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Date:   Thu, 20 Jul 2023 10:22:42 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Petr =?utf-8?B?VGVzYcWZw61r?= <petr@tesarici.cz>
Cc:     Christoph Hellwig <hch@lst.de>,
        Petr Tesarik <petrtesarik@huaweicloud.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Russell King <linux@armlinux.org.uk>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        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>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Juergen Gross <jgross@suse.com>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Petr Tesarik <petr.tesarik.ext@huawei.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Hans de Goede <hdegoede@redhat.com>,
        James Seo <james@equiv.tech>,
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        Kees Cook <keescook@chromium.org>,
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        Roberto Sassu <roberto.sassu@huaweicloud.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>
Subject: Re: [PATCH v4 8/8] swiotlb: search the software IO TLB only if a
 device makes use of it
Message-ID: <20230720082242.GA6932@lst.de>
References: <cover.1689261692.git.petr.tesarik.ext@huawei.com> <a8d31d3fffa0867dce2b44b98dc2714289edfdc9.1689261692.git.petr.tesarik.ext@huawei.com> <20230720064744.GA4395@lst.de> <20230720100238.58f11c06@meshulam.tesarici.cz>
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On Thu, Jul 20, 2023 at 10:02:38AM +0200, Petr Tesařík wrote:
> On Thu, 20 Jul 2023 08:47:44 +0200
> Christoph Hellwig <hch@lst.de> wrote:
> 
> > Any reason this can't just do a list_empty_careful on the list
> > instead of adding yet another field that grows struct device?
> 
> On which list?

dev->dma_io_tlb_mem->pools?

> 
> The dma_io_tlb_pools list only contains transient pools, but a device
> may use bounce buffers from a regular pool.

Oh, true.

> The dma_io_tlb_mem.pools list will always be non-empty, unless the
> system runs without SWIOTLB.
> 
> On a system which does have a SWIOTLB, the flag allows to differentiate
> between devices that actually use bounce buffers and devices that do
> not (e.g. because they do not have any addressing limitations).

Ok.