linux-kernel - Re: [PATCH v1 08/17] dma-mapping: add a dma_need

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Message-ID: <20241104091538.GB25041@lst.de>
Date: Mon, 4 Nov 2024 10:15:38 +0100
From: Christoph Hellwig <hch@....de>
To: Robin Murphy <robin.murphy@....com>
Cc: Leon Romanovsky <leon@...nel.org>, Jens Axboe <axboe@...nel.dk>,
	Jason Gunthorpe <jgg@...pe.ca>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Christoph Hellwig <hch@....de>,
	Sagi Grimberg <sagi@...mberg.me>, Keith Busch <kbusch@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Logan Gunthorpe <logang@...tatee.com>,
	Yishai Hadas <yishaih@...dia.com>,
	Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
	Kevin Tian <kevin.tian@...el.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Jérôme Glisse <jglisse@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
	linux-rdma@...r.kernel.org, iommu@...ts.linux.dev,
	linux-nvme@...ts.infradead.org, linux-pci@...r.kernel.org,
	kvm@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v1 08/17] dma-mapping: add a dma_need_unmap helper

On Thu, Oct 31, 2024 at 09:18:11PM +0000, Robin Murphy wrote:
>>   +/**
>> + * dma_need_unmap - does this device need dma_unmap_* operations
>> + * @dev: device to check
>> + *
>> + * If this function returns %false, drivers can skip calling dma_unmap_* after
>> + * finishing an I/O.  This function must be called after all mappings that might
>> + * need to be unmapped have been performed.
>
> In terms of the unmap call itself, why don't we just use dma_skip_sync to 
> short-cut dma_direct_unmap_*() and make sure it's as cheap as possible?
>
> In terms of not having to unmap implying not having to store addresses at 
> all, it doesn't seem super-useful when you still have to store them for 
> long enough to find out that you don't :/

I don't fully understand the comment, mostly because the way I read the
two sentences appear to contradict each other.

Bypassing dma_direct_unmap_ is not the important part, because it already
is pretty cheap.  Storing the addresses is not.

That being said now that we never check need_unmap in the iova path
it might make sense to not have a separate helper, but it needs to
be exposed and documented.