linux-kernel - Re: [PATCH v5 2/5] iommu/io-pgtable: Move Apple DART support to its own file

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Date:   Thu, 22 Sep 2022 23:05:10 +0900
From:   Hector Martin <marcan@...can.st>
To:     Janne Grunau <j@...nau.net>, iommu@...ts.linux.dev
Cc:     Konrad Dybcio <konrad.dybcio@...ainline.org>,
        asahi@...ts.linux.dev, Robin Murphy <robin.murphy@....com>,
        Sven Peter <sven@...npeter.dev>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        iommu@...ts.linux-foundation.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/5] iommu/io-pgtable: Move Apple DART support to its
 own file

On 16/09/2022 18.41, Janne Grunau wrote:
> The pte format used by the DARTs found in the Apple M1 (t8103) is not
> fully compatible with io-pgtable-arm. The 24 MSB are used for subpage
> protection (mapping only parts of page) and conflict with the address
> mask. In addition bit 1 is not available for tagging entries but disables
> subpage protection. Subpage protection could be useful to support a CPU
> granule of 4k with the fixed IOMMU page size of 16k.
> 
> The DARTs found on Apple M1 Pro/Max/Ultra use another different pte
> format which is even less compatible. To support an output address size
> of 42 bit the address is shifted down by 4. Subpage protection is
> mandatory and bit 1 signifies uncached mappings used by the display
> controller.
> 
> It would be advantageous to share code for all known Apple DART
> variants to support common features. The page table allocator for DARTs
> is less complex since it uses a two levels of translation table without
> support for huge pages.
> 
> Signed-off-by: Janne Grunau <j@...nau.net>
> Acked-by: Robin Murphy <robin.murphy@....com>
> Acked-by: Sven Peter <sven@...npeter.dev>
> 
> ---

[...]

> +static void *__dart_alloc_pages(size_t size, gfp_t gfp,
> +				    struct io_pgtable_cfg *cfg)
> +{
> +	struct device *dev = cfg->iommu_dev;
> +	int order = get_order(size);
> +	struct page *p;
> +
> +	VM_BUG_ON((gfp & __GFP_HIGHMEM));
> +	p = alloc_pages(gfp | __GFP_ZERO, order);
> +	if (!p)
> +		return NULL;
> +
> +	return page_address(p);
> +}

This throws a warning:

drivers/iommu/io-pgtable-dart.c: In function ‘__dart_alloc_pages’:
drivers/iommu/io-pgtable-dart.c:112:24: warning: unused variable ‘dev’
[-Wunused-variable]
  112 |         struct device *dev = cfg->iommu_dev;
      |                        ^~~

The fix is trivial, of course.

- Hector