linux-kernel - Re: [PATCH 1/1, v2] usb-use-kfifo-to-buffer-usb-generic-serial-writes.patch

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Message-Id: <200908271014.54282.oliver@neukum.org>
Date:	Thu, 27 Aug 2009 10:14:53 +0200
From:	Oliver Neukum <oliver@...kum.org>
To:	David VomLehn <dvomlehn@...co.com>, linux-usb@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	greg@...ah.com
Subject: Re: [PATCH 1/1, v2] usb-use-kfifo-to-buffer-usb-generic-serial-writes.patch


> +	/* send the data out the bulk port */
> +	result = usb_submit_urb(port->write_urb, GFP_ATOMIC);
> +	if (result) {
> +		dev_err(&port->dev,
> +			"%s - failed submitting write urb, error %d\n",
> +						__func__, result);
> +		/* don't have to grab the lock here, as we will
> +		   retry if != 0 */
> +		port->write_urb_busy = 0;
> +		status = result;

This looks deficient. If the first part of a transmission fails,
the fifo's remaining content should be discarded and if possible
an error returned to user space.

[..]
> @@ -487,8 +515,8 @@ void usb_serial_generic_write_bulk_callback(struct urb
> *urb) port->urbs_in_flight = 0;
>  		spin_unlock_irqrestore(&port->lock, flags);
>  	} else {
> -		/* Handle the case for single urb mode */
>  		port->write_urb_busy = 0;
> +		usb_serial_generic_write_start(port, 0);

This is a problem. This may fail due to a system suspend.
In that case you cannot depend on the next write restarting
IO. You need to restart IO in resume()


> @@ -970,6 +971,11 @@ int usb_serial_probe(struct usb_interface *interface,
>  			dev_err(&interface->dev, "No free urbs available\n");
>  			goto probe_error;
>  		}
> +		port->write_fifo = kfifo_alloc(PAGE_SIZE, GFP_KERNEL,
> +			&port->write_fifo_lock);

Whence do you take the fifo's size? What does this do to latency?

[..]
> @@ -96,6 +98,8 @@ struct usb_serial_port {
>  	unsigned char		*bulk_out_buffer;
>  	int			bulk_out_size;
>  	struct urb		*write_urb;
> +	struct kfifo		*write_fifo;
> +	spinlock_t		write_fifo_lock;

Do you really need a separate lock?

	Regards
		Oliver

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