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

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Message-ID: <20160222220756-mutt-send-email-mst@redhat.com>
Date:	Mon, 22 Feb 2016 22:14:50 +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:
> > > +static void fw_cfg_io_cleanup(void)
> > > +{
> > > +	if (fw_cfg_is_mmio) {
> > > +		iounmap(fw_cfg_dev_base);
> > > +		release_mem_region(fw_cfg_p_base, fw_cfg_p_size);
> > > +	} else {
> > > +		ioport_unmap(fw_cfg_dev_base);
> > > +		release_region(fw_cfg_p_base, fw_cfg_p_size);
> > > +	}
> > > +}
> > > +
> > > +/* arch-specific ctrl & data register offsets are not available in ACPI, DT */
> > 
> > So for all arches which support ACPI, I think this driver
> > should just rely on ACPI.
> 
> There was a discussion about that a few versions ago, and IIRC the
> conclusion was not to expect the firmware to contend for fw_cfg access
> after the guest kernel boots:
> 
> https://lkml.org/lkml/2015/10/5/283
> 

So it looks like NVDIMM at least wants to pass label data to guest -
for which fw cfg might be a reasonable choice.

I suspect things changed - fw cfg used to be very slow but we now have
DMA interface which makes it useful for a range of applications.

> (I even had a prototype version doing what you suggested, but per the above
> reference decided to drop it -- which IMHO is for the better, since otherwise
> I'd have had to ifdef between ACPI and non-ACPI versions of the driver --
> see https://lkml.org/lkml/2015/11/4/534 )

I'm not sure why you have these ifdefs - they are on the host, are they
not?

-- 
MST