Re: [RFC PATCH] MIPS: Provide cmpxchg64 for 32-bit builds

[James Hogan]

On Wed, Feb 14, 2018 at 09:36:33PM +0000, Keller, Jacob E wrote:
> > -----Original Message-----
> > From: Michael, Alice
> > Sent: Wednesday, February 14, 2018 1:03 PM
> > To: Guenter Roeck <linux@xxxxxxxxxxxx>; James Hogan <jhogan@xxxxxxxxxx>;
> > Keller, Jacob E <jacob.e.keller@xxxxxxxxx>
> > Cc: Ralf Baechle <ralf@xxxxxxxxxxxxxx>; linux-mips@xxxxxxxxxxxxxx; linux-
> > kernel@xxxxxxxxxxxxxxx; Kirsher, Jeffrey T <jeffrey.t.kirsher@xxxxxxxxx>; Shannon
> > Nelson <shannon.nelson@xxxxxxxxxx>
> > Subject: RE: [RFC PATCH] MIPS: Provide cmpxchg64 for 32-bit builds
> > 
> > As has previously been said, we're going to be removing the need for cmpxchg64.
> > But it takes a little bit of time and work to do so.  I'm adding the dev that is taking
> > care of the work back onto this email thread as well so he can see any concerns with
> > it.
> > 
> > Alice
> > 
> > -----Original Message-----
> > From: Guenter Roeck [mailto:groeck7@xxxxxxxxx] On Behalf Of Guenter Roeck
> > Sent: Monday, February 12, 2018 3:57 PM
> > To: James Hogan <jhogan@xxxxxxxxxx>
> > Cc: Ralf Baechle <ralf@xxxxxxxxxxxxxx>; linux-mips@xxxxxxxxxxxxxx; linux-
> > kernel@xxxxxxxxxxxxxxx; Michael, Alice <alice.michael@xxxxxxxxx>; Kirsher, Jeffrey T
> > <jeffrey.t.kirsher@xxxxxxxxx>; Shannon Nelson <shannon.nelson@xxxxxxxxxx>
> > Subject: Re: [RFC PATCH] MIPS: Provide cmpxchg64 for 32-bit builds
> > 
> > On Mon, Feb 12, 2018 at 11:42:02PM +0000, James Hogan wrote:
> > > Hi Guenter,
> > >
> > > On Mon, Feb 12, 2018 at 02:37:01PM -0800, Guenter Roeck wrote:
> > > > Since commit 60f481b970386 ("i40e: change flags to use 64 bits"),
> > > > the i40e driver uses cmpxchg64(). This causes mips:allmodconfig
> > > > builds to fail with
> > > >
> > > > drivers/net/ethernet/intel/i40e/i40e_ethtool.c:
> > > > 	In function 'i40e_set_priv_flags':
> > > > drivers/net/ethernet/intel/i40e/i40e_ethtool.c:4443:2: error:
> > > > 	implicit declaration of function 'cmpxchg64'
> > > >
> > > > Implement a poor-mans-version of cmpxchg64() to fix the problem for
> > > > 32-bit mips builds. The code is derived from sparc32, but only uses
> > > > a single spinlock.
> > >
> > > Will this be implemened for all 32-bit architectures which are
> > > currently missing cmpxchg64()?
> > >
> > No idea.
> > 
> > > If so, any particular reason not to do it in generic code?
> > >
> > Again, no idea. When the problem was previously seen on sparc32, it was
> > implemented there.
> > 
> > > If not then I think that driver should be fixed to either depend on
> > > some appropriate Kconfig symbol or to not use this API since it
> > > clearly isn't portable at the moment.
> > >
> > Good point.
> > 
> > > See also Shannon's comment about that specific driver:
> > > https://lkml.kernel.org/r/e7c934d7-e5f4-ee1b-0647-c31a98d9e944@oracle.
> > > com
> > >
> > 
> > Well, this was an RFC only. Feel free to ignore it.
> > 
> > FWIW, this is the second time that the call was introduced in the i40 driver.
> > After the first time the code was rewritten to avoid the problem, but now it came
> > back. Someone must really like it ;-). For my part, I may just blacklist the offending
> > driver in my builds; that is less than perfect, but much easier than having to deal with
> > the same problem over and over again. Guess I'll wait for a while and do just that if
> > the problem isn't fixed in a later RC.
> > 
> > Guenter
> 
> Hi,
> 
> I've been working on re-writing some of the code so that the need for a compare-and-exchange in the i40e_set_priv_flags() is not necessary. This mostly involved moving many flags out into an atomic bitops field instead, it should be posted to IWL soon.

Any update on this? Will a fix to the driver make it into 4.16 or is it
going to be too big a change?

As far as I can tell from grepping around, of the architectures which
support 32-bit SMP with PCI, these ones implement cmpxchg64 on 32-bit:

arch/arm
arch/ia64
arch/x86
arch/riscv (blindly implements using 64-bit instructions, broken?)
arch/parisc (with spinlock)
arch/sparc (with spinlock)

And these don't:

arch/arc
arch/mips
arch/powerpc
arch/sh
arch/xtensa

(I've excluded arches which are already being removed)

Cheers
James

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