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Date:   Mon, 24 Apr 2023 08:03:13 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Petr =?utf-8?B?VGVzYcWZw61r?= <petr@tesarici.cz>
Cc:     Robin Murphy <robin.murphy@arm.com>,
        Christoph Hellwig <hch@lst.de>,
        Petr Tesarik <petrtesarik@huaweicloud.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Borislav Petkov <bp@suse.de>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Damien Le Moal <damien.lemoal@opensource.wdc.com>,
        Kim Phillips <kim.phillips@amd.com>,
        "Steven Rostedt (Google)" <rostedt@goodmis.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        "open list:DMA MAPPING HELPERS" <iommu@lists.linux.dev>,
        Roberto Sassu <roberto.sassu@huawei.com>,
        Alexander Graf <graf@amazon.com>
Subject: Re: [RFC v1 3/4] swiotlb: Allow dynamic allocation of bounce
 buffers
Message-ID: <20230424060313.GB9805@lst.de>
References: <cover.1679309810.git.petr.tesarik.ext@huawei.com> <0334a54332ab75312c9de825548b616439dcc9f5.1679309810.git.petr.tesarik.ext@huawei.com> <20230328040724.GB25506@lst.de> <4268fa4e-4f0f-a2f6-a2a5-5b78ca4a073d@huaweicloud.com> <8cf7c515-9ce6-a2ed-0643-972aa3eba2fb@huaweicloud.com> <20230407055704.GD6803@lst.de> <20230407121555.4290a011@meshulam.tesarici.cz> <20230421150349.35966e0b@meshulam.tesarici.cz> <4bd8ce51-5874-0aa3-bc82-fec0cee9b8f1@arm.com> <20230421170934.185a03a0@meshulam.tesarici.cz>
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On Fri, Apr 21, 2023 at 05:09:34PM +0200, Petr Tesařík wrote:
> > > A. Only grouping those fields in their own struct?
> > > B. Or move the definition to another include file (cf. MAINTAINERS)?
> > > C. Or store a pointer in struct device?  
> > 
> > dev->dma_parms is already this, and IIRC still has some very old 
> > comments somewhere about consolidating the other DMA-related fields in 
> > there.
> 
> Thank you for the hint! I have actually seen dma_parms, but since it
> can be NULL and was initialized from various drivers, it did not occur
> to me that NULL simply means not DMA-capable.

Yes, dma_parms are still optional.  A much better hint is the dma_mask
itself, which for historic reasons is implemented in a completely
awfull way as a pointer to something stored in the containing struture,
which all kinds of platform devices or minor buses doing nasty hacks
there.