Re: [PATCH v8 1/4] firmware: introduce sysfs driver for QEMU's fw_cfg device

From: "Gabriel L. Somlo" <somlo@cmu.edu>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: gregkh@linuxfoundation.org, robh+dt@kernel.org,
	pawel.moll@arm.com, mark.rutland@arm.com,
	ijc+devicetree@hellion.org.uk, galak@codeaurora.org,
	arnd@arndb.de, lersek@redhat.com, ralf@linux-mips.org,
	rmk+kernel@arm.linux.org.uk, eric@anholt.net,
	hanjun.guo@linaro.org, zajec5@gmail.com, sudeep.holla@arm.com,
	agross@codeaurora.org, linux-api@vger.kernel.org,
	linux-kernel@vger.kernel.org, devicetree@vger.kernel.org,
	qemu-devel@nongnu.org, imammedo@redhat.com,
	peter.maydell@linaro.org, leif.lindholm@linaro.org,
	ard.biesheuvel@linaro.org, pbonzini@redhat.com,
	kraxel@redhat.com, ehabkost@redhat.com, luto@amacapital.net,
	stefanha@gmail.com, revol@free.fr, matt@codeblueprint.co.uk,
	rth@twiddle.net
Subject: Re: [PATCH v8 1/4] firmware: introduce sysfs driver for QEMU's fw_cfg device
Date: Sun, 21 Feb 2016 12:20:10 -0500	[thread overview]
Message-ID: <20160221172010.GA4711@GLSMBP.INI.CMU.EDU> (raw)
In-Reply-To: <20160221150904-mutt-send-email-mst@redhat.com>

On Sun, Feb 21, 2016 at 03:10:30PM +0200, Michael S. Tsirkin wrote:
> On Sun, Feb 21, 2016 at 08:06:17AM -0500, Gabriel L. Somlo wrote:
> > 
> > > 
> > > > +#if !(defined(FW_CFG_CTRL_OFF) && defined(FW_CTRL_DATA_OFF))
> > > > +# if (defined(CONFIG_ARM) || defined(CONFIG_ARM64))
> > > > +#  define FW_CFG_CTRL_OFF 0x08
> > > > +#  define FW_CFG_DATA_OFF 0x00
> > > > +# elif (defined(CONFIG_PPC_PMAC) || defined(CONFIG_SPARC32)) /* ppc/mac,sun4m */
> > > > +#  define FW_CFG_CTRL_OFF 0x00
> > > > +#  define FW_CFG_DATA_OFF 0x02
> > > > +# elif (defined(CONFIG_X86) || defined(CONFIG_SPARC64)) /* x86, sun4u */
> > > > +#  define FW_CFG_CTRL_OFF 0x00
> > > > +#  define FW_CFG_DATA_OFF 0x01
> > > > +# else
> > > > +#  warning "QEMU FW_CFG may not be available on this architecture!"
> > > > +#  define FW_CFG_CTRL_OFF 0x00
> > > > +#  define FW_CFG_DATA_OFF 0x01
> > > 
> > > Better not try hacks like this, they are hard
> > > to support down the road. Please only list what is tested and
> > > actually exposed by QEMU.
> > 
> > I was looking for a standard way to advertise register offsets within
> > the ioport or mmio region assigned to fw_cfg, but right now the answer
> > is "there isn't one", and "register offsets are an arch specific
> > detail". As such, the only reasonable way I saw was to copy the same
> > values used in the QEMU source.
> > 
> > See also:
> > https://lists.gnu.org/archive/html/qemu-devel/2015-11/msg05037.html
> >  
> > Thanks much,
> > --Gabriel
> 
> My point is you don't know what will qemu do on these
> other arches which do not at the moment have fw cfg.
> So don't try to guess!

Oh, you mean for the "else". I originally wanted to be able to compile
this on any architecture and wanted some dummy defaults I could
override on the command line. But now we're already restricting this
to known architectures only, so I'll send a patch turning the warning
into an error, and removing the #defines for the "else" branch above.

Sorry I misunderstood you the first time around :)

Thanks,
--Gabriel