linux-kernel - Re: [PATCH 03/16] PCI/P2PDMA: Attempt to set map

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Date:   Mon, 3 May 2021 11:22:34 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Logan Gunthorpe <logang@...tatee.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>,
        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 03/16] PCI/P2PDMA: Attempt to set map_type if it has not
 been set

On 5/3/21 9:17 AM, Logan Gunthorpe wrote:
>> 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.
>>
>> 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;
>>
>>
>>>    }
> 
> I agree that some of this has evolved in a way that some of the names
> are a bit odd now. Could definitely use a cleanup, but that's not really
> part of this series. When I have some time I can look at doing a cleanup
> series to help with some of this.

I'm OK with doing cleanup later. I just tend to call it out when I see it.

> 
>>>    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.
> 
> Before this patch, it was required that users of P2PDMA call
> pci_p2pdma_distance_many() in some form before calling
> pci_p2pdma_map_sg(). So, by convention, a usable map type had to already
> be in the cache. The warning was there to yell at anyone who wrote code
> that violated that convention.
> 
> This patch removes that convention and allows users to map P2PDMA pages
> sight unseen and if the mapping type isn't in the cache, then it will
> determine the mapping type at dma mapping time. Thus, the warning can be
> removed and the function can fail normally if the mapping is unsupported.
> 

Let's add some of those words to the commit description, perhaps, it's nice
to have. Obviously a minor point though.

thanks,
-- 
John Hubbard
NVIDIA