FAILED: patch "[PATCH] spi: atmel-qspi: Memory barriers after memory-mapped I/O" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x be92ab2de0ee1a13291c3b47b2d7eb24d80c0a2c
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025021003-crewmate-marxism-a03e@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From be92ab2de0ee1a13291c3b47b2d7eb24d80c0a2c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bence=20Cs=C3=B3k=C3=A1s?= <[email protected]>
Date: Thu, 19 Dec 2024 10:12:58 +0100
Subject: [PATCH] spi: atmel-qspi: Memory barriers after memory-mapped I/O
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The QSPI peripheral control and status registers are accessible via the SoC's APB bus, whereas MMIO transactions' data travels on the AHB bus.

Microchip documentation and even sample code from Atmel emphasises the need for a memory barrier before the first MMIO transaction to the AHB-connected QSPI, and before the last write to its registers via APB. This is achieved by the following lines in atmel_qspi_transfer():

	/* Dummy read of QSPI_IFR to synchronize APB and AHB accesses */
	(void)atmel_qspi_read(aq, QSPI_IFR);

However, the current documentation makes no mention to synchronization requirements in the other direction, i.e. after the last data written via AHB, and before the first register access on APB.

In our case, we were facing an issue where the QSPI peripheral would cease to send any new CSR (nCS Rise) interrupts, leading to a timeout in atmel_qspi_wait_for_completion() and ultimately this panic in higher levels:

ubi0 error: ubi_io_write: error -110 while writing 63108 bytes
to PEB 491:128, written 63104 bytes

After months of extensive research of the codebase, fiddling around the debugger with kgdb, and back-and-forth with Microchip, we came to the conclusion that the issue is probably that the peripheral is still busy receiving on AHB when the LASTXFER bit is written to its Control Register on APB, therefore this write gets lost, and the peripheral still thinks there is more data to come in the MMIO transfer. This was first formulated when we noticed that doubling the write() of QSPI_CR_LASTXFER seemed to solve the problem.

Ultimately, the solution is to introduce memory barriers after the AHB-mapped MMIO transfers, to ensure ordering.

index f46da363574f..8fdc9d27a95e 100644
@@ -661,13 +661,20 @@ static int atmel_qspi_transfer(struct spi_mem *mem,
 	(void)atmel_qspi_read(aq, QSPI_IFR);
 
 	/* Send/Receive data */
-	if (op->data.dir == SPI_MEM_DATA_IN)
+	if (op->data.dir == SPI_MEM_DATA_IN) {
 		memcpy_fromio(op->data.buf.in, aq->mem + offset,
 			      op->data.nbytes);
-	else
+
+		/* Synchronize AHB and APB accesses again */
+		rmb();
+	} else {
 		memcpy_toio(aq->mem + offset, op->data.buf.out,
 			    op->data.nbytes);
 
+		/* Synchronize AHB and APB accesses again */
+		wmb();
+	}
+
 	/* Release the chip-select */
 	atmel_qspi_write(QSPI_CR_LASTXFER, aq, QSPI_CR);