netdev - Re: [PATCH] isdn capi driver broken on 64 bit.

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Date:	Tue, 28 Aug 2007 08:38:21 -0700
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	Karsten Keil <kkeil@...e.de>
Cc:	isdn4linux@...tserv.isdn4linux.de, netdev@...r.kernel.org
Subject: Re: [PATCH] isdn capi driver broken on 64 bit.

On Tue, 28 Aug 2007 15:07:04 +0200
Karsten Keil <kkeil@...e.de> wrote:

> Hello Steven,
> 
> On Mon, Aug 27, 2007 at 09:16:55AM -0700, Stephen Hemminger wrote:
> > On Mon, 27 Aug 2007 13:02:26 +0200
> > Karsten Keil <kkeil@...e.de> wrote:
> > 
> > > On Fri, Aug 24, 2007 at 11:08:11AM -0700, Stephen Hemminger wrote:
> > > > The following driver API is broken on any architecture with 64 bit addresses.
> > > > because of cast that loses high bits.
> > > > 
> > > > Signed-off-by: Stephen Hemminger <shemminger@...ux-foundation.org>
> > > > 
> > > > 
> > > > --- a/drivers/isdn/capi/capidrv.c	2007-06-25 09:03:12.000000000 -0700
> > > > +++ b/drivers/isdn/capi/capidrv.c	2007-08-24 11:06:46.000000000 -0700
> > > > @@ -1855,6 +1855,9 @@ static int if_sendbuf(int id, int channe
> > > >  		return 0;
> > > >  	}
> > > >  	datahandle = nccip->datahandle;
> > > > +
> > > > +	/* This won't work on 64 bit! */
> > > > +	BUILD_BUG_ON(sizeof(skb->data) > sizeof(u32));
> > > >  	capi_fill_DATA_B3_REQ(&sendcmsg, global.ap.applid, card->msgid++,
> > > >  			      nccip->ncci,	/* adr */
> > > >  			      (u32) skb->data,	/* Data */
> > > 
> > > 
> > > NACK.
> > > 
> > > It is not a BUG.
> > > 
> > > This is OK, since this field must have a value and on 32 it has the correct
> > > one) On 64 bit this field is ignored (but also need a value, random data is
> > > bad as well).
> > 
> > If you are using it as a transaction ID, then you should generate one.
> > There is no guarantee that two skb's won't have the same 32 bit data value
> > on 64 bit.
> 
> No, it's the address of the data buffer, but nobody use it in linux kernel.
> A CAPI DATA_B3 message looks like this in Linux:
> 
> <8 Byte CAPI HEADER>
> <32 bit NCCI>
> <32 bit Pointer to DataBuffer>
> <16 bit length>
> <16 bit handle>
> <16 bit Flags>
> <Start of DataBuffer>
> 
> 
> So the payload data follows directely the Message header and inside
> Linux we use this to address the data. But we must fill the pointer
> with some data, so we use the correct value for 32 bit, for 64 bit
> it is wrong yes, but since the value is not used ..., I want to
> avoid using a special case for 64 bit (e.g. setiing it to zero).
> Other OS use seperate buffers for the payload data, thats the reason why
> this field exist.
> For 64 bit a (optional) extention exist, which place a 64 bit Pointer
> beween <16 bit Flags> and <Start DataBuffer> so you can adress seperate
> data buffers under 64 as well, but we do no use that inside kernel CAPI
> implementation.
> 

Okay, maybe you want to take that nice explanation and put
it as a comment as to what is going on there.
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