[lkml]   [2009]   [May]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH 5/5] usb_debug: EXPERIMENTAL - poll hcd device to force writes
Greg KH wrote:
>>> This patch takes the route of forcibly polling the hcd device to drain
>>> the urb queue and initiate the bulk write call backs.
>>> NOTE this patch is not signed off, it is a question of what is the
>>> right way to do this?
>> This patch is highly questionable.
> I agree.

That is why there is no signed-off on this patch. It is an
RFC/EXPERIMENTAL because it is not clear how to fix the problem. See
below for more discussion.

>> Is the idea to force the HCD to search for completed URBs before the
>> host controller has issued a completion interrupt? Wouldn't it be
>> better instead to use more URBs?
>> Besides, why should there be too many outstanding transmit URBs? A
>> normal serial debugging port running at 115200 baud can transmit no
>> more than 12 bytes per ms. You should be able to surpass that using
>> only three URBs! In fact, if you buffer up to 8 bytes per URB then
>> with only two URBs you could send 64 bytes per ms, which is equivalent
>> to 640000 baud. Do you really need to go more than (say) ten times
>> faster than that?
> Yeah, something seems wrong here.
> Jason, why is this really needed? With your ring buffer, you shouldn't
> need this at all anymore.
> Or if you do, just bump up the number outstanding urbs that are
> possible. Or the urb buffer size.

The URB queue has to be unacceptably large to make it work correctly
(>4000) The issue here stems from the register_console() function.
As a part of the registration with register_console, the usb_debug
device driver gets a huge flood of printk backlog. Because the urb
queue is not that deep we loose valuable printk logs. I would very much
like a way to force it to work correctly.

To answer Alan's question, the rs232 uart drivers don't process the
printk back log asynchronously. It is a direct write to the hardware
(see serial8250_console_putchar - drivers/serial/8250.c). That is why
there is no problem with lost printk data in the case of the uart drivers.

I could find no obvious safe way do to this synchronously with the USB
api, which is why I put together some way to "force it to work" with the
poll hook until we can figure out the right way to do this. The
mechanism for a console write is not the same as the standard tty
service, or at least that is the way it is today in the kernel.

Essentially I am seeking a synchronous write and wait for the usb serial
drivers when used as a system console. Right now in the usb serial API
there is no distinction between a console write and a tty write.
Perhaps that is what needs to change, to allow for some synchronous
behavior in usb_console_write()?

The usb_debug driver is not the only usb serial device that loses data
from the printk backlog, the ftdi_sio driver suffers the same problem.


 \ /
  Last update: 2009-05-06 18:03    [W:0.065 / U:11.412 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site