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Subject: Re: [PATCH 03/16] PCI/P2PDMA: Attempt to set map_type if it has not
 been set
To:     Logan Gunthorpe <logang@deltatee.com>,
        <linux-kernel@vger.kernel.org>, <linux-nvme@lists.infradead.org>,
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CC:     Stephen Bates <sbates@raithlin.com>,
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        Dan Williams <dan.j.williams@intel.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
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        Daniel Vetter <daniel.vetter@ffwll.ch>,
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        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>
From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <3834be62-3d1b-fc98-d793-e7dcb0a74624@nvidia.com>
Date:   Sun, 2 May 2021 12:58:38 -0700
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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.

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,
-- 
John Hubbard
NVIDIA

>   		return 0;
>   	}
>   }
>