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

[Marek Vasut]

On 6/12/26 7:52 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?

It is a mix of both, depending on the SoC. Old 32bit IPs are reused in 
64bit SoCs, sometimes old 32bit interconnect is reused in 64bit SoCs.

> 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.
> 
> Marek: I fear that a lot of things are going to depend on this config
> option being 'true' in the very near future, and with a lot of
> still-unidentified drivers with bugs unveiled by this LMB change (e.g.
> I spent much of last weekend chasing down a failure on RK3588 because
> U-Boot's SDHCI-SDMA impl. assumes 32-bit addresses) would it be better
> to make this setting default true for 2026.07 to buy us more time to
> find those cases?

We could do either that, or add more SoCs to this list.

What do others think ?

> I'm not very familiar with the rationale behind the
> original change ("lmb: Reinstate access to memory above ram_top") but
> it sounds like only some users need that access currently?

With LMB_LIMIT_DMA_BELOW_RAM_TOP disabled, TFTP above 4 GiB is not 
allowed, LMB will block that. If you have a lot of DRAM above 4 GiB and 
you want to TFTP large payloads there, it won't work, which is 
unfortunate (perfectly valid DRAM is wasted).

-- 
Best regards,
Marek Vasut