Greg KH wrote: > On Thu, Feb 14, 2008 at 07:55:44PM +1030, David Newall wrote: > >> The current 2.6 driver maintains it's own buffer. I think that's a bad >> thing: usbserial already buffers writes, and the extra buffer copy seems >> unnecessary, however it does solve the putchar problem. Buffered (i.e. >> by the 2.6 series pl2303 driver) data is written as soon as practicable, >> regardless of CTS/DTR. The same general workaround, but placed in >> pl2303_send seems correct to me; that is, stop submitting write urbs >> when the remote end lowers CTS/DTR, and trigger the resume from the >> interrupt callback (specifically in update_line_status.) >> > > Where does the usbserial core buffer writes on 2.6? The serial_write() > function just passes the data straight down to the usb-serial child > driver directly, no copying or buffering happens that I can see. >
You're right. I haven't examined the 2.6 stack as closely as the 2.4. I noticed the buffer in 2.6 pl2303, but didn't check 2.6 usb-serial. I think I prefer the buffer in usb-serial, because its centralised rather than in each driver, but I'm not going to step up to the plate and propose changing that! >> To make it clear: Even aside from the buffer in 2.6's pl2303.c, there's >> a race: An in-flight write URB can fill all hardware buffers, making >> unsafe what previously appeared to be a safe write. I think it's >> essential to delay submission of the URB on a stop-transmit condition. >> > > It's up to the individual driver to know when their buffers are filled > up. The big problem is, a lot of these cheap usb-serial devices (like > the pl2303) don't have a way to report the uart queue filled-state back > to the host, so things can easily get over-run as you have found out. > My understanding of the problem has developed over the last couple of days; going from wrist-deep to elbow-deep into the guts of things does that. There is a problem, and the solution I've been developing addresses it, but maybe there's a simpler answer. Hope to have a patch together soon. - To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
