linux-kernel - Re: [PATCH v2 1/3] Documentation: arm: add UEFI support documentation

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Message-ID: <20131003171053.GB6999@e106331-lin.cambridge.arm.com>
Date:	Thu, 3 Oct 2013 18:10:54 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Leif Lindholm <leif.lindholm@...aro.org>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"roy.franz@...aro.org" <roy.franz@...aro.org>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"grant.likely@...retlab.ca" <grant.likely@...retlab.ca>,
	"matt.fleming@...el.com" <matt.fleming@...el.com>,
	"msalter@...hat.com" <msalter@...hat.com>
Subject: Re: [PATCH v2 1/3] Documentation: arm: add UEFI support
 documentation

Hi Leif,

On Thu, Oct 03, 2013 at 12:24:39PM +0100, Leif Lindholm wrote:
> This patch provides documentation of the [U]EFI runtime services and
> configuration features.
> 
> Cc: linux-doc@...r.kernel.org
> Signed-off-by: Leif Lindholm <leif.lindholm@...aro.org>
> Acked-by: Rob Landley <rob@...dley.net>
> ---
>  Documentation/arm/00-INDEX |    3 +++
>  Documentation/arm/uefi.txt |   47 ++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 50 insertions(+)
>  create mode 100644 Documentation/arm/uefi.txt
> 
> diff --git a/Documentation/arm/00-INDEX b/Documentation/arm/00-INDEX
> index 4978456..87e01d1 100644
> --- a/Documentation/arm/00-INDEX
> +++ b/Documentation/arm/00-INDEX
> @@ -36,3 +36,6 @@ nwfpe/
>  	- NWFPE floating point emulator documentation
>  swp_emulation
>  	- SWP/SWPB emulation handler/logging description
> +
> +uefi.txt
> +	- [U]EFI configuration and runtime services documentation
> diff --git a/Documentation/arm/uefi.txt b/Documentation/arm/uefi.txt
> new file mode 100644
> index 0000000..e6e4d41
> --- /dev/null
> +++ b/Documentation/arm/uefi.txt
> @@ -0,0 +1,47 @@
> +UEFI, the Unified Extensible Firmware Interface is a speifcication
> +governing the behaviours of compatible firmware interfaces. It is
> +maintained by the UEFI Forum - http://www.uefi.org/.
> +
> +Since UEFI is an evolution of its predecessor 'EFI', the terms EFI and
> +UEFI are used somewhat interchangeably in this document and associated
> +source code.
> +
> +The implementation depends on receiving the UEFI runtime memory map and a
> +pointer to the System Table in a Flattened Device Tree - so is only available
> +with CONFIG_OF.
> +
> +It parses the FDT /chosen node for the following parameters:
> +- 'linux,efi-system-table':
> +  Physical address of the system table. (required)
> +  64-bit value since an ARMv7 plattform may support LPAE, and to facilitate

s/plattform/platform/

> +  code sharing with arm64. Top 32 bits will be ignored, since UEFI specification
> +  mandates a 1:1 mapping of all RAM.

You could use something like #size-cells to describe how big this is
going to be. Is this Linux-specific -- it looks like something provided
by EFI rather than the kernel itself.

> +- 'linux,efi-mmap':
> +  The EFI memory map as an embedded property. (required)
> +  An array of type EFI_MEMORY_DESCRIPTOR as described by the UEFI
> +  specification, current version described in Linux by efi_memory_desc_t.
> +  The memory map is represented in little-endian, not DT, byte order.
> +  This map needs to contain at least the regions to be preserved for runtime
> +  services, but would normally just be the map retreieved by calling UEFI
> +  GetMemoryMap() immediately before ExitBootServices().

This is a little scary. If the format is so complicated, should it
really be embedded? How big is this likely to be?

Given that this is in a format defined externally, this isn't really
Linux-specific. Maybe we need an "efi" pseudo-vendor prefix.

> +- 'linux,efi-mmap-desc-size':
> +  Size of each descriptor in the memory map. (override default)

What units is this in? How many u32 cells does this take up (one
presumably)?

> +- 'linux,efi-mmap-desc-ver':
> +  Memory descriptor format version. (override default)

Type, format, valid values and their meaning?

Thanks,
Mark.

> +
> +It also depends on early_memremap() to parse the UEFI configuration tables.
> +
> +For actually enabling [U]EFI support, enable:
> +- CONFIG_EFI=y
> +- CONFIG_EFI_VARS=y or m
> +
> +After the kernel has mapped the required regions into its address space,
> +a SetVirtualAddressMap() call is made into UEFI in order to update
> +relocations. This call must be performed with all the code in a 1:1
> +mapping. This implementation achieves this by temporarily disabling the
> +MMU for the duration of this call. This can only be done safely:
> +- before secondary CPUs are brought online.
> +- after early_initcalls have completed, since it uses setup_mm_for_reboot().
> +
> +For verbose debug messages, specify 'uefi_debug' on the kernel command
> +line.
> -- 
> 1.7.10.4
> 
> 
`> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@...ts.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
> 
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