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

[Michael S. Tsirkin]

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