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Date:   Thu, 20 Jul 2023 10:02:38 +0200
From:   Petr =?UTF-8?B?VGVzYcWZw61r?= <petr@tesarici.cz>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Petr Tesarik <petrtesarik@huaweicloud.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Russell King <linux@armlinux.org.uk>,
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        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>,
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        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>,
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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: <20230720100238.58f11c06@meshulam.tesarici.cz>
In-Reply-To: <20230720064744.GA4395@lst.de>
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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?

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

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).

Petr T