linux-kernel - Re: [PATCH 2/3] riscv: dts: spacemit: Move UARTs under dma-bus for K1

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Message-ID: <965d5dca-72b0-47da-bd9d-efeca16db826@iscas.ac.cn>
Date: Wed, 25 Jun 2025 11:21:28 +0800
From: Vivian Wang <wangruikang@...as.ac.cn>
To: Guodong Xu <guodong@...cstar.com>
Cc: Yixun Lan <dlan@...too.org>, Ze Huang <huangze@...t.edu.cn>,
 Alex Elder <elder@...cstar.com>, spacemit@...ts.linux.dev,
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>, Paul Walmsley
 <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>,
 Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>,
 Vivian Wang <uwu@...m.page>, devicetree@...r.kernel.org,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] riscv: dts: spacemit: Move UARTs under dma-bus for K1

On 6/24/25 20:21, Guodong Xu wrote:
> On Mon, Jun 23, 2025 at 6:04 PM Vivian Wang <wangruikang@...as.ac.cn> wrote:
>> UART devices in Spacemit K1 use dma-bus DMA translations. Move these
>> nodes under dma-bus to reflect this fact.
...
>>
>> -               sec_uart1: serial@...12000 {
>> -                       compatible = "spacemit,k1-uart", "intel,xscale-uart";
>> -                       reg = <0x0 0xf0612000 0x0 0x100>;
>> -                       interrupts = <43>;
>> -                       clock-frequency = <14857000>;
>> -                       reg-shift = <2>;
>> -                       reg-io-width = <4>;
>> -                       status = "reserved"; /* for TEE usage */
>> -               };
>> -
> Are you sure sec_uart1 should be placed under the dma_bus? Please double
> check. If sec_uart1 and other peripherals supported by the secure DMA
> (base address: 0xF0600000) share the same address mapping as dma_bus, and
> if they can be accessed by the linux kernel in some cases (as saying
> 'reserved'), then that makes sense. If not, better not move them.

Hi Guodong,

I have confirmed with SpacemiT that sec_uart1 uses dram_range4, a.k.a.
dma-bus, which is same as the rest of the UARTs.

Also, sec_uart1 is usable in e.g. Linux, but only if no TEE is in use.
Using status = "reserved" is the safe choice here. Platforms that do not
run a TEE (or anything similar that would disable access to the
0xf000_0000 to 0xffff_ffff range) and wishing to enable use of sec_uart1
can change the status to "okay".

Vivian "dramforever" Wang