linux-kernel - [PATCH v5 2/6] mmc: sdhci: allow drivers to pre-allocate bounce buffer

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Message-ID: <20260123095342.272505-3-yangzh0906@thundersoft.com>
Date: Fri, 23 Jan 2026 17:53:38 +0800
From: Albert Yang <yangzh0906@...ndersoft.com>
To: Ulf Hansson <ulf.hansson@...aro.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Ge Gordon <gordon.ge@....ai>,
	Arnd Bergmann <arnd@...db.de>
Cc: BST Linux Kernel Upstream Group <bst-upstream@...ai.top>,
	linux-mmc@...r.kernel.org,
	devicetree@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	Albert Yang <yangzh0906@...ndersoft.com>
Subject: [PATCH v5 2/6] mmc: sdhci: allow drivers to pre-allocate bounce buffer

Allow platform drivers to pre-allocate bounce buffer by checking if
host->bounce_buffer is already set before attempting allocation.

This enables platforms with specific DMA constraints (such as 32-bit
DMA on controllers that cannot access high memory) to use their own
reserved memory regions for the bounce buffer.

Suggested-by: Adrian Hunter <adrian.hunter@...el.com>
Signed-off-by: Albert Yang <yangzh0906@...ndersoft.com>
Acked-by: Adrian Hunter <adrian.hunter@...el.com>

---
Changes for v5:
- Split from platform series per Arnd's feedback

Changes for v4:
- New patch suggested by Adrian Hunter
---
 drivers/mmc/host/sdhci.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/mmc/host/sdhci.c b/drivers/mmc/host/sdhci.c
index ac7e11f37af7..dc5960e72a38 100644
--- a/drivers/mmc/host/sdhci.c
+++ b/drivers/mmc/host/sdhci.c
@@ -4193,6 +4193,12 @@ static void sdhci_allocate_bounce_buffer(struct sdhci_host *host)
 	unsigned int bounce_size;
 	int ret;
 
+	/* Drivers may have already allocated the buffer */
+	if (host->bounce_buffer) {
+		bounce_size = host->bounce_buffer_size;
+		max_blocks = bounce_size / 512;
+		goto out;
+	}
 	/*
 	 * Cap the bounce buffer at 64KB. Using a bigger bounce buffer
 	 * has diminishing returns, this is probably because SD/MMC
@@ -4241,6 +4247,7 @@ static void sdhci_allocate_bounce_buffer(struct sdhci_host *host)
 
 	host->bounce_buffer_size = bounce_size;
 
+out:
 	/* Lie about this since we're bouncing */
 	mmc->max_segs = max_blocks;
 	mmc->max_seg_size = bounce_size;
-- 
2.43.0