linux-kernel - Re: [PATCH 5/8] arm64: use ZONE_DMA on DMA addressing limited devices

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Date:   Thu, 1 Aug 2019 17:07:54 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Catalin Marinas <catalin.marinas@....com>
Cc:     phill@...pberryi.org, devicetree@...r.kernel.org,
        linux-rpi-kernel@...ts.infradead.org, f.fainelli@...il.com,
        frowand.list@...il.com, eric@...olt.net, marc.zyngier@....com,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        robh+dt@...nel.org, wahrenst@....net, mbrugger@...e.com,
        akpm@...ux-foundation.org, hch@....de,
        linux-arm-kernel@...ts.infradead.org, m.szyprowski@...sung.com
Subject: Re: [PATCH 5/8] arm64: use ZONE_DMA on DMA addressing limited devices

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
> 
>