linux-kernel - Re: [RFC 1/5] RISC-V: Implement arch_sync

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Date:   Mon, 26 Jul 2021 08:56:57 +0200
From:   Christoph Hellwig <hch@....de>
To:     Atish Patra <atish.patra@....com>
Cc:     linux-kernel@...r.kernel.org, Albert Ou <aou@...s.berkeley.edu>,
        Christoph Hellwig <hch@....de>, devicetree@...r.kernel.org,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Frank Rowand <frowand.list@...il.com>,
        Guo Ren <guoren@...ux.alibaba.com>,
        iommu@...ts.linux-foundation.org, linux-riscv@...ts.infradead.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Rob Herring <robh+dt@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Tobias Klauser <tklauser@...tanz.ch>
Subject: Re: [RFC 1/5] RISC-V: Implement arch_sync_dma* functions

> +#ifdef CONFIG_RISCV_DMA_NONCOHERENT
> +struct riscv_dma_cache_sync {
> +	void (*cache_invalidate)(phys_addr_t paddr, size_t size);
> +	void (*cache_clean)(phys_addr_t paddr, size_t size);
> +	void (*cache_flush)(phys_addr_t paddr, size_t size);
> +};
> +
> +void riscv_dma_cache_sync_set(struct riscv_dma_cache_sync *ops);
> +#endif

As told a bunch of times before: doing indirect calls here is a
performance nightmare.  Use something that actually does perform
horribly like alternatives.  Or even delay implementing that until
we need it and do a plain direct call for now.

static void __dma_sync(phys_addr_t paddr, size_t size, enum dma_data_direction dir)
> +{
> +	if ((dir == DMA_FROM_DEVICE) && (dma_cache_sync->cache_invalidate))
> +		dma_cache_sync->cache_invalidate(paddr, size);
> +	else if ((dir == DMA_TO_DEVICE) && (dma_cache_sync->cache_clean))
> +		dma_cache_sync->cache_clean(paddr, size);
> +	else if ((dir == DMA_BIDIRECTIONAL) && dma_cache_sync->cache_flush)
> +		dma_cache_sync->cache_flush(paddr, size);
> +}

The seletion of flush types is completely broken.  Take a look at the
comment in arch/arc/mm/dma.c above arch_sync_dma_for_device for a good
explanation.

> +void arch_dma_prep_coherent(struct page *page, size_t size)
> +{
> +	void *flush_addr = page_address(page);
> +
> +	memset(flush_addr, 0, size);

arch_dma_prep_coherent is not supposed to modify the content of
the data.