linux-kernel - Re: [PATCH 2/2] USB: ehci: tegra: Align DMA transfers to 32 bytes

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Date:	Mon, 24 Jan 2011 11:36:55 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Robert Morell <rmorell@...dia.com>
cc:	David Brownell <dbrownell@...rs.sourceforge.net>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Benoit Goby <benoit@...roid.com>,
	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Ming Lei <tom.leiming@...il.com>,
	Jacob Pan <jacob.jun.pan@...el.com>,
	Oliver Neukum <oliver@...kum.org>,
	Olof Johansson <olof@...om.net>,
	Erik Gilling <konkers@...roid.com>,
	Colin Cross <ccross@...roid.com>, <linux-usb@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH 2/2] USB: ehci: tegra: Align DMA transfers to 32 bytes

On Thu, 20 Jan 2011, Robert Morell wrote:

> The Tegra2 USB controller doesn't properly deal with misaligned DMA
> buffers, causing corruption.  This is especially prevalent with USB
> network adapters, where skbuff alignment is often in the middle of a
> 4-byte dword.
> 
> To avoid this, allocate a temporary buffer for the DMA if the provided
> buffer isn't sufficiently aligned.
> 
> Signed-off-by: Robert Morell <rmorell@...dia.com>
> ---
>  drivers/usb/host/ehci-tegra.c |   92 +++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 92 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/usb/host/ehci-tegra.c b/drivers/usb/host/ehci-tegra.c
> index 2341904..7cdfc65 100644
> --- a/drivers/usb/host/ehci-tegra.c
> +++ b/drivers/usb/host/ehci-tegra.c
> @@ -32,6 +32,8 @@
>  #define TEGRA_USB_USBMODE_HOST			(3 << 0)
>  #define TEGRA_USB_PORTSC1_PTC(x)		(((x) & 0xf) << 16)
>  
> +#define TEGRA_USB_DMA_ALIGN 32
> +
>  struct tegra_ehci_context {
>  	bool valid;
>  	u32 command;
> @@ -461,6 +463,94 @@ static int tegra_ehci_bus_resume(struct usb_hcd *hcd)
>  }
>  #endif
>  
> +struct temp_buffer {
> +	void *kmalloc_ptr;
> +	void *old_xfer_buffer;
> +	u8 data[0];
> +};
> +
> +static void free_temp_buffer(struct urb *urb, int status)
> +{
> +	enum dma_data_direction dir;
> +	struct temp_buffer *temp;
> +
> +	if (!(urb->transfer_flags & URB_ALIGNED_TEMP_BUFFER))
> +		return;
> +
> +	dir = usb_urb_dir_in(urb) ? DMA_FROM_DEVICE : DMA_TO_DEVICE;
> +
> +	temp = container_of(urb->transfer_buffer, struct temp_buffer,
> +			    data);
> +
> +	if (dir == DMA_FROM_DEVICE && !status)
> +		memcpy(temp->old_xfer_buffer, temp->data,
> +			urb->transfer_buffer_length);

Even if status is nonzero, there may be valid data in the buffer.  You 
should skip that test.

No other problems that I can see.

Alan Stern


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