netdev - Re: [PATCH 03/12] net: ll_temac: Fix support for 64-bit platforms

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Date:   Fri, 26 Apr 2019 15:30:56 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Esben Haabendal <esben@...nix.com>, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Michal Simek <michal.simek@...inx.com>,
        Yang Wei <yang.wei9@....com.cn>,
        YueHaibing <yuehaibing@...wei.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/12] net: ll_temac: Fix support for 64-bit platforms

On Sat, 27 Apr 2019 00:02:26 +0200, Andrew Lunn wrote:
> On Fri, Apr 26, 2019 at 02:08:56PM -0700, Jakub Kicinski wrote:
> > On Fri, 26 Apr 2019 22:59:12 +0200, Andrew Lunn wrote:  
> > > On Fri, Apr 26, 2019 at 11:40:13AM -0700, Jakub Kicinski wrote:  
> > > > On Fri, 26 Apr 2019 09:32:22 +0200, Esben Haabendal wrote:    
> > > > > The use of buffer descriptor APP4 field (32-bit) for storing skb pointer
> > > > > obviously does not work on 64-bit platforms.
> > > > > As APP3 is also unused, we can use that to store the other half of 64-bit
> > > > > pointer values.
> > > > > 
> > > > > Contrary to what is hinted at in commit message of commit 15bfe05c8d63
> > > > > ("net: ethernet: xilinx: Mark XILINX_LL_TEMAC broken on 64-bit")
> > > > > there are no other pointers stored in cdmac_bd.
> > > > > 
> > > > > Signed-off-by: Esben Haabendal <esben@...nix.com>    
> > > > 
> > > > This is a bit strange, the driver stores the host's virtual address into
> > > > the HW descriptor?    
> 
> Lets try that again
> 
> Hi Jakub

:)

> > > Hi Jukub  
> > 
> > I need to start keeping track of all the ways my name gets spelled :)
> > I find it entertaining :)  
> 
> Sorry. 
> 
> And i prefer entertaining over offended :-)

Certainly no offence taken! :)

> > > This is reasonably common. You need some sort of cookie which links
> > > the hardware descriptor to the skbuf it points to. The hardware makes
> > > no use of it, it is just a cookie.  
> > 
> > Right, but accesses to HW descriptor memory ring are significantly 
> > more expensive, especially on platforms which are not coherent with 
> > DMA operations (everything but x86?)
> >
> > A preferable design is to have two descriptor rings - one for HW
> > descriptors and one for software context, no?  
> 
> Modern drivers do that. But this driver seems to be quite old.  And if
> you look at what it is used on, PPC & MICROBLAZE, they are old
> architectures, i don't think hardware access are that as expensive as
> for modern architectures.

True, my comment was certainly more of a suggestion than a blocker.

Looking closer at the series it kind of looks like a soft IP.
Esben, is there anything architecture specific here?  Should we perhaps
drop the dependency on the architectures in patch 6 completely?