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Date: Tue, 8 Sep 2015 12:46:08 -0400
From: "Kevin O'Connor" <kevin@koconnor.net>
To: Gerd Hoffmann <kraxel@redhat.com>
Cc: Marc =?iso-8859-1?Q?Mar=ED?= <markmb@redhat.com>,
        Stefan Hajnoczi <stefanha@gmail.com>,
        linux-kernel <linux-kernel@vger.kernel.org>, Drew <drjones@redhat.com>,
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        Rob Herring <rob.herring@linaro.org>,
        Mark Rutland <mark.rutland@arm.com>, Alexander Graf <agraf@suse.de>,
        devicetree@vger.kernel.org
Subject: Re: [PATCH v2] QEMU fw_cfg DMA interface documentation
Message-ID: <20150908164608.GA9169@morn.lan>
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On Mon, Sep 07, 2015 at 01:08:29PM +0200, Gerd Hoffmann wrote:
> > It's just simplicity. If you want to read a few times from the same
> > field (like in ACPI tables, read the data size and then the data), you
> > need a way to enable and disable the selector and manage the current
> > offset for that entry. This is already provided with the "old"
> > interface.
> 
> Could be handled with a 'select' control bit.  Only when set select
> entry and reset offset to zero.

I think two features would help "round off" the new fw_cfg DMA
proposal: add a select bit as you describe (that uses the 16 most
significant bits of the "control" field for the "select entry" when
the bit is set), and define a static signature (eg, "QEMU CFG") when
reading the 64bit MMIO dma register.

Both are optional features that don't change the fundamental
interface; I was thinking of sending them as two patches on top of
Marc's next version of his patch series (if no one else gets to it
first).

-Kevin