From: "Maciej W. Rozycki" <macro@orcam.me.uk>
To: Niklas Schnelle <schnelle@linux.ibm.com>
Cc: Arnd Bergmann <arnd@arndb.de>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
linux-pci@vger.kernel.org, Marc Kleine-Budde <mkl@pengutronix.de>,
Arnd Bergmann <arnd@kernel.org>,
"David S. Miller" <davem@davemloft.net>,
Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
Michael Grzeschik <m.grzeschik@pengutronix.de>,
Wolfgang Grandegger <wg@grandegger.com>,
Jesse Brandeburg <jesse.brandeburg@intel.com>,
Tony Nguyen <anthony.l.nguyen@intel.com>,
Ralf Baechle <ralf@linux-mips.org>,
"open list:NETWORKING DRIVERS" <netdev@vger.kernel.org>,
"open list:CAN NETWORK DRIVERS" <linux-can@vger.kernel.org>,
"moderated list:INTEL ETHERNET DRIVERS"
<intel-wired-lan@lists.osuosl.org>,
open
Subject: Re: [RFC v2 21/39] net: add HAS_IOPORT dependencies
Date: Tue, 3 May 2022 14:36:21 +0100 (BST) [thread overview]
Message-ID: <alpine.DEB.2.21.2205031359490.64520@angie.orcam.me.uk> (raw)
In-Reply-To: <867e70df01fc938abf93ffa15a3f1989a8fb136b.camel@linux.ibm.com>
On Tue, 3 May 2022, Niklas Schnelle wrote:
> > The driver works just fine with MMIO where available, so if `inb'/`outb'
> > do get removed, then only parts that rely on port I/O need to be disabled.
> > In fact there's already such provision there in drivers/net/fddi/defxx.c
> > for TURBOchannel systems (CONFIG_TC), which have no port I/O space either:
> >
> > #if defined(CONFIG_EISA) || defined(CONFIG_PCI)
> > #define dfx_use_mmio bp->mmio
> > #else
> > #define dfx_use_mmio true
> > #endif
> >
> > so I guess it's just the conditional that will have to be changed to:
> >
> > #ifdef CONFIG_HAS_IOPORT
> >
> > replacing the current explicit bus dependency list. The compiler will
> > then optimise away all the port I/O stuff (though I suspect dummy function
> > declarations may be required for `inb'/`outb', etc.).
[...]
> With dfx_use_mmio changed as you propose above things compile on s390
> which previously ran into missing (now __compile_error()) inl() via
> dfx_port_read_long() -> dfx_inl() -> inl().
Great, thanks for checking! And I note referring `__compile_error' is
roughly equivalent to a dummy declaration, so you've got that part sorted.
> Looking at the other uses of dfx_use_mmio I notice however that in
> dfx_get_bars(), inb() actually gets called when dfx_use_mmio is true.
> This happens if dfx_bus_eisa is also true. Now that variable is just
> the cached result of DFX_BUS_EISA(dev) which is defined to 0 if
> CONFIG_EISA is unset. I'm not 100% sure if going through a local
> variable is still considered trivial enough dead code elimination, at
> least it works for meâ„¢. I did also check the GCC docs and they
> explicitly say that __attribute__(error) is supposed to be used when
> dead code elimination gets rid of the error paths.
Yeah, dead code elimination is supposed to handle such cases. The local
automatic variable is essentially a syntactic feature not to use the same
expression inline over and over throughout a function (for clarity the
variable should probably be declared `const', but that is not essential)
and it is up to the compiler whether to reuse the value previously
calculated or to re-evaluate the expression.
> I think we also need a "depends on HAS_IOPORT" for "config HAVE_EISA"
> just as I'm adding for "config ISA".
Oh absolutely! There's the slot-specific port I/O space that is used to
identify EISA option cards in device discovery, so no EISA device will
ever work without port I/O. Have a look at `decode_eisa_sig' in
drivers/eisa/eisa-bus.c for the very obvious code. Note that some ISA
cards can be configured to appear as EISA devices as well (I have a 3c509B
Ethernet NIC set up that way).
Maciej
prev parent reply other threads:[~2022-05-03 13:36 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20220429135108.2781579-1-schnelle@linux.ibm.com>
2022-04-29 13:50 ` [RFC v2 21/39] net: add HAS_IOPORT dependencies Niklas Schnelle
2022-04-29 14:13 ` Marc Kleine-Budde
2022-05-01 22:48 ` Maciej W. Rozycki
2022-05-03 12:45 ` Niklas Schnelle
2022-05-03 13:36 ` Maciej W. Rozycki [this message]
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