linux-kernel - Re: [PATCH 5/5] iommu/dart: Assert !locked when configuring

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Message-ID: <756ba6d7-b18e-4139-851b-496b1820117a@arm.com>
Date: Tue, 11 Feb 2025 18:41:00 +0000
From: Robin Murphy <robin.murphy@....com>
To: Alyssa Rosenzweig <alyssa@...enzweig.io>, Sven Peter
 <sven@...npeter.dev>, Janne Grunau <j@...nau.net>,
 Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>
Cc: asahi@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
 iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/5] iommu/dart: Assert !locked when configuring

On 2025-02-10 7:39 pm, Alyssa Rosenzweig wrote:
> Configuration is only possible and needed for non-locked DARTs and will
> fail for locked DARTs. We cannot try -- assert that we do not.

Except now we absolutely will - if a locked DART and its client device 
are advertised to Linux, instead of gracefully refusing to touch it, 
we'll now attach the client to a DMA domain, firing a barrage of 
multiple WARNs in the process, and give it DMA ops which still cannot 
work. I'm not really convinced this series on its own leaves us in a 
better position than we're already in now... :/

How hideous is the rest of what's required to actually make this usable?

Thanks,
Robin.

> Signed-off-by: Alyssa Rosenzweig <alyssa@...enzweig.io>
> ---
>   drivers/iommu/apple-dart.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/apple-dart.c b/drivers/iommu/apple-dart.c
> index 29b627b38e8c37afd2b6a72865f43d24b633834a..87eb87bb2f5158d000a2c2fc801b722a2262c941 100644
> --- a/drivers/iommu/apple-dart.c
> +++ b/drivers/iommu/apple-dart.c
> @@ -309,6 +309,7 @@ apple_dart_hw_enable_translation(struct apple_dart_stream_map *stream_map)
>   	struct apple_dart *dart = stream_map->dart;
>   	int sid;
>   
> +	WARN_ON(stream_map->dart->locked);
>   	for_each_set_bit(sid, stream_map->sidmap, dart->num_streams)
>   		writel(dart->hw->tcr_enabled, dart->regs + DART_TCR(dart, sid));
>   }
> @@ -318,6 +319,7 @@ static void apple_dart_hw_disable_dma(struct apple_dart_stream_map *stream_map)
>   	struct apple_dart *dart = stream_map->dart;
>   	int sid;
>   
> +	WARN_ON(stream_map->dart->locked);
>   	for_each_set_bit(sid, stream_map->sidmap, dart->num_streams)
>   		writel(dart->hw->tcr_disabled, dart->regs + DART_TCR(dart, sid));
>   }
> @@ -328,7 +330,7 @@ apple_dart_hw_enable_bypass(struct apple_dart_stream_map *stream_map)
>   	struct apple_dart *dart = stream_map->dart;
>   	int sid;
>   
> -	WARN_ON(!stream_map->dart->supports_bypass);
> +	WARN_ON(stream_map->dart->locked || !stream_map->dart->supports_bypass);
>   	for_each_set_bit(sid, stream_map->sidmap, dart->num_streams)
>   		writel(dart->hw->tcr_bypass,
>   		       dart->regs + DART_TCR(dart, sid));
> @@ -340,6 +342,7 @@ static void apple_dart_hw_set_ttbr(struct apple_dart_stream_map *stream_map,
>   	struct apple_dart *dart = stream_map->dart;
>   	int sid;
>   
> +	WARN_ON(stream_map->dart->locked);
>   	WARN_ON(paddr & ((1 << dart->hw->ttbr_shift) - 1));
>   	for_each_set_bit(sid, stream_map->sidmap, dart->num_streams)
>   		writel(dart->hw->ttbr_valid |
> @@ -353,6 +356,7 @@ static void apple_dart_hw_clear_ttbr(struct apple_dart_stream_map *stream_map,
>   	struct apple_dart *dart = stream_map->dart;
>   	int sid;
>   
> +	WARN_ON(stream_map->dart->locked);
>   	for_each_set_bit(sid, stream_map->sidmap, dart->num_streams)
>   		writel(0, dart->regs + DART_TTBR(dart, sid, idx));
>   }
>