linux-kernel - Re: [PATCH 3/3] iommu: apple-dart: Add 4-level page table support

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Message-Id: <2d3e818d-5bc3-4156-a0c6-6d756f814c86@app.fastmail.com>
Date: Sat, 16 Aug 2025 16:19:33 +0200
From: "Janne Grunau" <j@...nau.net>
To: "Sven Peter" <sven@...nel.org>,
 "Alyssa Rosenzweig" <alyssa@...enzweig.io>, "Neal Gompa" <neal@...pa.dev>,
 "Joerg Roedel" <joro@...tes.org>, "Will Deacon" <will@...nel.org>,
 "Robin Murphy" <robin.murphy@....com>
Cc: asahi@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
 iommu@...ts.linux.dev, LKML <linux-kernel@...r.kernel.org>,
 "Hector Martin" <marcan@...can.st>
Subject: Re: [PATCH 3/3] iommu: apple-dart: Add 4-level page table support

Hej,

On Sat, Aug 16, 2025, at 15:50, Sven Peter wrote:
> On 14.08.25 10:40, Janne Grunau wrote:
>> From: Hector Martin <marcan@...can.st>
>>
>> The T8110 variant DART implementation on T602x SoCs indicates an IAS
>> of 42, which requires an extra page table level. The extra level is
>> optional, but let's implement it.
>>
>> Since the driver failed at IO page table creation with 42-bit IAS add
>> "apple,t6020-dart" as separate compatible using the T8110 HW data.
>
> Is the commit description outdated? I don't see this change anywhere.

yes, I decided to handle this as missing feature / bug. Both end up with
the same result and as far as we can tell it is fully compatible.
Removed locally.

>> Later it might be useful to restrict this based on the actual
>> attached devices, since most won't need that much address space
>> anyway.
>>
>> Signed-off-by: Hector Martin <marcan@...can.st> Signed-off-by: Janne
>> Grunau <j@...nau.net>
>> ---
>>   drivers/iommu/apple-dart.c | 23 +++++++++++++++++------ 1 file
>>   changed, 17 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/iommu/apple-dart.c b/drivers/iommu/apple-dart.c
>> index e72a93e78e26ca61b233c83d439dbdfadf040fc6..bb48e8603d6c84bcf107-
>> 294d851c2f2fc1273298 100644 --- a/drivers/iommu/apple-dart.c +++ b/drivers/iommu/apple-
>> dart.c @@ -133,6 +133,7 @@  #define DART_T8110_TCR
>> 0x1000  #define DART_T8110_TCR_REMAP            GENMASK(11, 8)
>> #define DART_T8110_TCR_REMAP_EN         BIT(7) +#define
>> DART_T8110_TCR_FOUR_LEVEL       BIT(3)  #define
>> DART_T8110_TCR_BYPASS_DAPF      BIT(2)  #define
>> DART_T8110_TCR_BYPASS_DART      BIT(1)  #define
>> DART_T8110_TCR_TRANSLATE_ENABLE BIT(0) @@ -177,6 +178,7 @@ struct
>> apple_dart_hw {     u32 tcr_enabled;     u32 tcr_disabled;     u32
>> tcr_bypass;
>> +    u32 tcr_4level;
>>
>>      u32 ttbr; u32 ttbr_valid; @@ -217,6 +219,7 @@ struct apple_dart
>>      { u32 pgsize; u32 num_streams; u32 supports_bypass : 1;
>> +    u32 four_level : 1;
>>
>>      struct iommu_group *sid2group[DART_MAX_STREAMS]; struct
>>      iommu_device iommu; @@ -305,13 +308,16 @@ static struct
>>      apple_dart_domain *to_dart_domain(struct iommu_domain *dom) }
>>
>>   static void -apple_dart_hw_enable_translation(struct
>>   apple_dart_stream_map *stream_map)
>>   +apple_dart_hw_enable_translation(struct apple_dart_stream_map
>>   *stream_map, int levels) {   struct apple_dart *dart = stream_map-
>>   >dart;   int sid;
>>
>> +    WARN_ON(levels != 3 && levels != 4);
>> +    WARN_ON(levels == 4 && !dart->four_level); for_each_set_bit(sid,
>>      stream_map->sidmap, dart->num_streams)
>> -            writel(dart->hw->tcr_enabled, dart->regs +
>>              DART_TCR(dart, sid));
>> +            writel(dart->hw->tcr_enabled | (levels == 4 ? dart->hw-
>>              >tcr_4level : 0),
>> +                   dart->regs + DART_TCR(dart, sid));
>
> This is a bit hard to read, I'd prefer an explicit if (dart->hw-
> >tcr_4level) here.

you mean `if (levels == 4)`? `dart->hw->tcr_4level` will be `BIT(3)` for
t8110 darts even when they use just 3 page table levels.

Changed locally to

u32 tcr = dart->hw->tcr_enabled; if (levels == 4)        tcr |= dart->hw-
>tcr_4level;

and then writel(tcr, ...) in the loop.

I've change prefix of all commits in this series to "iommu/apple-dart"
and "iommu/io-pgtable-dart".

thanks,
Janne