[PATCH 1/7] lmb: default LMB_LIMIT_DMA_BELOW_RAM_TOP for 64-bit Mediatek SoCs

[Sam Edwards]

On Fri, Jun 12, 2026 at 3:13 PM David Lechner <dlechner at baylibre.com> wrote:
>
> On 6/12/26 4:30 PM, David Lechner wrote:
> > On 6/12/26 12:52 PM, Sam Edwards wrote:
> >> On Fri, Jun 12, 2026 at 10:32 AM David Lechner <dlechner at baylibre.com> wrote:
> >>>
> >>> Add 64-bit Mediatek ARM SoCs to the default y setting of
> >>> LMB_LIMIT_DMA_BELOW_RAM_TOP. These platforms have a 4GiB DMA limit, e.g.
> >>> for the SD/MMC controller.
> >>
> >> Hi David,
> >>
> >> Is the 4GiB DMA limit due to the interconnect itself or are there
> >> merely a few blocks that can't speak 64-bit? If the latter, I think
> >> it'd be clearer to associate the setting with the culprit driver(s)
> >> rather than the whole platform, so that it can be removed if/when
> >> those drivers begin to use bounce buffers.
> >>
> > For MediaTek, I think it is mostly specific IP blocks (e.g. MMC in the
> > mt8xxx targets in and NETSYS in mt79xx targets covered by this series are
> > the ones I am aware of).
> >
> > Weijie Gao, maybe you have more information about this?
> >
> >
>
> Sam,
>
> Just to make sure I understood you correctly, your suggestion is to
> do this instead?

Yep -- to me, that says "this driver currently has broken 64-bit DMA"
rather than "this platform can't ever tolerate 64-bit allocations."

> ---
> diff --git a/drivers/mmc/Kconfig b/drivers/mmc/Kconfig
> index 24bd16ad5f3..e2c5a54f925 100644
> --- a/drivers/mmc/Kconfig
> +++ b/drivers/mmc/Kconfig
> @@ -858,6 +858,7 @@ config MMC_MTK
>         bool "MediaTek SD/MMC Card Interface support"
>         depends on ARCH_MEDIATEK || ARCH_MTMIPS || ARCH_AIROHA
>         depends on OF_CONTROL
> +       select LMB_LIMIT_DMA_BELOW_RAM_TOP
>         help
>           This selects the MediaTek(R) Secure digital and Multimedia card Interface.
>           If you have a machine with a integrated SD/MMC card reader, say Y here.
> diff --git a/drivers/net/mtk_eth/Kconfig b/drivers/net/mtk_eth/Kconfig
> index 5d4e54ab90e..7d58f542bde 100644
> --- a/drivers/net/mtk_eth/Kconfig
> +++ b/drivers/net/mtk_eth/Kconfig
> @@ -5,6 +5,7 @@ config MEDIATEK_ETH
>         select PHYLIB
>         select DM_GPIO
>         select DM_RESET
> +       select LMB_LIMIT_DMA_BELOW_RAM_TOP
>         help
>           This Driver support MediaTek Ethernet GMAC
>           Say Y to enable support for the MediaTek Ethernet GMAC.
> diff --git a/lib/Kconfig b/lib/Kconfig
> index 27ad43125e4..cf13ac1bdad 100644
> --- a/lib/Kconfig
> +++ b/lib/Kconfig
> @@ -1298,7 +1298,7 @@ config SPL_LMB_ARCH_MEM_MAP
>  config LMB_LIMIT_DMA_BELOW_RAM_TOP
>         bool
>         depends on LMB
> -       default y if ARCH_BCM283X || (ARCH_MEDIATEK && ARM64)
> +       default y if ARCH_BCM283X
>         help
>           Some architectures can not DMA above ram_top boundary,
>           which is after 4 GiB or 32-bit boundary too. Limit the
>