Re: [PATCH 05/16] dma-mapping: Introduce dma_map_sg_p2pdma()

From: Logan Gunthorpe <logang@deltatee.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
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>,
	"Christian König" <christian.koenig@amd.com>,
	"John Hubbard" <jhubbard@nvidia.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>
Subject: Re: [PATCH 05/16] dma-mapping: Introduce dma_map_sg_p2pdma()
Date: Tue, 27 Apr 2021 16:49:58 -0600	[thread overview]
Message-ID: <74473159-b4d7-89c8-9dae-7e983b22ef2b@deltatee.com> (raw)
In-Reply-To: <20210427192232.GO2047089@ziepe.ca>

On 2021-04-27 1:22 p.m., Jason Gunthorpe wrote:
> On Thu, Apr 08, 2021 at 11:01:12AM -0600, Logan Gunthorpe wrote:
>> dma_map_sg() either returns a positive number indicating the number
>> of entries mapped or zero indicating that resources were not available
>> to create the mapping. When zero is returned, it is always safe to retry
>> the mapping later once resources have been freed.
>>
>> Once P2PDMA pages are mixed into the SGL there may be pages that may
>> never be successfully mapped with a given device because that device may
>> not actually be able to access those pages. Thus, multiple error
>> conditions will need to be distinguished to determine weather a retry
>> is safe.
>>
>> Introduce dma_map_sg_p2pdma[_attrs]() with a different calling
>> convention from dma_map_sg(). The function will return a positive
>> integer on success or a negative errno on failure.
>>
>> ENOMEM will be used to indicate a resource failure and EREMOTEIO to
>> indicate that a P2PDMA page is not mappable.
>>
>> The __DMA_ATTR_PCI_P2PDMA attribute is introduced to inform the lower
>> level implementations that P2PDMA pages are allowed and to warn if a
>> caller introduces them into the regular dma_map_sg() interface.
> 
> So this new API is all about being able to return an error code
> because auditing the old API is basically terrifying?
> 
> OK, but why name everything new P2PDMA? It seems nicer to give this
> some generic name and have some general program to gradually deprecate
> normal non-error-capable dma_map_sg() ?
> 
> I think that will raise less questions when subsystem people see the
> changes, as I was wondering why RW was being moved to use what looked
> like a p2pdma only API.
> 
> dma_map_sg_or_err() would have been clearer
> 
> The flag is also clearer as to the purpose if it is named
> __DMA_ATTR_ERROR_ALLOWED

I'm not opposed to these names. I can use them for v2 if there are no
other opinions.

Logan