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

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Message-ID: <20230407055704.GD6803@lst.de>
Date:   Fri, 7 Apr 2023 07:57:04 +0200
From:   Christoph Hellwig <hch@....de>
To:     Petr Tesarik <petrtesarik@...weicloud.com>
Cc:     Christoph Hellwig <hch@....de>, Jonathan Corbet <corbet@....net>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....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>, petr@...arici.cz,
        Alexander Graf <graf@...zon.com>
Subject: Re: [RFC v1 3/4] swiotlb: Allow dynamic allocation of bounce
 buffers

On Tue, Mar 28, 2023 at 02:43:03PM +0200, Petr Tesarik wrote:
> Oh, wait! I can do at least something for devices which do not use
> swiotlb at all.
> 
> If a device does not use bounce buffers, it cannot pass an address
> that belongs to the swiotlb. Consequently, the potentially
> expensive check can be skipped. This avoids the dynamic lookup
> penalty for devices which do not need the swiotlb.
> 
> Note that the counter always remains zero if dma_io_tlb_mem is
> NULL, so the NULL check is not required.

Hmm, that's yet another atomic for each map/unmap, and bloats
struct device.

(Btw, in case anyone is interested, we really need to get started
on moving the dma fields out of struct device into a sub-struct
only allocated for DMA capable busses)