linux-kernel - Re: [RFT] rtl8187: use DMA-aware buffers with usb_control

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Date:	Sat, 9 May 2009 06:57:31 -0700
From:	Greg KH <greg@...ah.com>
To:	Eric Valette <eric.valette@...e.fr>
Cc:	Hin-Tak Leung <hintak.leung@...il.com>,
	"John W. Linville" <linville@...driver.com>,
	linux-wireless@...r.kernel.org,
	Larry Finger <Larry.Finger@...inger.net>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [RFT] rtl8187: use DMA-aware buffers with usb_control_msg

On Sat, May 09, 2009 at 11:38:32AM +0200, Eric Valette wrote:
> The patch fix the DMA warning and the driver seems to work (just
> associated it) but I must say that the allocation failure handling path
> and the fact that we use now kmalloc for allocating a few bytes in such
> a routine makes me worry about possible negative performance impact
> unless theses routines are used only in a slow configuration path (did
> no took time to red the code due to many other problems).

usb_control messages are slow and should not be on the "fast path" of
any data being sent through the device.  Any overhead of the
kmalloc/kfree is totally eaten up by the actual transmission turn around
time of the message itself, so you don't have to worry about the
performance impact.

thanks for testing.

> BTW if someone know who I should send this attached patch for DRM git, I
> would gladly forward it.

To the drm maintainer?

thanks,

greg k-h
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