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Subject: Re: [PATCH 03/16] PCI/P2PDMA: Attempt to set map_type if it has not
 been set
To:     John Hubbard <jhubbard@nvidia.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
        linux-block@vger.kernel.org, linux-pci@vger.kernel.org,
        linux-mm@kvack.org, iommu@lists.linux-foundation.org
Cc:     Stephen Bates <sbates@raithlin.com>,
        Christoph Hellwig <hch@lst.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        Matthew Wilcox <willy@infradead.org>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Jakowski Andrzej <andrzej.jakowski@intel.com>,
        Minturn Dave B <dave.b.minturn@intel.com>,
        Jason Ekstrand <jason@jlekstrand.net>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Xiong Jianxin <jianxin.xiong@intel.com>,
        Bjorn Helgaas <helgaas@kernel.org>,
        Ira Weiny <ira.weiny@intel.com>,
        Robin Murphy <robin.murphy@arm.com>
References: <20210408170123.8788-1-logang@deltatee.com>
 <20210408170123.8788-4-logang@deltatee.com>
 <3834be62-3d1b-fc98-d793-e7dcb0a74624@nvidia.com>
From:   Don Dutile <ddutile@redhat.com>
Message-ID: <87a2b8e9-f0ef-ec23-8427-3022a86b0ec5@redhat.com>
Date:   Tue, 11 May 2021 12:05:49 -0400
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On 5/2/21 3:58 PM, John Hubbard wrote:
> On 4/8/21 10:01 AM, Logan Gunthorpe wrote:
>> Attempt to find the mapping type for P2PDMA pages on the first
>> DMA map attempt if it has not been done ahead of time.
>>
>> Previously, the mapping type was expected to be calculated ahead of
>> time, but if pages are to come from userspace then there's no
>> way to ensure the path was checked ahead of time.
>>
>> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
>> ---
>>   drivers/pci/p2pdma.c | 12 +++++++++---
>>   1 file changed, 9 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
>> index 473a08940fbc..2574a062a255 100644
>> --- a/drivers/pci/p2pdma.c
>> +++ b/drivers/pci/p2pdma.c
>> @@ -825,11 +825,18 @@ EXPORT_SYMBOL_GPL(pci_p2pmem_publish);
>>   static enum pci_p2pdma_map_type pci_p2pdma_map_type(struct pci_dev *provider,
>>                               struct pci_dev *client)
>>   {
>> +    enum pci_p2pdma_map_type ret;
>> +
>>       if (!provider->p2pdma)
>>           return PCI_P2PDMA_MAP_NOT_SUPPORTED;
>>   -    return xa_to_value(xa_load(&provider->p2pdma->map_types,
>> -                   map_types_idx(client)));
>> +    ret = xa_to_value(xa_load(&provider->p2pdma->map_types,
>> +                  map_types_idx(client)));
>> +    if (ret != PCI_P2PDMA_MAP_UNKNOWN)
>> +        return ret;
>> +
>> +    return upstream_bridge_distance_warn(provider, client, NULL,
>> +                         GFP_ATOMIC);
>
> Returning a "bridge distance" from a "get map type" routine is jarring,
> and I think it is because of a pre-existing problem: the above function
> is severely misnamed. Let's try renaming it (and the other one) to
> approximately:
>
>     upstream_bridge_map_type_warn()
>     upstream_bridge_map_type()
>
> ...and that should fix that. Well, that, plus tweaking the kernel doc
> comments, which are also confused. I think someone started off thinking
> about distances through PCIe, but in the end, the routine boils down to
> just a few situations that are not distances at all.
>
+1. didn't like the 'distance' check  for a 'connection check" in the beginning, and looks like this is the time to clean it out.
:)

> Also, the above will read a little better if it is written like this:
>
>     ret = xa_to_value(xa_load(&provider->p2pdma->map_types,
>                   map_types_idx(client)));
>
>     if (ret == PCI_P2PDMA_MAP_UNKNOWN)
>         ret = upstream_bridge_map_type_warn(provider, client, NULL,
>                             GFP_ATOMIC);
>
>     return ret;
>
>
>>   }
>>     static int __pci_p2pdma_map_sg(struct pci_p2pdma_pagemap *p2p_pgmap,
>> @@ -877,7 +884,6 @@ int pci_p2pdma_map_sg_attrs(struct device *dev, struct scatterlist *sg,
>>       case PCI_P2PDMA_MAP_BUS_ADDR:
>>           return __pci_p2pdma_map_sg(p2p_pgmap, dev, sg, nents);
>>       default:
>> -        WARN_ON_ONCE(1);
>
> Why? Or at least, why, in this patch? It looks like an accidental
> leftover from something, seeing as how it is not directly related to the
> patch, and is not mentioned at all.
>
>
> thanks,