linux-kernel - Re: [PATCH 10/16] dma-mapping: Add flags to dma_map

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Message-ID: <185f39c0-d71c-5d24-9d87-951a72c3532d@deltatee.com>
Date:   Tue, 11 May 2021 10:19:01 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Don Dutile <ddutile@...hat.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
Cc:     Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian König <christian.koenig@....com>,
        John Hubbard <jhubbard@...dia.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 10/16] dma-mapping: Add flags to dma_map_ops to indicate
 PCI P2PDMA support



On 2021-05-11 10:06 a.m., Don Dutile wrote:
> On 4/8/21 1:01 PM, Logan Gunthorpe wrote:
>> Add a flags member to the dma_map_ops structure with one flag to
>> indicate support for PCI P2PDMA.
>>
>> Also, add a helper to check if a device supports PCI P2PDMA.
>>
>> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
>> ---
>>   include/linux/dma-map-ops.h |  3 +++
>>   include/linux/dma-mapping.h |  5 +++++
>>   kernel/dma/mapping.c        | 18 ++++++++++++++++++
>>   3 files changed, 26 insertions(+)
>>
>> diff --git a/include/linux/dma-map-ops.h b/include/linux/dma-map-ops.h
>> index 51872e736e7b..481892822104 100644
>> --- a/include/linux/dma-map-ops.h
>> +++ b/include/linux/dma-map-ops.h
>> @@ -12,6 +12,9 @@
>>   struct cma;
>>   
>>   struct dma_map_ops {
>> +	unsigned int flags;
>> +#define DMA_F_PCI_P2PDMA_SUPPORTED     (1 << 0)
>> +
> I'm not a fan of in-line define's; if we're going to add a flags field to the dma-ops
> -- and logically it'd be good to have p2pdma go through the dma-ops struct --
> then let's move this up in front of the dma-ops description.

Already changed for v2.

> And now that the dma-ops struct is being 'opened' for p2pdma, should p2pdma ops be added
> to this struct, so all this work can be mimic'd/reflected/leveraged/refactored for CXL, GenZ, etc. p2pdma in (the near?) future?

v2 no longer has a specific op for p2pdma. We are now using
dma_map_sgtable() which already has the error return we need.

I think any work to support CXL, GenZ, etc will need to be done when
they add their own support. I can't and shouldn't guess at their needs now.

Logan