Alan Cox <alan@xxxxxxxxxx> writes: > On Thu, Jun 22, 2006 at 11:33:15AM +0400, Sergei Organov wrote: >> One more question, if I get memory for N bytes with >> tty_prepare_flip_string() then store M (M <= N) bytes into the buffer, >> how do I tell tty layer that only M bytes are in fact stored? > > You don't. > >> [I'm thinking about eliminating buffers allocation for urbs as well as >> data copy when transferring data from USB subsystem to flip buffers of >> the tty subsystem. Currently the flow is: > > The tty buffers may not be DMAable I saw this warning, but didn't think that USB urbs must be given DMAable memory. Does "transfer_buffer: This buffer has to be allocated as a non-pageable contiguous physical memory block" imply the memory must be DMAable? If it doesn't, then do tty buffers meet the above cited requirement? > >> and it seems that using tty buffers directly is a better idea: >> >> - allocate N-bytes data buffer from tty and use it for urb >> - submit the urb to USB subsystem >> - flip M-bytes of data (M <= N) in USB receive callback. > > Its doable in theory but is it worth it ? Well, I don't actually know, -- does your experience suggest it is not? I thought that saving roughly 30% memory (ttys require x2 memory due to flags, right?) plus eliminating memcpy(), could be a valuable improvement, especially for embedded/low end systems and/or USB2 speeds. If you don't see some hard to avoid obstacles making it difficult to implement, I'll probably try to modify tty layer to be able to support such a feature to see what it gives. [My daily job is programming real-time applications for embedded systems, and time to time I do consider Linux a platform for our future embedded development. Recent progress of the kernel towards preemptibility and real-time performance is very promising, so I'm somewhat interested party indeed.] -- Sergei. - To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
