On 2020/09/22 16:33, Oliver Neukum wrote: > Am Dienstag, den 22.09.2020, 10:56 +0900 schrieb Tetsuo Handa: >> On 2020/09/21 19:52, Oliver Neukum wrote: >> To understand it, I must understand why it is safe to defer error reporting. > > It is not. There is nothing to understand here. If user space needs > a guarantee that data has been pushed out without an error, it will > have to call fsync() >> I'm querying you about characteristics of data passed to wdm_write(). >> Without knowing the difference between writing to cdc-wdm driver and normal file on >> some filesystem, I can't judge whether it is acceptable to defer reporting errors. > > That is simply not a decision you or I make. The man page clearly > says that it is acceptable. If user space does not like that, it must > call fsync() after write(). Then, cdc-wdm driver did not implement fsync() was a big problem. Userspace needs to be careful not to give up upon -EINVAL when running on older kernels which did not implement wdm_fsync(). The remaining concern would be how to handle unresponding hardware, for blocking wdm_write()/wdm_read() and wdm_fsync() are using wait_event_interruptible(). If the caller do not have a mean to send a signal, the caller might hung up forever when the hardware stopped responding. Please add a comment that userspace needs to be careful when calling these functions.
