linux-kernel - Re: [PATCH v3 5/8] mmc: sdhci: add Black Sesame Technologies BST C1200 controller driver

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Message-Id: <689f58fe-6339-4a3e-9aad-175bff7681f8@app.fastmail.com>
Date: Mon, 18 Aug 2025 14:08:20 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Ulf Hansson" <ulf.hansson@...aro.org>,
 "yangzh0906@...ndersoft.com" <yangzh0906@...ndersoft.com>
Cc: "Krzysztof Kozlowski" <krzk@...nel.org>, "Rob Herring" <robh@...nel.org>,
 krzk+dt@...nel.org, "Conor Dooley" <conor+dt@...nel.org>,
 "Catalin Marinas" <catalin.marinas@....com>, "Will Deacon" <will@...nel.org>,
 "Adrian Hunter" <adrian.hunter@...el.com>,
 "Robin Murphy" <robin.murphy@....com>, ding.wang@....ai,
 "gordon.ge" <gordon.ge@....ai>, bst-upstream <bst-upstream@...ai.top>,
 linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
 "linux-mmc @ vger . kernel . org" <linux-mmc@...r.kernel.org>,
 soc@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 5/8] mmc: sdhci: add Black Sesame Technologies BST C1200
 controller driver

On Mon, Aug 18, 2025, at 12:48, Ulf Hansson wrote:
> On Tue, 12 Aug 2025 at 14:31, Albert Yang <yangzh0906@...ndersoft.com> wrote:

>> +       /*
>> +        * Silicon constraints for BST C1200:
>> +        * - System RAM base is 0x800000000 (above 32-bit addressable range)
>> +        * - The eMMC controller DMA engine is limited to 32-bit addressing
>> +        * - SMMU cannot be used on this path due to hardware design flaws
>> +        * - These are fixed in silicon and cannot be changed in software
>> +        *
>> +        * Bus/controller mapping:
>> +        * - No registers are available to reprogram the address mapping
>> +        * - The 32-bit DMA limit is a hard constraint of the controller IP
>> +        *
>> +        * Given these constraints, an SRAM-based bounce buffer in the 32-bit
>> +        * address space is required to enable eMMC DMA on this platform.
>> +        */
>> +       err = bst_sdhci_reallocate_bounce_buffer(host);
>> +       if (err) {
>> +               dev_err(&pdev->dev, "Failed to allocate bounce buffer: %d\n", err);
>> +               goto err_remove_host;
>> +       }
>
> FYI, I will be awaiting a confirmation from Arnd to be with the above
> hack, before I queue this up.

The explanations here are clear enough to me,

Acked-by: Arnd Bergmann <arnd@...db.de>