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

[Hans de Goede]

Hi,

On 02-05-15 15:21, Ian Campbell wrote:
> 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.

I can understand where you're coming from, the problem is that despite
various mails to the arm kernel mailing list no one from the upstream
kernel seems to be looking into this, and I've no clue where to even
begun looking.

In the mean time any sunxi boards users using a 1024x600 framebuffer (and
likely other resulutions, 1024x600 is just one known way to trigger the
problem), trying to boot using the latest upstream kernel end up with
a non working system, without any clue as to why the kernel dies
(as it dies really early). This also means that people trying to bring
up new boards will not succeed, costing us potential contributors.

All in all I really believe that this is something which we should
workaround, also keep in mind that what we're doing is quite special
(clearly otherwise the bug would hit more users), we are not just curving
out X MB of RAM at the top, but exactly the amount we need, and no more,
lots of boards likely just carve out 8 / 16 MiB and thus are fine.

Regards,

Hans




>
>> Cc: Stefan Agner <stefan at agner.ch>
>> Signed-off-by: Hans de Goede <hdegoede at 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");
>
>