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Fri, 8 Aug 2025 05:35:51 -0400 (EDT) X-Mailer: MessagingEngine.com Webmail Interface Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-ThreadId: Tf7f6c885039edc57 Date: Fri, 08 Aug 2025 11:35:07 +0200 From: "Arnd Bergmann" To: "yangzh0906@thundersoft.com" Cc: "Adrian Hunter" , "gordon.ge" , linux-kernel@vger.kernel.org, "linux-mmc @ vger . kernel . org" , "Ulf Hansson" Message-Id: <8413fb10-3526-42f8-aae9-382931aed7b2@app.fastmail.com> In-Reply-To: <20250808083916.959079-1-yangzh0906@thundersoft.com> References: <05e13ba9-c1fe-450a-a159-9693edce0a23@app.fastmail.com> <20250808083916.959079-1-yangzh0906@thundersoft.com> Subject: Re: [PATCH v2 5/8] mmc: sdhci: add Black Sesame Technologies BST C1200 controller driver Content-Type: text/plain Content-Transfer-Encoding: 7bit On Fri, Aug 8, 2025, at 10:39, Albert Yang wrote: > On Fri, Jul 11, 2025, at 08:55, Arnd Bergmann wrote: > > After investigating the approaches you suggested, I need to clarify the > hardware constraints we face: > > The BST C1200 SoC has fundamental hardware limitations: > - System RAM starts at 0x8.00000000 (above 32-bit addressable range) > - The eMMC controller's DMA engine is limited to 32-bit addressing > - The SMMU does not function for address translation in this path due to > hardware design flaws > - These limitations were finalized during silicon design and cannot be > modified in software > > Regarding your suggestion about programming the bus controller mapping, > we have thoroughly reviewed the BST C1200 datasheet and confirmed: > - No accessible registers exist to reprogram the address mapping > - The 32-bit DMA limitation is a hard constraint of the controller IP > > Given these silicon-level constraints, the current bounce buffer approach > represents the only viable solution to provide eMMC functionality on this > platform. Thanks for checking and confirming. I think what you had in v2 is probably close to what we want then. Please update the comments in the code that explain the workaround and send it as v3 so we can have another look. Arnd