Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
To:     Bjorn Helgaas <helgaas@kernel.org>
Cc:     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,
        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>,
        Ira Weiny <iweiny@intel.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Don Dutile <ddutile@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Daniel Vetter <daniel.vetter@ffwll.ch>
References: <20201110232513.GA705726@bjorn-Precision-5520>
From:   Logan Gunthorpe <logang@deltatee.com>
Message-ID: <b3918b58-1c46-4f9c-fcae-37fd47b6bfde@deltatee.com>
Date:   Tue, 10 Nov 2020 16:42:03 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.4.0
MIME-Version: 1.0
In-Reply-To: <20201110232513.GA705726@bjorn-Precision-5520>
Content-Type: text/plain; charset=utf-8
Content-Language: en-CA
Content-Transfer-Encoding: 7bit
Subject: Re: [RFC PATCH 03/15] PCI/P2PDMA: Introduce
 pci_p2pdma_should_map_bus() and pci_p2pdma_bus_offset()
Precedence: bulk


On 2020-11-10 4:25 p.m., Bjorn Helgaas wrote:
> On Fri, Nov 06, 2020 at 10:00:24AM -0700, Logan Gunthorpe wrote:
>> Introduce pci_p2pdma_should_map_bus() which is meant to be called by
>> dma map functions to determine how to map a given p2pdma page.
> 
> s/dma/DMA/ for consistency (also below in function comment)
> 
>> pci_p2pdma_bus_offset() is also added to allow callers to get the bus
>> offset if they need to map the bus address.
>>
>> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
>> ---
>>  drivers/pci/p2pdma.c       | 46 ++++++++++++++++++++++++++++++++++++++
>>  include/linux/pci-p2pdma.h | 11 +++++++++
>>  2 files changed, 57 insertions(+)
>>
>> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
>> index ea8472278b11..9961e779f430 100644
>> --- a/drivers/pci/p2pdma.c
>> +++ b/drivers/pci/p2pdma.c
>> @@ -930,6 +930,52 @@ void pci_p2pdma_unmap_sg_attrs(struct device *dev, struct scatterlist *sg,
>>  }
>>  EXPORT_SYMBOL_GPL(pci_p2pdma_unmap_sg_attrs);
>>  
>> +/**
>> + * pci_p2pdma_bus_offset - returns the bus offset for a given page
>> + * @page: page to get the offset for
>> + *
>> + * Must be passed a pci p2pdma page.
> 
> s/pci/PCI/
> 
>> + */
>> +u64 pci_p2pdma_bus_offset(struct page *page)
>> +{
>> +	struct pci_p2pdma_pagemap *p2p_pgmap = to_p2p_pgmap(page->pgmap);
>> +
>> +	WARN_ON(!is_pci_p2pdma_page(page));
>> +
>> +	return p2p_pgmap->bus_offset;
>> +}
>> +EXPORT_SYMBOL_GPL(pci_p2pdma_bus_offset);
>> +
>> +/**
>> + * pci_p2pdma_should_map_bus - determine if a dma mapping should use the
>> + *	bus address
>> + * @dev: device doing the DMA request
>> + * @pgmap: dev_pagemap structure for the mapping
>> + *
>> + * Returns 1 if the page should be mapped with a bus address, 0 otherwise
>> + * and -1 the device should not be mapping P2PDMA pages.
> 
> I think this is missing a word.
> 
> I'm not really sure how to interpret the "should" in
> pci_p2pdma_should_map_bus().  If this returns -1, does that mean the
> patches *cannot* be mapped?  They *could* be mapped, but you really
> *shouldn't*?  Something else?
> 
> 1 means page should be mapped with bus address.  0 means ... what,
> exactly?  It should be mapped with some different address?

1 means it must be mapped with a bus address
0 means it may be mapped normally (through the IOMMU or just with a
direct physical address)
-1 means it cannot be mapped and should fail (ie. if it must go through
the IOMMU, but the IOMMU is not in the whitelist).

> Sorry these are naive questions because I don't know how all this
> works.

Thanks for the review. Definitely points out some questionable language
that I used. I'll reword this if/when it goes further.

Logan