linux-kernel - Re: [PATCH 5/7] MIPS: use the generic uncached segment support in dma-direct

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Date:   Tue, 30 Apr 2019 20:10:43 +0000
From:   Paul Burton <paul.burton@...s.com>
To:     Christoph Hellwig <hch@....de>
CC:     Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>,
        Ley Foon Tan <lftan@...era.com>,
        Michal Simek <monstr@...str.eu>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>
Subject: Re: [PATCH 5/7] MIPS: use the generic uncached segment support in
 dma-direct

Hi Christoph,

On Tue, Apr 30, 2019 at 07:00:30AM -0400, Christoph Hellwig wrote:
> diff --git a/arch/mips/mm/dma-noncoherent.c b/arch/mips/mm/dma-noncoherent.c
> index f9549d2fbea3..f739f42c9d3c 100644
> --- a/arch/mips/mm/dma-noncoherent.c
> +++ b/arch/mips/mm/dma-noncoherent.c
> @@ -44,33 +44,26 @@ static inline bool cpu_needs_post_dma_flush(struct device *dev)
>  	}
>  }
>  
> -void *arch_dma_alloc(struct device *dev, size_t size,
> -		dma_addr_t *dma_handle, gfp_t gfp, unsigned long attrs)
> +void arch_dma_prep_coherent(struct page *page, size_t size)
>  {
> -	void *ret;
> -
> -	ret = dma_direct_alloc_pages(dev, size, dma_handle, gfp, attrs);
> -	if (ret && !(attrs & DMA_ATTR_NON_CONSISTENT)) {
> -		dma_cache_wback_inv((unsigned long) ret, size);
> -		ret = (void *)UNCAC_ADDR(ret);
> -	}
> +	if (!PageHighMem(page))
> +		dma_cache_wback_inv((unsigned long)page_address(page), size);
> +}

This series looks like a nice cleanup to me - the one thing that puzzles
me is the !PageHighMem check above.

As far as I can see arch_dma_prep_coherent() should never be called with
a highmem page, so would it make more sense to either drop this check or
perhaps wrap it in a WARN_ON()?

Thanks,
    Paul