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

From: Tomasz Figa <tfiga@chromium.org>
To: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Ezequiel Garcia <ezequiel@collabora.com>,
	Linux Media Mailing List <linux-media@vger.kernel.org>,
	Hans Verkuil <hans.verkuil@cisco.com>,
	kernel@collabora.com,
	Nicolas Dufresne <nicolas.dufresne@collabora.com>,
	"open list:ARM/Rockchip SoC..."
	<linux-rockchip@lists.infradead.org>,
	Heiko Stuebner <heiko@sntech.de>, Jonas Karlman <jonas@kwiboo.se>,
	Boris Brezillon <boris.brezillon@collabora.com>,
	Paul Kocialkowski <paul.kocialkowski@bootlin.com>,
	Alexandre Courbot <acourbot@chromium.org>,
	fbuergisser@chromium.org,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	stable@vger.kernel.org
Subject: Re: [PATCH v2 1/7] media: hantro: Set DMA max segment size
Date: Fri, 26 Jul 2019 00:41:48 +0900	[thread overview]
Message-ID: <CAAFQd5BxQJBNqMnS1bCVXz-9+dCkw0g4xmiPLYgtVCJx_pbRPg@mail.gmail.com> (raw)
In-Reply-To: <1564069001.3006.1.camel@pengutronix.de>

On Fri, Jul 26, 2019 at 12:36 AM Philipp Zabel <p.zabel@pengutronix.de> wrote:
>
> On Thu, 2019-07-25 at 11:17 -0300, Ezequiel Garcia wrote:
> > From: Francois Buergisser <fbuergisser@chromium.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