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

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Message-ID: <CAAFQd5Ar18TvFE3jXBuphpCiwWAb8O99wqkiwEVDSs3+MXvW0w@mail.gmail.com>
Date:   Tue, 12 Sep 2023 16:10:59 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Hui Fang <hui.fang@....com>
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 4:01 PM Hui Fang <hui.fang@....com> wrote:
>
> 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".

Is your DMA device restricted only to the bottom-most 4 GB (32-bit DMA
address)? If yes, would it make sense to also allocate from that area
rather than bouncing the memory?

Best regards,
Tomasz