[Ftdi-usb-sio-devel] Bug in ftdi_sio driver. Re: low_latency flag for ftdi_sio

[Boris Z. <bs_...@ya...>]

Hello,

Well I've found how I could possibly reduce 
latency: writing to latency_timer file under sysfs.
BUT reading/writing this file crashes the kernel. It
happens because these lines

struct usb_serial_port *port = to_usb_serial_port(dev);
struct ftdi_private *priv = usb_get_serial_port_data(port);

in all sysfs attribute callbacks produce wrong pointer to
usb_serial_port, because "struct device" pointer passed to
device_create_file in "create_sysfs_attrs" doesn't points
to "struct device" embedded in usb_serial_port.(In my case
above "priv" pointer turned out to be NULL and was trapped
by kernel producing Oops, but kernel could crash even
earlier while dereferencing "port" pointer). This happens
with FC6's kernel 2.6.20.

best regards,
Boris 



Boris Zingerman <bs_...@ya...> wrote: Hello,

I have some device that uses ftdi chip
to communicate with host computer. I set
low latency flag in this manner

struct serial_struct serial_info;
ioctl(m_fd, TIOCGSERIAL, &serial_info)  ;
serial_info.flags |= ASYNC_LOW_LATENCY;
ioctl(m_fd, TIOCSSERIAL, &serial_info) ;

setserial reports that low_latency flag is
indeed set, but round-trip time for short commands
is about 15 msec. This device has also regular RS232
input and the same code produces 1.5msec 
roundtrip time for the same commands (ten times
faster !!). Is low_latency flag has no effect on ftdi_sio
driver ? If so how can I improve latency.

Best regards,
Boris Zingerman

   

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