linux-kernel - Re: [PATCH 05/16] dma-mapping: Introduce dma_map_sg

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Date:   Tue, 27 Apr 2021 16:49:58 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
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>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        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 05/16] dma-mapping: Introduce dma_map_sg_p2pdma()



On 2021-04-27 1:22 p.m., Jason Gunthorpe wrote:
> On Thu, Apr 08, 2021 at 11:01:12AM -0600, Logan Gunthorpe wrote:
>> dma_map_sg() either returns a positive number indicating the number
>> of entries mapped or zero indicating that resources were not available
>> to create the mapping. When zero is returned, it is always safe to retry
>> the mapping later once resources have been freed.
>>
>> Once P2PDMA pages are mixed into the SGL there may be pages that may
>> never be successfully mapped with a given device because that device may
>> not actually be able to access those pages. Thus, multiple error
>> conditions will need to be distinguished to determine weather a retry
>> is safe.
>>
>> Introduce dma_map_sg_p2pdma[_attrs]() with a different calling
>> convention from dma_map_sg(). The function will return a positive
>> integer on success or a negative errno on failure.
>>
>> ENOMEM will be used to indicate a resource failure and EREMOTEIO to
>> indicate that a P2PDMA page is not mappable.
>>
>> The __DMA_ATTR_PCI_P2PDMA attribute is introduced to inform the lower
>> level implementations that P2PDMA pages are allowed and to warn if a
>> caller introduces them into the regular dma_map_sg() interface.
> 
> So this new API is all about being able to return an error code
> because auditing the old API is basically terrifying?
> 
> OK, but why name everything new P2PDMA? It seems nicer to give this
> some generic name and have some general program to gradually deprecate
> normal non-error-capable dma_map_sg() ?
> 
> I think that will raise less questions when subsystem people see the
> changes, as I was wondering why RW was being moved to use what looked
> like a p2pdma only API.
> 
> dma_map_sg_or_err() would have been clearer
> 
> The flag is also clearer as to the purpose if it is named
> __DMA_ATTR_ERROR_ALLOWED

I'm not opposed to these names. I can use them for v2 if there are no
other opinions.

Logan