[RFC 1/1] ARM: bootm: recalculate decompression space

[Sascha Hauer]

Hi Stefan,

On Wed, Jun 22, 2016 at 04:49:04PM +0200, Stefan Müller-Klieser wrote:
> According to the kernel documentation it is recommended to place the
> compressed image between 32MiB and 128MiB. We will conform to this if we
> have at least 64MiB of RAM. If this is not the case, we fall back to the
> old scheme but take the relocated image into account.
> This is required because of the ARM default kernel config changes
> regarding RODATA layout, which lead to an increased compression factor
> of the kernel image.
> 
> Signed-off-by: Stefan Müller-Klieser <s.mueller-klieser at phytec.de>
> ---
>  arch/arm/lib/bootm.c | 24 +++++++++++++++---------
>  1 file changed, 15 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/arm/lib/bootm.c b/arch/arm/lib/bootm.c
> index 803aa94..d4fe9a4 100644
> --- a/arch/arm/lib/bootm.c
> +++ b/arch/arm/lib/bootm.c
> @@ -73,18 +73,24 @@ static int get_kernel_addresses(size_t image_size,
>  {
>  	unsigned long mem_start, mem_size;
>  	int ret;
> -	size_t image_decomp_size;
> -	unsigned long spacing;
> +	unsigned long decomp_space, spacing;

I renamed back decomp_space to image_decomp_size which leads to this
easier to read patch:

> diff --git a/arch/arm/lib/bootm.c b/arch/arm/lib/bootm.c
> index 803aa94..8630f2c 100644
> --- a/arch/arm/lib/bootm.c
> +++ b/arch/arm/lib/bootm.c
> @@ -81,10 +81,17 @@ static int get_kernel_addresses(size_t image_size,
>  		return ret;
>  
>  	/*
> -	 * We don't know the exact decompressed size so just use a conservative
> -	 * default of 4 times the size of the compressed image.
> +	 * The kernel documentation "Documentation/arm/Booting" advises
> +	 * to place the compressed image outside of the lowest 32MiB to
> +	 * avoid relocation. We should do this if we have at least 64MiB
> +	 * of ram. If we have less space, we assume a maximum
> +	 * compression factor of 4 plus 1. The latter factor is the
> +	 * space for the relocated image.

Why space for the relocated image? Our mission is to place the image
where it doesn't have to be relocated.

>  	 */
> -	image_decomp_size = PAGE_ALIGN(image_size * 4);
> +	if (mem_size >= SZ_64M)
> +		image_decomp_size = SZ_32M;
> +	else
> +		image_decomp_size = PAGE_ALIGN(image_size * 5);

	image_decomp_size = PAGE_ALIGN(image_size * 5);
	if (mem_size >= SZ_64M)
		image_decomp_size = max(image_decomp_size, SZ_32M);

>  
>  	/*
>  	 * By default put oftree/initrd close behind compressed kernel image to
> @@ -111,7 +118,7 @@ static int get_kernel_addresses(size_t image_size,
>  		spacing += image_decomp_size;
>  	}
>  
> -	*mem_free = PAGE_ALIGN(*load_address + image_size + spacing);
> +	*mem_free = PAGE_ALIGN(*load_address + image_decomp_size + spacing);

Now why this change? We have two cases to consider. First, when the
*load_address is unspecified, we aim for this setup:

|- uncompressed image -||- compressed image -||- spacing -||- free for initrd/oftree -|

The code should be correct for this case without this hunk:

*load_address = mem_start + image_decomp_size
*mem_free = *load_address + image_size + spacing

Second case is when the load address is specified. In this case we
adjust the spacing by image_decomp_size, this also seems to be done
correctly in the current code.

So I think this hunk is wrong.

Instead we should consider adding the following at the end:

	if (mem_size > SZ_256M)
		*mem_free = max(*mem_free, mem_start + SZ_128M);

This would make sure to follow the recommendation to put the
initrd/device tree at 128MiB.

Sascha

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