linux-kernel - RE: [EXT] Re: [PATCH] media: videobuf2-dma-sg: limit the sg segment size

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Message-ID: <DB9PR04MB92846D75C93A9B2B8C8A998D87F1A@DB9PR04MB9284.eurprd04.prod.outlook.com>
Date:   Tue, 12 Sep 2023 07:01:28 +0000
From:   Hui Fang <hui.fang@....com>
To:     Tomasz Figa <tfiga@...omium.org>
CC:     Anle Pan <anle.pan@....com>,
        "m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
        "mchehab@...nel.org" <mchehab@...nel.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jindong Yue <jindong.yue@....com>,
        Xuegang Liu <xuegang.liu@....com>
Subject: RE: [EXT] Re: [PATCH] media: videobuf2-dma-sg: limit the sg segment
 size

On Tue, Sep 12 2023 at 11:22 AM Tomasz Figa <tfiga@...omium.org>
> On Mon, Sep 11, 2023 at 3:13 PM Hui Fang <hui.fang@....com> wrote:
> >
> > On Wed, Sep 6, 2023 at 18:28 PM Tomasz Figa <tfiga@...omium.org>
> wrote:
> > > That all makes sense, but it still doesn't answer the real question
> > > on why swiotlb ends up being used. I think you may want to trace
> > > what happens in the DMA mapping ops implementation on your system
> > > causing it to use swiotlb.
> >
> > Add log and feed invalid data to low buffer on purpose, it's confirmed
> > that swiotlb is actually used.
> >
> 
> Yes, that we already know. But why?


The physical address of v4l2 buffer is large than 4G (5504139264), so the swiotlb is used.
"[  846.570271][  T138] software IO TLB: ==== swiotlb_bounce: DMA_TO_DEVICE,
 dst 000000004589fa38, src 00000000c6d7e8d8, srcPhy 5504139264, size 4096".

BRs,
Fang Hui