linux-kernel - Re: [RFC PATCH v2 06/11] dma-direct: Support PCI P2PDMA pages in dma-direct map

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Date:   Tue, 16 Mar 2021 09:54:53 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Robin Murphy <robin.murphy@....com>, 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,
        Minturn Dave B <dave.b.minturn@...el.com>,
        John Hubbard <jhubbard@...dia.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Christian König <christian.koenig@....com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jason Ekstrand <jason@...kstrand.net>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Dan Williams <dan.j.williams@...el.com>,
        Stephen Bates <sbates@...thlin.com>,
        Jakowski Andrzej <andrzej.jakowski@...el.com>,
        Xiong Jianxin <jianxin.xiong@...el.com>
Subject: Re: [RFC PATCH v2 06/11] dma-direct: Support PCI P2PDMA pages in
 dma-direct map_sg



On 2021-03-16 1:58 a.m., Christoph Hellwig wrote:
> On Fri, Mar 12, 2021 at 11:27:46AM -0700, Logan Gunthorpe wrote:
>> So then we reject the patches that make that change. Seems like an odd
>> argument to say that we can't do something that won't cause problems
>> because someone might use it as an example and do something that will
>> cause problems. Reject the change that causes the problem.
> 
> No, the problem is a mess of calling conventions.  A calling convention
> returning 0 for error, positive values for success is fine.  One returning
> a negative errno for error and positive values for success is fine a well.
> One returning 0 for the usual errors and negativ errnos for an unusual
> corner case is just a complete mess.

Fair enough. I can try implementing a dma_map_sg_p2p() roughly as Robin
suggested that has a more reasonable calling convention.

Most of your other feedback seems easy enough so I'll address it in a
future series.

Thanks,

Logan