linux-kernel - Re: [PATCH 3/4] cp210x: Store part number

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Date:	Mon, 27 Jul 2015 08:50:39 +0200
From:	Petr Tesarik <ptesarik@...e.com>
To:	Oliver Neukum <oneukum@...e.com>
Cc:	Johan Hovold <johan@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"open list:USB SERIAL SUBSYSTEM" <linux-usb@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>,
	Petr Tesarik <ptesarik@...e.cz>
Subject: Re: [PATCH 3/4] cp210x: Store part number

On Sun, 26 Jul 2015 15:32:54 +0200
Oliver Neukum <oneukum@...e.com> wrote:

> On Fri, 2015-07-24 at 08:48 +0200, Petr Tesarik wrote:
> > @@ -872,6 +886,12 @@ static int cp210x_startup(struct usb_serial
> > *serial)
> >  
> >         usb_set_serial_data(serial, spriv);
> >  
> > +       /* Get the 1-byte part number of the cp210x device */
> > +       cp210x_control_msg(serial->port[0], CP210X_VENDOR_SPECIFIC,
> > +                          REQTYPE_DEVICE_TO_HOST, CP210X_GET_PARTNUM,
> > +                          &partnum, 1, USB_CTRL_GET_TIMEOUT);
> > +       spriv->bPartNumber = partnum & 0xFF;
> 
> DMA on the stack. That breaks the cache coherency rules.
> You must kmalloc the buffer.

I don't understand. While you're right that I copied this part from
Sillicon Labs' driver without much thinking, and &spriv->bPartNumber
can be used directly, I can't see any DMA on stack. FWIW
cp210x_control_msg always allocates a buffer using kcalloc:

        buf = kcalloc(length, sizeof(__le32), GFP_KERNEL);
/* ... */
        result = usb_control_msg(serial->dev, usb_rcvctrlpipe(serial->dev, 0),
                                 request, requesttype, value,
                                 spriv->bInterfaceNumber, buf, size, timeout);

Is that what you mean?

TIA,
Petr T
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