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Date:   Thu, 31 Aug 2023 14:51:09 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Jonathan Corbet <corbet@lwn.net>
Cc:     Petr Tesarik <petrtesarik@huaweicloud.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Russell King <linux@armlinux.org.uk>,
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        Dave Hansen <dave.hansen@linux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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        Christoph Hellwig <hch@lst.de>,
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        Robin Murphy <robin.murphy@arm.com>,
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        open list <linux-kernel@vger.kernel.org>,
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        Roberto Sassu <roberto.sassu@huaweicloud.com>, petr@tesarici.cz
Subject: Re: [PATCH v7 9/9] swiotlb: search the software IO TLB only if the
 device makes use of it
Message-ID: <20230831125109.GA11562@lst.de>
References: <cover.1690871004.git.petr.tesarik.ext@huawei.com> <adea71bd1fa8660d4c3157a562431ad8127016d4.1690871004.git.petr.tesarik.ext@huawei.com> <87a5uz3ob8.fsf@meer.lwn.net>
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On Wed, Aug 09, 2023 at 03:20:43PM -0600, Jonathan Corbet wrote:
> >  	spin_unlock_irqrestore(&dev->dma_io_tlb_lock, flags);
> >  
> > -	/* Pairs with smp_rmb() in swiotlb_find_pool(). */
> > -	smp_wmb();
> >  found:
> > +	dev->dma_uses_io_tlb = true;
> > +	/* Pairs with smp_rmb() in is_swiotlb_buffer() */
> > +	smp_wmb();
> > +
> 
> ...and here you set it if swiotlb is used.
> 
> But, as far as I can tell, you don't actually *use* this field anywhere.
> What am I missing?

It's very much unused.  Petr, I guess you wanted to use this in
is_swiotlb_buffer to avoid the lookup unless required.  Can you send
a follow up?