On Sat, Mar 15, 2025 at 07:20:37PM +0800, daixin_tkzc wrote: > I'm sorry you may have misunderstood me. > > > HCTSIZ register only reflects the transfer size for the Host Channel (between host and device). The dwc_otg manual explains it as follows： > Non-Scatter/Gather DMA Mode: > Transfer Size (XferSize) > For an OUT, this field is the number of data bytes the host sends > during the transfer. > For an IN, this field is the buffer size that the application has > Reserved for the transfer. The application is expected to program > this field as an integer multiple of the maximum packet size for IN > transactions (periodic and non-periodic). In that case, the dwc_otg driver needs to use a 512-byte bounce buffer. The driver must _guarantee_ that no more than 13 bytes will be written to the URB's transfer_buffer if the URB's transfer_length is 13. If the hardware cannot provide this guarantee then the driver must work around the hardware's deficiencies. That is how the kernel's USB API is designed. Alan Stern
