Re: [TECH TOPIC] Reaching consensus on CONFIG_HIGHMEM phaseout

Nicolas Ferre
2025-09-19T14:42:04Z

Hi Geert,

On 19/09/2025 at 09:17, Geert Uytterhoeven wrote:
> Hi Arnd,
>
> On Thu, 18 Sept 2025 at 15:13, Arnd Bergmann <arnd@arndb.de> wrote:
>> On Wed, Sep 17, 2025, at 14:59, Jason Gunthorpe wrote:
>>> On Tue, Sep 09, 2025 at 11:23:37PM +0200, Arnd Bergmann wrote:
>>>
>>>> I'm still collecting information about which of the remaining highmem
>>>> users plan to keep updating their kernels and for what reason.
>>>
>>> On this topic of removing some parts of highmem, can we say goodbye to
>>> kmap_high_get()? Only ARM uses it and only for
>>> !cache_is_vipt_nonaliasing() systems.
>>
>> Good idea. I think we are almost there, just need to verify that
>> there is actually no impact for existing users. I already knew
>> that there is very little highmem usage on ARMv6 and earlier, but
>> I tried to recheck all platforms that might be affected:
>
>> * Microchip SAM9x7 is the newest ARMv5 chip, clearly does
>> get kernel updates, and the only one I can think of with
>> DDR3 support, but seems to be limited to 256MB total memory.
>
> Are they limited to DDR3?

For sam9x75:
- DDR2
- DDR3 (DLL Off/On mode)
- DDR3L (DLL Off/On mode)

For (older) sam9x60:
- LPDDR1
- DDR2
- SDRAM & Mobile SDRAM
> IIRC, someone (you? ;-) told me at ELCE that Microchip keeps on spinning
> new variants of old SoCs, to accommodate the changing DDR landscape
> and market. So perhaps they also accept larger RAM sizes?
No, in fact we address 256 M Bytes max on sam9x60 and sam9x75 as well.

Regards,
Nicolas