linux-kernel - Re: [PATCH 1/2] Use kfifo to buffer USB generic serial writes

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Date:	Fri, 14 Aug 2009 22:20:39 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	David VomLehn <dvomlehn@...co.com>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	greg@...ah.com
Subject: Re: [PATCH 1/2] Use kfifo to buffer USB generic serial writes

> The first part of patch uses the kfifo infrastructure to implement a write
> FIFO that accurately returns the amount of space available in the buffer. The
> second makes a minor change to kfifo_put() and __kfifo_put() to add the "const"
> attribute to their input buffer pointers.

Nice, fixes the bug, uses the proper infrastructure and comes out as
loads less code and far easier to read


> +	/* TODO: Much of the time there won't be a write pending and won't be
> +	 * any data ahead of the current chunk. In those cases, we could save
> +	 * a copy by putting the data directly into the URB transfer buffer.
> +	 * This adds some complexity, is it worth it for serial I/O? */
> +	count = kfifo_put(port->write_fifo, buf, count);

If your device has a single transmit URB and isn't doing 2-3MBit the
answer will be no - its in cache anyway and the real hit will be the
writeout for the USB controller to DMA it.


> +		spin_lock_irqsave(&port->write_fifo_lock, flags);
> +		chars = __kfifo_len(port->write_fifo);
> +		spin_unlock_irqrestore(&port->write_fifo_lock, flags);

Any reason for not using kfifo_len() ?

Alan
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