linux-kernel - Re: [PATCH v5 02/10] iommu/vt-d: Use per-device dma

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Date:   Wed, 20 Nov 2019 11:44:37 +0100
From:   Christoph Hellwig <hch@....de>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Christoph Hellwig <hch@....de>,
        David Woodhouse <dwmw2@...radead.org>,
        Joerg Roedel <joro@...tes.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>, ashok.raj@...el.com,
        jacob.jun.pan@...el.com, alan.cox@...el.com, kevin.tian@...el.com,
        mika.westerberg@...ux.intel.com, Ingo Molnar <mingo@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        pengfei.xu@...el.com,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Jonathan Corbet <corbet@....net>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>
Subject: Re: [PATCH v5 02/10] iommu/vt-d: Use per-device dma_ops

On Fri, Nov 15, 2019 at 08:57:32AM +0800, Lu Baolu wrote:
> Hi,
>
> On 11/14/19 4:14 PM, Christoph Hellwig wrote:
>> On Thu, Nov 14, 2019 at 01:14:11PM +0800, Lu Baolu wrote:
>>> Could you please educate me what dma_supported() is exactly for? Will
>>> it always get called during boot? When will it be called?
>>
>> ->dma_supported is set when setting either the dma_mask or
>> dma_coherent_mask. These days it serves too primary purposes: reject
>> too small masks that can't be addressed, and provide any hooks needed
>> in the driver based on the mask.
>
> Thanks! So ->dma_supported might not be called before driver maps buffer
> and start DMA. Right?

It is supposed to, yes.