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From: Thomas Huth <thuth@redhat.com>
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Date: Thu, 26 Nov 2015 11:57:20 +0100
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Subject: Re: [Qemu-devel] [RESEND RFC 2/6] device_tree: introduce
 load_device_tree_from_sysfs
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To: Eric Auger <eric.auger@linaro.org>, eric.auger@st.com, qemu-devel@nongnu.org, qemu-arm@nongnu.org, alex.williamson@redhat.com, peter.maydell@linaro.org
Cc: thomas.lendacky@amd.com, b.reynal@virtualopensystems.com, patches@linaro.org, suravee.suthikulpanit@amd.com, pbonzini@redhat.com, christoffer.dall@linaro.org, David Gibson <david@gibson.dropbear.id.au>

On 19/11/15 16:22, Eric Auger wrote:
> This function returns the host device tree blob from sysfs
> (/sys/firmware/devicetree/base).
> 
> This has a runtime dependency on the dtc binary. This functionality
> is useful for platform device passthrough where the host device tree
> needs to be parsed to feed information into the guest device tree.
> 
> Signed-off-by: Eric Auger <eric.auger@linaro.org>
> ---
>  device_tree.c                | 40 ++++++++++++++++++++++++++++++++++++++++
>  include/sysemu/device_tree.h |  1 +
>  2 files changed, 41 insertions(+)
> 
> diff --git a/device_tree.c b/device_tree.c
> index a9f5f8e..58a5329 100644
> --- a/device_tree.c
> +++ b/device_tree.c
> @@ -117,6 +117,46 @@ fail:
>      return NULL;
>  }
>  
> +/**
> + * load_device_tree_from_sysfs
> + *
> + * extract the dt blob from host sysfs
> + * this has a runtime dependency on the dtc binary
> + */
> +void *load_device_tree_from_sysfs(void)
> +{
> +    char cmd[] = "dtc -I fs -O dtb /sys/firmware/devicetree/base";
> +    FILE *pipe;
> +    void *fdt;
> +    int ret, actual_dt_size;
> +
> +    pipe = popen(cmd, "r");
> +    if (!pipe) {
> +        error_report("%s: Error when executing dtc", __func__);
> +        return NULL;
> +    }

The Device Tree Compiler binary is normally only installed on
developer's machines, so I somewhat doubt that it is a good idea to rely
on the availability of that binary in QEMU? Maybe you should rather
extend libfdt to support such a feature?

 Thomas