Re: [PATCH] Airprime driver improvements to allow full speed EvDO transfers

[Greg KH]

On Thu, Jul 13, 2006 at 10:20:19PM +0400, Sergei Organov wrote:
> Alan Cox <alan@xxxxxxxxxxxxxxxxxxx> writes:
> > On Iau, 2006-07-13 at 18:17 +0400, Sergei Organov wrote:
> >> This problem may occur with any tty driver that doesn't stop to insert
> >> data into the tty buffers in throttled state. And yes, there are such
> >> drivers in the tree. Before Paul's changes, the tty just dropped bytes
> >> that aren't accepted by ldisc, so this problem had no chances to arise.
> >
> > You must honour throttle. 
> 
> I do, it's Greg who doesn't ;) BTW, isn't it OK to just check for
> tty->throttled where appropriate if I don't have anything special to do
> at unthrottled/throttled transition time?
> 
> > That has always been the case. At all times you should attempt to
> > homour tty->receive_room and also ->throttle. If you don't it breaks.
> 
> Yes, but the difference is what "it" actually is. Loosing some
> characters at some rare or "might never in fact happen conditions" is
> one "it", and exhausting kernel memory at (even more) rare conditions is
> a different "it", isn't it?
> 
> Besides, if the throttle() is that important and failure to handle it is
> a big mistake, why is it optional then? I mean why struct tty_operations
> with throttle field set to NULL is accepted in the first place? The same
> question is applicable to the struct usb_serial_driver.

Yes, I didn't realize it was required.  If it is, we should add this
change.

> >> latter cases drivers that insert too much data without pushing to ldisc
> >> may cause similar problem. Anyway, you definitely know better what to do
> >> about it.
> >
> > Might be a good idea to put a limiter in before 2.6.18 proper just to
> > trap any other drivers that have that bug. At least printk a warning and
> > refuse the allocation once there is say 64K queued. That way the driver
> > author gets a hint all is not well.
> 
> I'm afraid that the limit won't work well as a hint for driver
> developers that didn't honour throttle, as real applications do usually
> read from the files they open, and therefore the problem most probably
> won't be noticed for a long time.
> 
> Provided the limiter is put, why not to make it a variable with 64K
> default?  Driver writers that for whatever reason decide they need more
> in buffers will be able to change that, but then it will be their
> deliberate decision, not just underestimation of consequences of failure
> to handle throttle() due to a lack of knowledge.
> 
> Actually I think that the first thing to decide is if memory usage by
> tty should be bounded or not, and if yes, should it be per-tty limit, or
> total memory usage by all the ttys limit, or both. Those decisions I'd
> probably base on how other kernel subsystems behave (TCP stack is the
> first that comes to mind, and AFAIK buffering for every socket is
> limited). Due to lack of broad knowledge of the kernel, I won't try to
> insist on any solution, even though my experience in embedded systems
> programming cries for bounded model.
> 
> And at the end, I'm going to RTFM ;)
> 
> The comment to the throttle routine in the kernel tree says:
> 
>  * 	This routine notifies the tty driver that input buffers for
>  * 	the line discipline are close to full, and it should somehow
>  * 	signal that no more characters should be sent to the tty.
> 
> "Linux Device Drivers" 3-d edition says:
> 
>  The throttle function is called when the tty core???s input buffers are
>  getting full. The tty driver should try to signal to the device that
>  no more characters should be sent to it.
> 
> None of these two suggests there could be such a global consequences of
> attempting to insert data to the throttled tty as exhausted kernel
> memory. The kernel version reads more strict to me, but LDD one is
> apparently how people indeed understand it.

Well, as I wrote that chapter in LDD, that was how I understood it :)

> BTW, I'm curious if Greg wasn't aware throttle must be handled, or just
> decided that it's not worth to, as neither generic nor airprime
> usb-serial drivers handle throttle.

I wasn't aware that it was required.

> Besides, the example tiny_tty.c driver from the LDD doesn't handle
> throttle either.

I wrote that too :)

thanks,

greg k-h
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