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Date: Wed, 20 Jul 2011 10:36:12 -0400
From: "John W. Linville" <linville@tuxdriver.com>
To: Kalle Valo <kvalo@qca.qualcomm.com>
Cc: Ben Hutchings <bhutchings@solarflare.com>, netdev@vger.kernel.org,
	linux-wireless@vger.kernel.org
Subject: Re: [PATCH] ethtool: fix ethtool_get_regs() to work with zero length
 registers
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On Wed, Jul 20, 2011 at 02:58:20PM +0300, Kalle Valo wrote:
> On 07/20/2011 02:38 PM, Ben Hutchings wrote:
> > On Wed, 2011-07-20 at 12:18 +0300, Kalle Valo wrote:
> >> cfg80211 exports zero length register size as it currently only uses
> >> struct ethtool_regs.version to export struct wiphy.hw_version.
> > [...]
> > 
> > The ethtool_regs::version field represents the version of the register
> > dump format.  This may or may not relate to a hardware version.

This seems like a strange claim to make...?

struct ethtool_regs {
	__u32	cmd;
	__u32	version; /* driver-specific, indicates different chips/revs */
	__u32	len; /* bytes */
	__u8	data[0];
};

That "indicates different chips/revs" comment has been there at least
as long as the kernel has been in git (back to the 2.6.12 era).

> > If you don't actually provide a register dump then don't implement this
> > operation.
> 
> Then we have a problem as cfg80211 exports the hw version without any
> register dumps:
> 
> static int cfg80211_get_regs_len(struct net_device *dev)
> {
> 	/* For now, return 0... */
> 	return 0;
> }
> 
> static void cfg80211_get_regs(struct net_device *dev, struct
> ethtool_regs *regs,
> 			void *data)
> {
> 	struct wireless_dev *wdev = dev->ieee80211_ptr;
> 
> 	regs->version = wdev->wiphy->hw_version;
> 	regs->len = 0;
> }
> 
> And this has been there a long time already. How cfg80211 should export
> hw version if this is not a proper way?

The ethool binary already has support for the at76c50x_usb driver,
which uses this very mechanism in exactly this way.  I know this
worked previously, although I don't know what might have changed to
break it...?

John
-- 
John W. Linville		Someday the world will need a hero, and you
linville@tuxdriver.com			might be all we have.  Be ready.