[U-Boot] [PATCH] sunxi: display: Align end of memory to work around a linux-4.0 bug

[Ian Campbell <ijc+uboot@hellion.org.uk>]

On Fri, 2015-04-24 at 20:39 +0200, Hans de Goede wrote:
> Linux-4.0 as shipped has a bug causing it to not boot if the end of memory
> is not aligned to a multiple of 2 MiB. For details see the linux-arm
> mailing list post titled:
> "Memory size unaligned to section boundary"
> http://www.spinics.net/lists/arm-kernel/msg413811.html
> 
> This is something which specifically hits the sunxi display driver because
> we carve out the exact needed framebuffer size at the top of mem, this
> commit works around this issue by aligning the carve out.

I'm afraid I don't like this, we shouldn't be working around Linux bugs
in the firmware, especially when both are Free software. Lets just fix
Linux and get the fix into the appropriate stable trees and in the
meantime tell people to avoid this buggy kernel.

The problem with this sort of thing is that it is very hard to get rid
of these workarounds, even once the underlying issue is fixed and we no
longer care about the versions with the bug OS authors (including
non-Linux OSes) can inadvertently come to rely on the quirky behaviour,
(i.e. the work around masks other bugs). Hence we end up in a
quirks-race as everyone works around the other parties last workaround.

If there is to be a workaround instead of a fix then it should be for
Linux to align memory to 2MB boundaries if that is what it requires.

> Cc: Stefan Agner <stefan@agner.ch>
> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
> ---
>  drivers/video/sunxi_display.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/drivers/video/sunxi_display.c b/drivers/video/sunxi_display.c
> index 95cfe94..4607269 100644
> --- a/drivers/video/sunxi_display.c
> +++ b/drivers/video/sunxi_display.c
> @@ -1278,6 +1278,19 @@ int sunxi_simplefb_setup(void *blob)
>  	 */
>  	start = gd->bd->bi_dram[0].start;
>  	size = gd->bd->bi_dram[0].size - sunxi_display.fb_size;
> +
> +	/*
> +	 * Linux-4.0 as shipped has a bug causing it not boot if the end
> +	 * of memory is not aligned to a multiple of 2 MiB. For details
> +	 * see the linux-arm mailing list post titled:
> +	 * "Memory size unaligned to section boundary"
> +	 * http://www.spinics.net/lists/arm-kernel/msg413811.html
> +	 *
> +	 * This workaround should be removed once the bug has been fixed
> +	 * and we no longer care about the Linux versions with the bug.
> +	 */
> +	size &= ~(2 * 1024 * 1024 - 1);
> +
>  	ret = fdt_fixup_memory_banks(blob, &start, &size, 1);
>  	if (ret) {
>  		eprintf("Cannot setup simplefb: Error reserving memory\n");