linux-kernel - Re: [RFC PATCH v3 4/6] swiotlb: Add restricted DMA alloc/free support.

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Date:   Wed, 13 Jan 2021 13:48:47 +0100
From:   Christoph Hellwig <hch@....de>
To:     Claire Chang <tientzu@...omium.org>
Cc:     robh+dt@...nel.org, mpe@...erman.id.au, benh@...nel.crashing.org,
        paulus@...ba.org, joro@...tes.org, will@...nel.org,
        frowand.list@...il.com, konrad.wilk@...cle.com,
        boris.ostrovsky@...cle.com, jgross@...e.com,
        sstabellini@...nel.org, hch@....de, m.szyprowski@...sung.com,
        robin.murphy@....com, grant.likely@....com, xypron.glpk@....de,
        treding@...dia.com, mingo@...nel.org, bauerman@...ux.ibm.com,
        peterz@...radead.org, gregkh@...uxfoundation.org,
        saravanak@...gle.com, rafael.j.wysocki@...el.com,
        heikki.krogerus@...ux.intel.com, andriy.shevchenko@...ux.intel.com,
        rdunlap@...radead.org, dan.j.williams@...el.com,
        bgolaszewski@...libre.com, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        iommu@...ts.linux-foundation.org, xen-devel@...ts.xenproject.org,
        tfiga@...omium.org, drinkcat@...omium.org
Subject: Re: [RFC PATCH v3 4/6] swiotlb: Add restricted DMA alloc/free
 support.

> +#ifdef CONFIG_SWIOTLB
> +	if (unlikely(dev->dma_io_tlb_mem))
> +		return swiotlb_alloc(dev, size, dma_handle, attrs);
> +#endif

Another place where the dma_io_tlb_mem is useful to avoid the ifdef.

> -phys_addr_t swiotlb_tbl_map_single(struct device *hwdev, phys_addr_t orig_addr,
> -		size_t mapping_size, size_t alloc_size,
> -		enum dma_data_direction dir, unsigned long attrs)
> +static int swiotlb_tbl_find_free_region(struct device *hwdev,
> +					dma_addr_t tbl_dma_addr,
> +					size_t alloc_size,
> +					unsigned long attrs)

> +static void swiotlb_tbl_release_region(struct device *hwdev, int index,
> +				       size_t size)

This refactoring should be another prep patch.


> +void *swiotlb_alloc(struct device *dev, size_t size, dma_addr_t *dma_handle,
> +		    unsigned long attrs)

I'd rather have the names convey there are for the per-device bounce
buffer in some form.

> +	struct io_tlb_mem *mem = dev->dma_io_tlb_mem;

While we're at it I wonder if the io_tlb is something we could change
while we're at it.  Maybe replace io_tlb_mem with struct swiotlb
and rename the field in struct device to dev_swiotlb?

> +	int index;
> +	void *vaddr;
> +	phys_addr_t tlb_addr;
> +
> +	size = PAGE_ALIGN(size);
> +	index = swiotlb_tbl_find_free_region(dev, mem->start, size, attrs);
> +	if (index < 0)
> +		return NULL;
> +
> +	tlb_addr = mem->start + (index << IO_TLB_SHIFT);
> +	*dma_handle = phys_to_dma_unencrypted(dev, tlb_addr);
> +
> +	if (!dev_is_dma_coherent(dev)) {
> +		unsigned long pfn = PFN_DOWN(tlb_addr);
> +
> +		/* remove any dirty cache lines on the kernel alias */
> +		arch_dma_prep_coherent(pfn_to_page(pfn), size);

Can we hook in somewhat lower level in the dma-direct code so that all
the remapping in dma-direct can be reused instead of duplicated?  That
also becomes important if we want to use non-remapping uncached support,
e.g. on mips or x86, or the direct changing of the attributes that Will
planned to look into for arm64.