linux-kernel - Re: [PATCH 5/6] usb-hcd,usb-console: poll hcd device to force usb console writes

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Date:	Tue, 9 Mar 2010 10:08:02 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Jason Wessel <jason.wessel@...driver.com>
cc:	gregkh@...e.de, <linux-usb@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Alan Cox <alan@...ux.intel.com>,
	Oliver Neukum <oliver@...kum.org>
Subject: Re: [PATCH 5/6] usb-hcd,usb-console: poll hcd device to force usb
 console writes

On Tue, 9 Mar 2010, Jason Wessel wrote:

> This patch tries to solve the problem that data is lost because there
> are too many outstanding transmit urb's while trying to execute
> printk's to a console.  The same is true if you try something like
> "echo t > /proc/sysrq-trigger".
> 
> This patch takes the route of forcibly polling the hcd device to drain
> the urb queue in order to initiate the bulk write call backs.  This
> only happens if the device is a usb serial console device that sets
> the max_in_flight_urbs to a non zero value in the serial device
> structure.
> 
> A few millisecond penalty will get incurred to allow the hcd controller
> to complete a write urb, else the console data is thrown away.
> 
> The max_in_flight_urbs was reduced in the usb_debug driver because it
> is highly desired to push things out to the console in a timely
> fashion and there is no need to have a queue that large for the
> interrupt driven mode of operation when used through the tty
> interface.

> --- a/drivers/usb/core/hcd.c
> +++ b/drivers/usb/core/hcd.c
> @@ -1973,6 +1973,7 @@ irqreturn_t usb_hcd_irq (int irq, void *__hcd)
>  	local_irq_restore(flags);
>  	return rc;
>  }
> +EXPORT_SYMBOL_GPL(usb_hcd_irq);

Is there any reason for EXPORTing this?  It doesn't get used anywhere 
else.

> index a3cdb09..703f407 100644
> --- a/drivers/usb/core/hcd.h
> +++ b/drivers/usb/core/hcd.h
> @@ -318,6 +318,7 @@ extern void usb_put_hcd(struct usb_hcd *hcd);
>  extern int usb_add_hcd(struct usb_hcd *hcd,
>  		unsigned int irqnum, unsigned long irqflags);
>  extern void usb_remove_hcd(struct usb_hcd *hcd);
> +extern void usb_hcd_poll_irq(struct usb_device *udev);
>  
>  struct platform_device;
>  extern void usb_hcd_platform_shutdown(struct platform_device *dev);

It might be better to put this in a "public" header file; that is, 
something under include/linux/usb or include/linux/usb.h.  You could 
even take the "_hcd" part out of the name, because the interface uses 
only a struct usb_device, not a struct usb_hcd.

Alan Stern

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