linux-kernel - Re: [PATCH v2 1/7] media: hantro: Set DMA max segment size

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Date:   Fri, 26 Jul 2019 00:41:48 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Philipp Zabel <p.zabel@...gutronix.de>
Cc:     Ezequiel Garcia <ezequiel@...labora.com>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Hans Verkuil <hans.verkuil@...co.com>, kernel@...labora.com,
        Nicolas Dufresne <nicolas.dufresne@...labora.com>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        Heiko Stuebner <heiko@...ech.de>,
        Jonas Karlman <jonas@...boo.se>,
        Boris Brezillon <boris.brezillon@...labora.com>,
        Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        Alexandre Courbot <acourbot@...omium.org>,
        fbuergisser@...omium.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH v2 1/7] media: hantro: Set DMA max segment size

On Fri, Jul 26, 2019 at 12:36 AM Philipp Zabel <p.zabel@...gutronix.de> wrote:
>
> On Thu, 2019-07-25 at 11:17 -0300, Ezequiel Garcia wrote:
> > From: Francois Buergisser <fbuergisser@...omium.org>
> >
> > The Hantro codec is typically used in platforms with an IOMMU,
> > so we need to set a proper DMA segment size.
>
> ... to make sure the DMA-mapping subsystem produces contiguous mappings?
>
> > Devices without an
> > IOMMU will still fallback to default 64KiB segments.
>
> I don't understand this comment. The default max_seg_size may be 64 KiB,
> but if we are always setting it to DMA_BUT_MASK(32), there is no falling
> back.
>

DMA mask and segment size are two completely orthogonal parameters.
Please check https://elixir.bootlin.com/linux/v5.3-rc1/source/drivers/iommu/dma-iommu.c#L740
for an example of how the latter is used.

Best regards,
Tomasz