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SubjectRe: [PATCH 5/8] arm64: use ZONE_DMA on DMA addressing limited devices
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On 2019-08-01 4:44 pm, Nicolas Saenz Julienne wrote:
> On Wed, 2019-07-31 at 18:07 +0100, Catalin Marinas wrote:
>> On Wed, Jul 31, 2019 at 05:47:48PM +0200, Nicolas Saenz Julienne wrote:
>>> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
>>> index 1c4ffabbe1cb..f5279ef85756 100644
>>> --- a/arch/arm64/mm/init.c
>>> +++ b/arch/arm64/mm/init.c
>>> @@ -50,6 +50,13 @@
>>> s64 memstart_addr __ro_after_init = -1;
>>> EXPORT_SYMBOL(memstart_addr);
>>>
>>> +/*
>>> + * We might create both a ZONE_DMA and ZONE_DMA32. ZONE_DMA is needed if
>>> there
>>> + * are periferals unable to address the first naturally aligned 4GB of ram.
>>> + * ZONE_DMA32 will be expanded to cover the rest of that memory. If such
>>> + * limitations doesn't exist only ZONE_DMA32 is created.
>>> + */
>>
>> Shouldn't we instead only create ZONE_DMA to cover the whole 32-bit
>> range and leave ZONE_DMA32 empty? Can__GFP_DMA allocations fall back
>> onto ZONE_DMA32?
>
> Hi Catalin, thanks for the review.
>
> You're right, the GFP_DMA page allocation will fail with a nasty dmesg error if
> ZONE_DMA is configured but empty. Unsurprisingly the opposite situation is fine
> (GFP_DMA32 with an empty ZONE_DMA32).

Was that tested on something other than RPi4 with more than 4GB of RAM?
(i.e. with a non-empty ZONE_NORMAL either way)

Robin.

> I switched to the scheme you're suggesting for the next version of the series.
> The comment will be something the likes of this:
>
> /*
> * We create both a ZONE_DMA and ZONE_DMA32. ZONE_DMA's size is decided based
> * on whether the SoC's peripherals are able to address the first naturally
> * aligned 4 GB of ram.
> *
> * If limited, ZONE_DMA covers that area and ZONE_DMA32 the rest of that 32 bit
> * addressable memory.
> *
> * If not ZONE_DMA is expanded to cover the whole 32 bit addressable memory and
> * ZONE_DMA32 is left empty.
> */
>
> Regards,
> Nicolas
>
>

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