linux-kernel - Re: [RFC v1 3/4] swiotlb: Allow dynamic allocation of bounce buffers

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Date:   Mon, 24 Apr 2023 08:03:13 +0200
From:   Christoph Hellwig <hch@....de>
To:     Petr Tesařík <petr@...arici.cz>
Cc:     Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@....de>,
        Petr Tesarik <petrtesarik@...weicloud.com>,
        Jonathan Corbet <corbet@....net>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Borislav Petkov <bp@...e.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Damien Le Moal <damien.lemoal@...nsource.wdc.com>,
        Kim Phillips <kim.phillips@....com>,
        "Steven Rostedt (Google)" <rostedt@...dmis.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:DMA MAPPING HELPERS" <iommu@...ts.linux.dev>,
        Roberto Sassu <roberto.sassu@...wei.com>,
        Alexander Graf <graf@...zon.com>
Subject: Re: [RFC v1 3/4] swiotlb: Allow dynamic allocation of bounce
 buffers

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.