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

[Tomasz Figa]

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