On Mon, 20 May 2024 at 23:24, Cord Amfmgm <dmamf...@gmail.com> wrote: > On Mon, May 20, 2024 at 12:05 PM Peter Maydell <peter.mayd...@linaro.org> > wrote: >> For the "zero length buffer" case, do you have a more detailed >> pointer to the bit of the spec that says that "cbp = be + 1" is a >> valid way to specify a zero length buffer? Table 4-2 in the copy I >> have says for CurrentBufferPointer "a value of 0 indicates >> a zero-length data packet or that all bytes have been transferred", >> and the sample host OS driver function QueueGeneralRequest() >> later in the spec handles a 0 length packet by setting >> TD->HcTD.CBP = TD->HcTD.BE = 0; >> (which our emulation's code does handle). > > > Reading the spec carefully, a CBP set to 0 should always mean the zero-length > buffer case (or that all bytes have been transferred, so the buffer is now > zero-length - the same thing).
Yeah, I can see the argument for "we should treat all cbp == 0 as zero-length buffer, not just cbp == be == 0". > Table 4-2 is the correct reference, and this part is clear. It's the part you > quoted. "Contains the physical address of the next memory location that will > be accessed for transfer to/from the endpoint. A value of 0 indicates a > zero-length data packet or that all bytes have been transferred." > > Table 4-2 has this additional nugget that may be confusingly worded, for > BufferEnd: "Contains physical address of the last byte in the buffer for this > TD" Mmm, but for a zero length buffer there is no "last byte" to have this be the physical address of. > And here's an example buffer of length 0 -- you probably already know what > I'm going to do here: > > char buf[0]; > char * CurrentBufferPointer = &buf[0]; > char * BufferEnd = &buf[-1]; // "address of the last byte in the buffer" > // The OHCI Host Controller than advances CurrentBufferPointer like this: > CurrentBufferPointer += 0 > // After the transfer: > // CurrentBufferPointer = &buf[0]; > // BufferEnd = &buf[-1]; Right, but why do you think this is valid, rather than being a guest software bug? My reading of the spec is that it's pretty clear about how to say "zero length buffer", and this isn't it. Is there some real-world guest OS that programs the OHCI controller this way that we're trying to accommodate? thanks -- PMM
