[PATCH v1 3/7] mci: sdhci: honor BROKEN_ADMA_ZEROLEN_DESC / NO_ENDATTR_IN_NOPDESC quirks

[Johannes Schneider <johannes.schneider@leica-geosystems.com>]

The ADMA2 table builder always emitted a 64 KiB chunk as a length-0 descriptor
(the 16-bit length field encodes 65536 as 0) and terminated the table with a
separate NOP-END descriptor. Some controllers mishandle these forms; Linux
gates the workarounds behind SDHCI_QUIRK_BROKEN_ADMA_ZEROLEN_DESC (cap the
length below 64 KiB so it is never 0) and SDHCI_QUIRK_NO_ENDATTR_IN_NOPDESC
(mark END on the last transfer descriptor instead of a trailing NOP-END).

Add both quirks to the shared SDHCI layer and honor them in the table builder.
Controllers that set neither keep the previous behaviour exactly, so this is a
no-op for everyone until a host driver opts in.

Assisted-by: Claude Opus 4.8 (1M context)
Assisted-by: GitHub Copilot CLI (gpt-5.5)
Signed-off-by: Johannes Schneider <johannes.schneider@leica-geosystems.com>
---
 drivers/mci/sdhci.c | 35 ++++++++++++++++++++++++-----------
 drivers/mci/sdhci.h |  4 ++++
 2 files changed, 28 insertions(+), 11 deletions(-)

diff --git a/drivers/mci/sdhci.c b/drivers/mci/sdhci.c
index ccd5d4b83c..8ed8a59342 100644
--- a/drivers/mci/sdhci.c
+++ b/drivers/mci/sdhci.c
@@ -646,31 +646,44 @@ static int sdhci_adma_write_desc(struct sdhci *host, void **desc,
 static int sdhci_adma_build_table(struct sdhci *host, dma_addr_t addr,
 				  unsigned int len)
 {
+	unsigned int max_len = SDHCI_ADMA2_MAX_LEN;
 	void *desc = host->adma_table;
 	int ret;
 
+	/*
+	 * Some controllers (e.g. the i.MX uSDHC) cannot handle a length-0
+	 * descriptor, which the 16-bit length field uses to encode the full
+	 * SDHCI_ADMA2_MAX_LEN. Cap the chunk just below that so the length is
+	 * never 0.
+	 */
+	if (host->quirks & SDHCI_QUIRK_BROKEN_ADMA_ZEROLEN_DESC)
+		max_len = SDHCI_ADMA2_MAX_LEN - 4;
+
 	while (len) {
-		unsigned int chunk = min_t(unsigned int, len,
-					   SDHCI_ADMA2_MAX_LEN);
+		unsigned int chunk = min_t(unsigned int, len, max_len);
+		unsigned int attr = ADMA2_TRAN_VALID;
 
+		len -= chunk;
 		/*
-		 * The length field is 16-bit; a length of 0 encodes
-		 * SDHCI_ADMA2_MAX_LEN bytes per the SD Host Controller
-		 * specification.
+		 * Controllers that ignore the END attribute in a trailing NOP
+		 * descriptor want END on the last transfer descriptor instead
+		 * (SDHCI_QUIRK_NO_ENDATTR_IN_NOPDESC).
 		 */
-		ret = sdhci_adma_write_desc(host, &desc, addr, chunk & 0xffff,
-					    ADMA2_TRAN_VALID);
+		if (!len && (host->quirks & SDHCI_QUIRK_NO_ENDATTR_IN_NOPDESC))
+			attr |= ADMA2_END;
+
+		ret = sdhci_adma_write_desc(host, &desc, addr, chunk & 0xffff, attr);
 		if (ret)
 			return ret;
 
 		addr += chunk;
-		len -= chunk;
 	}
 
-	/* Append a terminating descriptor (nop, end, valid). */
-	ret = sdhci_adma_write_desc(host, &desc, 0, 0, ADMA2_NOP_END_VALID);
+	if (host->quirks & SDHCI_QUIRK_NO_ENDATTR_IN_NOPDESC)
+		return 0;
 
-	return ret;
+	/* Append a terminating descriptor (nop, end, valid). */
+	return sdhci_adma_write_desc(host, &desc, 0, 0, ADMA2_NOP_END_VALID);
 }
 
 void sdhci_setup_data_dma(struct sdhci *sdhci, struct mci_data *data,
diff --git a/drivers/mci/sdhci.h b/drivers/mci/sdhci.h
index 558469e8ab..cb3b6d2b17 100644
--- a/drivers/mci/sdhci.h
+++ b/drivers/mci/sdhci.h
@@ -315,6 +315,10 @@ struct sdhci {
 	bool v4_mode;		/* Host Version 4 Enable */
 
 	unsigned int quirks;
+/* Controller ignores the END attribute in a NOP ADMA2 descriptor */
+#define SDHCI_QUIRK_NO_ENDATTR_IN_NOPDESC	BIT(6)
+/* Controller cannot handle a length-0 (== 64 KiB) ADMA2 descriptor */
+#define SDHCI_QUIRK_BROKEN_ADMA_ZEROLEN_DESC	BIT(11)
 #define SDHCI_QUIRK_MISSING_CAPS		BIT(27)
 	unsigned int quirks2;
 /* The system physically doesn't support 1.8v, even if the host does */
-- 
2.43.0