linux-kernel - Re: [PATCH v2 15/22] dma-direct: Support PCI P2PDMA pages in dma-direct map

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Date:   Fri, 14 May 2021 10:17:48 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Stephen Bates <sbates@...thlin.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian König <christian.koenig@....com>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Jakowski Andrzej <andrzej.jakowski@...el.com>,
        Minturn Dave B <dave.b.minturn@...el.com>,
        Jason Ekstrand <jason@...kstrand.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xiong Jianxin <jianxin.xiong@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH v2 15/22] dma-direct: Support PCI P2PDMA pages in
 dma-direct map_sg



On 2021-05-14 7:57 a.m., Christoph Hellwig wrote:
>> +	for_each_sg(sgl, sg, nents, i) {
>> +		if (sg_is_dma_pci_p2pdma(sg)) {
>> +			sg_dma_unmark_pci_p2pdma(sg);
>> +		} else  {
> 
> Double space here.  We also don't really need the curly braces to start
> with.
> 
>> +	struct pci_p2pdma_map_state p2pdma_state = {};
>> +	enum pci_p2pdma_map_type map;
>>  	struct scatterlist *sg;
>> +	int i, ret;
>>  
>>  	for_each_sg(sgl, sg, nents, i) {
>> +		if (is_pci_p2pdma_page(sg_page(sg))) {
>> +			map = pci_p2pdma_map_segment(&p2pdma_state, dev, sg);
>> +			switch (map) {
> 
> Why not just:
> 
> 			switch (pci_p2pdma_map_segment(&p2pdma_state, dev,
> 					sg)) {
> 
> (even better with a shorter name for p2pdma_state so that it all fits on
> a single line)?
> 
>> +			case PCI_P2PDMA_MAP_BUS_ADDR:
>> +				continue;
>> +			case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE:
>> +				/*
>> +				 * Mapping through host bridge should be
>> +				 * mapped normally, thus we do nothing
>> +				 * and continue below.
>> +				 */
> 
> I have a bit of a hard time parsing this comment.
> 
>> +		if (sg->dma_address == DMA_MAPPING_ERROR) {
>> +			ret = -EINVAL;
>>  			goto out_unmap;
>> +		}
>>  		sg_dma_len(sg) = sg->length;
>>  	}
>>  
>> @@ -411,7 +443,7 @@ int dma_direct_map_sg(struct device *dev, struct scatterlist *sgl, int nents,
>>  
>>  out_unmap:
>>  	dma_direct_unmap_sg(dev, sgl, i, dir, attrs | DMA_ATTR_SKIP_CPU_SYNC);
>> -	return -EINVAL;
>> +	return ret;
> 
> Maybe just initialize ret to -EINVAL at declaration time to simplify this
> a bit?
>

All fair points, will fix in v3.

Logan