linux-kernel - Re: [PATCH v2] QEMU fw_cfg DMA interface documentation

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Message-ID: <20150908164608.GA9169@morn.lan>
Date:	Tue, 8 Sep 2015 12:46:08 -0400
From:	Kevin O'Connor <kevin@...onnor.net>
To:	Gerd Hoffmann <kraxel@...hat.com>
Cc:	Marc Marí <markmb@...hat.com>,
	Stefan Hajnoczi <stefanha@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Drew <drjones@...hat.com>, Laszlo <lersek@...hat.com>,
	Arnd Bergmann <arnd@...db.de>,
	Rob Herring <rob.herring@...aro.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Graf <agraf@...e.de>, devicetree@...r.kernel.org
Subject: Re: [PATCH v2] QEMU fw_cfg DMA interface documentation

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
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