linux-kernel - Re: [PATCH 8/12] x86-64: update iommu/dma mapping functions to sg helpers

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Message-ID: <464322BC.50101@panasas.com>
Date:	Thu, 10 May 2007 16:48:44 +0300
From:	Benny Halevy <bhalevy@...asas.com>
To:	Jens Axboe <jens.axboe@...cle.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 8/12] x86-64: update iommu/dma mapping functions to sg
 helpers

Jens Axboe wrote:
<snip>
> @@ -323,13 +324,17 @@ static int __dma_map_cont(struct scatterlist *sg, int start, int stopat,
>  {
>  	unsigned long iommu_start = alloc_iommu(pages);
>  	unsigned long iommu_page = iommu_start; 
> -	int i;
> +	struct scatterlist *s;
> +	int i, nelems;
>  
>  	if (iommu_start == -1)
>  		return -1;
> +
> +	nelems = stopat - start;
> +	while (start--)
> +		sg = sg_next(sg);

Ouch.  This will suck if we merge many times in a long list.
How about keeping and passing start as a struct scatterlist * rather
than an index? (see attached example, (compiles, bu untested though)


>  	
> -	for (i = start; i < stopat; i++) {
> -		struct scatterlist *s = &sg[i];
> +	for_each_sg(sg, s, nelems, i) {
>  		unsigned long pages, addr;
>  		unsigned long phys_addr = s->dma_address;


There seems to be a bug hiding here as now i is in [0, nelems) now,
not [start, stopat) so "if (i == start)" below should turn into "if (i == 0)"

this is fixed by comparing pointers instead way in the attached example.

>  		
> @@ -360,12 +365,14 @@ static inline int dma_map_cont(struct scatterlist *sg, int start, int stopat,
>  		      struct scatterlist *sout,
>  		      unsigned long pages, int need)
>  {
> -	if (!need) { 
> +	if (!need) {
>  		BUG_ON(stopat - start != 1);
> -		*sout = sg[start]; 
> -		sout->dma_length = sg[start].length; 
> +		while (--start)
> +			sg = sg_next(sg);

same efficiency issue as above.

> +		*sout = *sg;
> +		sout->dma_length = sg->length;
>  		return 0;
> -	} 
> +	}
>  	return __dma_map_cont(sg, start, stopat, sout, pages);
>  }
>  		
<snip>


View attachment "pci-gart-sg_chain.patch" of type "text/plain" (3264 bytes)