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Subject: Re: [PATCH 04/16] PCI/P2PDMA: Refactor pci_p2pdma_map_type() to take
 pagmap and device
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>,
        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>,
        Don Dutile <ddutile@redhat.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>
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From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <f07f0ca7-9772-5b3b-4cea-9defcefaaf8b@nvidia.com>
Date:   Mon, 3 May 2021 11:31:26 -0700
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On 5/3/21 9:30 AM, Logan Gunthorpe wrote:
> 
> 
> On 2021-05-02 2:41 p.m., John Hubbard wrote:
>> On 4/8/21 10:01 AM, Logan Gunthorpe wrote:
>>> All callers of pci_p2pdma_map_type() have a struct dev_pgmap and a
>>> struct device (of the client doing the DMA transfer). Thus move the
>>> conversion to struct pci_devs for the provider and client into this
>>> function.
>>
>> Actually, this is the wrong direction to go! All of these pre-existing
>> pci_*() functions have a small problem already: they are dealing with
>> struct device, instead of struct pci_dev. And so refactoring should be
>> pushing the conversion to pci_dev *up* the calling stack, not lower as
>> the patch here proposes.
>>
>> Also, there is no improvement in clarity by passing in (pgmap, dev)
>> instead of the previous (provider, client). Now you have to do more type
>> checking in the leaf function, which is another indication of a problem.
>>
>> Let's go that direction, please? Just convert to pci_dev much higher in
>> the calling stack, and you'll find that everything fits together better.
>> And it's OK to pass in extra params if that turns out to be necessary,
>> after all.
> 
> No, I disagree with this and it seems a bit confused. This change is

I am not confused here, no. Other places, yes, but not at this moment. :)

> allowing callers to call the function with what they have and doing more
> checks inside the called function. This allows for *less* checks in the
> leaf function, not more checks. (I mean, look at the patch itself, it
> puts a bunch of checks in both call sites into the callee and makes the
> code a lot cleaner -- it's removing more lines than it adds).
> 
> Similar argument can be made with the pci_p2pdma_distance_many() (which
> I assume you are referring to). If the function took struct pci_dev
> instead of struct device, every caller would need to do all checks and
> conversions to struct pci_dev. That is not an improvement.
> 


IMHO, it is better to have all of the pci_*() functions dealing with pci_dev
instead of dev, but it is also true that this is a larger change, so I
won't press the point too hard right now.

The reason I commented was that this refactoring goes in the opposite
direction that I would be going in, if I were to start "improving" this
part of the kernel, via refactoring.

Anyway, I'll leave it alone.

thanks,
-- 
John Hubbard
NVIDIA