linux-kernel - Re: [PATCH v2 2/4] ARC: allow to use IOC and non-IOC DMA devices simultaneously

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Date:   Mon, 13 Aug 2018 16:24:02 +0000
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        "hch@....de" <hch@....de>
Subject: Re: [PATCH v2 2/4] ARC: allow to use IOC and non-IOC DMA devices
 simultaneously

On 07/30/2018 09:26 AM, Eugeniy Paltsev wrote:
> @@ -1263,11 +1254,7 @@ void __init arc_cache_init_master(void)
>  	if (is_isa_arcv2() && ioc_enable)
>  		arc_ioc_setup();
>  
> -	if (is_isa_arcv2() && ioc_enable) {
> -		__dma_cache_wback_inv = __dma_cache_wback_inv_ioc;
> -		__dma_cache_inv = __dma_cache_inv_ioc;
> -		__dma_cache_wback = __dma_cache_wback_ioc;
> -	} else if (is_isa_arcv2() && l2_line_sz && slc_enable) {
> +	if (is_isa_arcv2() && l2_line_sz && slc_enable) {
>  		__dma_cache_wback_inv = __dma_cache_wback_inv_slc;
>  		__dma_cache_inv = __dma_cache_inv_slc;
>  		__dma_cache_wback = __dma_cache_wback_slc;
> diff --git a/arch/arc/mm/dma.c b/arch/arc/mm/dma.c
> index cefb776a99ff..4d1466905e48 100644
> --- a/arch/arc/mm/dma.c
> +++ b/arch/arc/mm/dma.c
> @@ -33,19 +33,7 @@ void *arch_dma_alloc(struct device *dev, size_t size, dma_addr_t *dma_handle,
>  	if (!page)
>  		return NULL;
>  
> -	/*
> -	 * IOC relies on all data (even coherent DMA data) being in cache
> -	 * Thus allocate normal cached memory
> -	 *
> -	 * The gains with IOC are two pronged:
> -	 *   -For streaming data, elides need for cache maintenance, saving
> -	 *    cycles in flush code, and bus bandwidth as all the lines of a
> -	 *    buffer need to be flushed out to memory
> -	 *   -For coherent data, Read/Write to buffers terminate early in cache
> -	 *   (vs. always going to memory - thus are faster)
> -	 */
> -	if ((is_isa_arcv2() && ioc_enable) ||
> -	    (attrs & DMA_ATTR_NON_CONSISTENT))
> +	if (attrs & DMA_ATTR_NON_CONSISTENT)
>  		need_coh = 0;
>  
>  	/*
> @@ -95,8 +83,7 @@ void arch_dma_free(struct device *dev, size_t size, void *vaddr,
>  	struct page *page = virt_to_page(paddr);
>  	int is_non_coh = 1;
>  
> -	is_non_coh = (attrs & DMA_ATTR_NON_CONSISTENT) ||
> -			(is_isa_arcv2() && ioc_enable);
> +	is_non_coh = (attrs & DMA_ATTR_NON_CONSISTENT);
>  
>  	if (PageHighMem(page) || !is_non_coh)
>  		iounmap((void __force __iomem *)vaddr);
> @@ -182,3 +169,20 @@ void arch_sync_dma_for_cpu(struct device *dev, phys_addr_t paddr,
>  		break;
>  	}
>  }

I think we have some shenanigans with @ioc_enable now.
Do note that it was more of a debug hack using when the hw feature was introduced
to be able to run same kernel on various FPGA bitfiles but just flicking a global
variable via the debugger.

So per code below, if @ioc_enable is NOT set, we still use software assisted cache
maintenance, but dma_{alloc,free} don't do use that variable. Have you tried
testing the combination when @ioc_enable is set to 0 before boot ? And is that works ?

> +
> +void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
> +			const struct iommu_ops *iommu, bool coherent)
> +{
> +	/*
> +	 * IOC hardware snoops all DMA traffic keeping the caches consistent
> +	 * with memory - eliding need for any explicit cache maintenance of
> +	 * DMA buffers - so we can use dma_direct cache ops.
> +	 */
> +	if (is_isa_arcv2() && ioc_enable && coherent) {
> +		set_dma_ops(dev, &dma_direct_ops);
> +		dev_info(dev, "use dma_direct_ops cache ops\n");
> +	} else {
> +		set_dma_ops(dev, &dma_noncoherent_ops);
> +		dev_info(dev, "use dma_noncoherent_ops cache ops\n");
> +	}
> +}