that caused driver to queue a usb_request with length set to 512. Host sent more data than the expected 5 bytes, why did host do that ? And if that data was needed, why didn't userspace read() more than 5 ? -- balbi -- 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
