Re: [PATCH v6] ARM: boot: Obtain start of physical memory from DTB

[Lukasz Stelmach]

It was <2020-05-19 wto 14:12>, when Russell King - ARM Linux admin wrote:
> On Tue, May 19, 2020 at 02:49:57PM +0200, Lukasz Stelmach wrote:
>> It was <2020-05-19 wto 13:27>, when Russell King - ARM Linux admin wrote:
>>> On Tue, May 19, 2020 at 02:20:25PM +0200, Lukasz Stelmach wrote:
>>>> It was <2020-05-19 wto 12:43>, when Russell King - ARM Linux admin wrote:
>>>>> On Tue, May 19, 2020 at 01:21:09PM +0200, Geert Uytterhoeven wrote:
>>>>>> On Tue, May 19, 2020 at 11:46 AM Russell King - ARM Linux admin
>>>>>> <linux@xxxxxxxxxxxxxxx> wrote:
>>>>>>> On Tue, May 19, 2020 at 11:44:17AM +0200, Geert Uytterhoeven wrote:
>>>>>>>> On Tue, May 19, 2020 at 10:54 AM Lukasz Stelmach <l.stelmach@xxxxxxxxxxx> wrote:
>>>>>>>>> It was <2020-04-29 Åro 10:21>, when Geert Uytterhoeven wrote:
>>>>>>>>>> Currently, the start address of physical memory is obtained by masking
>>>>>>>>>> the program counter with a fixed mask of 0xf8000000.  This mask value
>>>>>>>>>> was chosen as a balance between the requirements of different platforms.
>>>>>>>>>> However, this does require that the start address of physical memory is
>>>>>>>>>> a multiple of 128 MiB, precluding booting Linux on platforms where this
>>>>>>>>>> requirement is not fulfilled.
>>>>>>>>>>
>>>>>>>>>> Fix this limitation by obtaining the start address from the DTB instead,
>>>>>>>>>> if available (either explicitly passed, or appended to the kernel).
>>>>>>>>>> Fall back to the traditional method when needed.
>> [...]
>>>>>>>>> Apparently reading physical memory layout from DTB breaks crashdump
>>>>>>>>> kernels. A crashdump kernel is loaded into a region of memory, that is
>>>>>>>>> reserved in the original (i.e. to be crashed) kernel. The reserved
>>>>>>>>> region is large enough for the crashdump kernel to run completely inside
>>>>>>>>> it and don't modify anything outside it, just read and dump the remains
>>>>>>>>> of the crashed kernel. Using the information from DTB makes the
>>>>>>>>> decompressor place the kernel outside of the dedicated region.
>>>>>>>>>
>>>>>>>>> The log below shows that a zImage and DTB are loaded at 0x18eb8000 and
>>>>>>>>> 0x193f6000 (physical). The kernel is expected to run at 0x18008000, but
>>>>>>>>> it is decompressed to 0x00008000 (see r4 reported before jumping from
>>>>>>>>> within __enter_kernel). If I were to suggest something, there need to be
>>>>>>>>> one more bit of information passed in the DTB telling the decompressor
>>>>>>>>> to use the old masking technique to determain kernel address. It would
>>>>>>>>> be set in the DTB loaded along with the crashdump kernel.
>> [...]
>>>>>>>> Describing "to use the old masking technique" sounds a bit hackish to me.
>>>>>>>> I guess it cannot just restrict the /memory node to the reserved region,
>>>>>>>> as the crashkernel needs to be able to dump the remains of the crashed
>>>>>>>> kernel, which lie outside this region.
>>>>>>>
>>>>>>> Correct.
>>>>>>>
>>>>>>>> However, something under /chosen should work.
>>>>>>>
>>>>>>> Yet another sticky plaster...
>>>>>> 
>>>>>> IMHO the old masking technique is the hacky solution covered by
>>>>>> plasters.
>>>>>
>>>>> One line of code is not "covered by plasters".  There are no plasters.
>>>>> It's a solution that works for 99.99% of people, unlike your approach
>>>>> that has had a stream of issues over the last four months, and has
>>>>> required many reworks of the code to fix each one.  That in itself
>>>>> speaks volumes about the suitability of the approach.
>>>> 
>>>> As I have been working with kexec code (patches soon) I would like to
>>>> defend the DT approach a bit. It allows to avoid zImage relocation when
>>>> a decompressed kernel is larger than ~128MiB. In such case zImage isn't
>>>> small either and moving it around takes some time.
>>>
>>> ... which is something that has been supported for a very long time,
>>> before the days of DT.
>> 
>> How? If a decompressed kernel requires >128M and a bootloader would like
>> to put a zImage high enough to *avoid* copying it once again, then the
>> decompressor can't see any memory below the 128M window it starts in and
>> can't decompress the kernel there.
>
> Do you have such a large kernel?  It would be rather inefficient as
> branch instructions could not be used; every function call would have
> to be indirect.  The maximum is +/- 32MB for a branch.

This number includes data, particularly initramfs which may be linked
into the kernel. Assuming kernel <32MB (15MB like min ATM) we are left
with slightly more than 100MB. Which isn't that much if you would like
it to be root file system for your device.

Of course initramfs could be loaded separately by a bootloader and yet
having both kernel and initramfs in one file has some advantages.

I am not here to argue. It's your call.

>> If we do not care about copying
>> zImage, then, indeed, everything works fine as it is today. You are
>> most probably right 99% doesn't require 128M kernel, but the case is
>> IMHO obvious enough, that it should be adressed somehow.
>
> If I have a kernel in excess of 4GB... "it should be addressed somehow"!

I believe software and hardware limitations should not be compared this
way. 

-- 
Åukasz Stelmach
Samsung R&D Institute Poland
Samsung Electronics

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