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

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Date:	Wed, 2 Sep 2015 09:20:12 +0100
From:	Stefan Hajnoczi <stefanha@...il.com>
To:	Marc Marí <markmb@...hat.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Drew <drjones@...hat.com>, "Kevin O'Connor" <kevin@...onnor.net>,
	Gerd Hoffmann <kraxel@...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, Aug 31, 2015 at 10:11 AM, Marc Marí <markmb@...hat.com> wrote:
> Add fw_cfg DMA interface specfication in the fw_cfg documentation.
>
> Signed-off-by: Marc Marí <markmb@...hat.com>
> ---
>  Documentation/devicetree/bindings/arm/fw-cfg.txt | 51 +++++++++++++++++++++++-
>  1 file changed, 50 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/devicetree/bindings/arm/fw-cfg.txt b/Documentation/devicetree/bindings/arm/fw-cfg.txt
> index 953fb64..766ddbe 100644
> --- a/Documentation/devicetree/bindings/arm/fw-cfg.txt
> +++ b/Documentation/devicetree/bindings/arm/fw-cfg.txt
> @@ -45,6 +45,53 @@ blob to be read from the data register has size 4, and it is to be interpreted
>  as a uint32_t value in little endian byte order. The current value
>  (corresponding to the above outer protocol) is zero.
>
> +If bit 1 of the feature bitmap is set, the DMA interface is present. This
> +can be used through the 64-bit wide address register.
> +
> +The address register is in big-endian format. The value for the register is 0
> +at startup and after an operation. A write to the lower half triggers an
> +operation. This means, that operations with 32-bit addresses can be triggered
> +with just one write, whereas operations with 64-bit addresses can be triggered
> +with one 64-bit write or two 32-bit writes, starting with the higher part.
> +
> +In this register, a physical RAM address to a FWCfgDmaAccess structure should
> +be written. This is the format of the FWCfgDmaAccess structure:
> +
> +typedef struct FWCfgDmaAccess {
> +    uint32_t control;
> +    uint32_t length;
> +    uint64_t address;
> +} FWCfgDmaAccess;

I think including the selector field would be nice to avoid extra
register accesses, but I'm not that familiar with fw_cfg so maybe
there's a reason not to include that field.

> +The fields of the structure are in big endian mode, and the field at the lowest
> +address is the "control" field.
> +
> +The "control" field has the following bits:
> + - Bit 0: Error
> + - Bit 1: Read
> + - Bit 2: Skip
> +
> +When an operation is triggered, if the "control" field has bit 1 set, a read
> +operation will be performed. "length" bytes for the current selector and
> +offset will be copied into the address specified by the "address" field.

Minor clarification:
s/address/physical RAM address/

Reviewed-by: Stefan Hajnoczi <stefanha@...hat.com>
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