linux-kernel - Re: [PATCH v8 1/4] firmware: introduce sysfs driver for QEMU's fw

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Date:	Sun, 21 Feb 2016 15:10:30 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	"Gabriel L. Somlo" <somlo@....edu>
Cc:	gregkh@...uxfoundation.org, robh+dt@...nel.org, pawel.moll@....com,
	mark.rutland@....com, ijc+devicetree@...lion.org.uk,
	galak@...eaurora.org, arnd@...db.de, lersek@...hat.com,
	ralf@...ux-mips.org, rmk+kernel@....linux.org.uk, eric@...olt.net,
	hanjun.guo@...aro.org, zajec5@...il.com, sudeep.holla@....com,
	agross@...eaurora.org, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
	qemu-devel@...gnu.org, imammedo@...hat.com,
	peter.maydell@...aro.org, leif.lindholm@...aro.org,
	ard.biesheuvel@...aro.org, pbonzini@...hat.com, kraxel@...hat.com,
	ehabkost@...hat.com, luto@...capital.net, stefanha@...il.com,
	revol@...e.fr, matt@...eblueprint.co.uk, rth@...ddle.net
Subject: Re: [PATCH v8 1/4] firmware: introduce sysfs driver for QEMU's
 fw_cfg device

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!

-- 
MST