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Date: Mon, 7 Nov 2022 10:46:03 +0100
From: Christoph Hellwig <hch@lst.de>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Arnd Bergmann <arnd@arndb.de>, Christoph Hellwig <hch@lst.de>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Will Deacon <will@kernel.org>, Marc Zyngier <maz@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Ard Biesheuvel <ardb@kernel.org>,
	Isaac Manjarres <isaacmanjarres@google.com>,
	Saravana Kannan <saravanak@google.com>,
	Alasdair Kergon <agk@redhat.com>, Daniel Vetter <daniel@ffwll.ch>,
	Joerg Roedel <joro@8bytes.org>, Mark Brown <broonie@kernel.org>,
	Mike Snitzer <snitzer@kernel.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Robin Murphy <robin.murphy@arm.com>, linux-mm@kvack.org,
	iommu@lists.linux.dev, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3 03/13] iommu/dma: Force bouncing of the size is not
 cacheline-aligned
Message-ID: <20221107094603.GB6055@lst.de>
References: <20221106220143.2129263-1-catalin.marinas@arm.com> <20221106220143.2129263-4-catalin.marinas@arm.com>
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User-Agent: Mutt/1.5.17 (2007-11-01)

> +static inline bool dma_sg_kmalloc_needs_bounce(struct device *dev,
> +					       struct scatterlist *sg, int nents,
> +					       enum dma_data_direction dir)
> +{
> +	struct scatterlist *s;
> +	int i;
> +
> +	if (!IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) ||
> +	    dir == DMA_TO_DEVICE || dev_is_dma_coherent(dev))
> +		return false;

This part should be shared with dma-direct in a well documented helper.

> +	for_each_sg(sg, s, nents, i) {
> +		if (dma_kmalloc_needs_bounce(dev, s->length, dir))
> +			return true;
> +	}

And for this loop iteration I'd much prefer it to be out of line, and
also not available in a global helper.

But maybe someone can come up with a nice tweak to the dma-iommu
code to not require the extra sglist walk anyway.