spi: atmel-qspi: Memory barriers after memory-mapped I/O

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.

Fixes: d5433de ("mtd: spi-nor: atmel-quadspi: Add spi-mem support to atmel-quadspi")
Cc: Hari.PrasathGE@microchip.com
Cc: Mahesh.Abotula@microchip.com
Cc: Marco.Cardellini@microchip.com
Cc: stable@vger.kernel.org # c0a0203: ("spi: atmel-quadspi: Create `atmel_qspi_ops`"...)
Cc: stable@vger.kernel.org # 6.x.y
Signed-off-by: Bence Csókás <csokas.bence@prolan.hu>
Link: https://patch.msgid.link/20241219091258.395187-1-csokas.bence@prolan.hu
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: Bence Csókás

drivers/spi/atmel-quadspi.c

@@ -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);