[PATCH v2] tty: pl011: Work around QDF2400 E44 stuck BUSY bit

[Christopher Covington]

The Qualcomm Datacenter Technologies QDF2400 family of SoCs contains a
custom (non-PrimeCell) implementation of the SBSA UART. Occasionally the
BUSY bit in the Flag Register gets stuck as 1, erratum 44 for both 2432v1
and 2400v1 SoCs. Checking that the Transmit FIFO Empty (TXFE) bit is 0,
instead of checking that the BUSY bit is 1, works around the issue. To
facilitate this substitution when UART AMBA Port (UAP) data is available,
introduce vendor-specific inversion of Feature Register bits. For the
earlycon case, implement alternative putc and early_write functions.
Similar to what how ARMv8 ACPI PCI quirks are detected during MCFG parsing,
check the OEM fields of the Serial Port Console Redirection (SPCR) ACPI
table to determine if the current platform is known to be affected by the
erratum.

Signed-off-by: Christopher Covington <cov@xxxxxxxxxxxxxx>
Acked-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxx>
---
Changes in v2:
* Improved comments per Timur
* Removed #ifdef CONFIG_QCOM_QDF2400_ERRATUM_44 per Robin
* Moved from MIDR based affected system detection to ACPI based
    per Mark and Robin
* Combined earlycon and UAP-available workarounds back into one patch
* Keeping Russell's Acked-by because I don't think I've drastically
    changed the PL011 bits, but please let me know if I'm wrong
* Happy Valentine's Day!
---
 Documentation/arm64/silicon-errata.txt |  2 +
 drivers/acpi/spcr.c                    |  8 ++++
 drivers/tty/serial/amba-pl011.c        | 67 ++++++++++++++++++++++++++++++----
 3 files changed, 70 insertions(+), 7 deletions(-)

diff --git a/Documentation/arm64/silicon-errata.txt b/Documentation/arm64/silicon-errata.txt
index a71b8095dbd8..bc3d086bc624 100644
--- a/Documentation/arm64/silicon-errata.txt
+++ b/Documentation/arm64/silicon-errata.txt
@@ -68,3 +68,5 @@ stable kernels.
 |                |                 |                 |                             |
 | Qualcomm Tech. | Falkor v1       | E1003           | QCOM_FALKOR_ERRATUM_1003    |
 | Qualcomm Tech. | Falkor v1       | E1009           | QCOM_FALKOR_ERRATUM_1009    |
+| Qualcomm Tech. | QDF2432v1 UART  | SoC E44         | N/A                         |
+| Qualcomm Tech. | QDF2400v1 UART  | SoC E44         | N/A                         |
diff --git a/drivers/acpi/spcr.c b/drivers/acpi/spcr.c
index b8019c4c1d38..59064f10353f 100644
--- a/drivers/acpi/spcr.c
+++ b/drivers/acpi/spcr.c
@@ -93,6 +93,14 @@ int __init parse_spcr(bool earlycon)
 		goto done;
 	}
 
+	if (!memcmp(table->header.oem_id, "QCOM  ", ACPI_OEM_ID_SIZE))
+		if (!memcmp(table->header.oem_table_id, "QDF2432 ",
+				ACPI_OEM_TABLE_ID_SIZE) ||
+				(!memcmp(table->header.oem_table_id,
+				"QDF2400 ", ACPI_OEM_TABLE_ID_SIZE) &&
+				table->header.oem_revision == 0))
+			uart = "qdf2400_e44";
+
 	snprintf(opts, sizeof(opts), "%s,%s,0x%llx,%d", uart, iotype,
 		 table->serial_port.address, baud_rate);
 
diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index d4171d71a258..4208012d52b2 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -97,6 +97,7 @@ struct vendor_data {
 	unsigned int		fr_dsr;
 	unsigned int		fr_cts;
 	unsigned int		fr_ri;
+	unsigned int		inv_fr;
 	bool			access_32b;
 	bool			oversampling;
 	bool			dma_threshold;
@@ -141,6 +142,30 @@ static struct vendor_data vendor_sbsa = {
 	.fixed_options		= true,
 };
 
+/*
+ * Erratum 44 for QDF2432v1 and QDF2400v1 SoCs describes the BUSY bit as
+ * occasionally getting stuck as 1. To avoid the potential for a hang, check
+ * TXFE == 0 instead of BUSY == 1. This may not be suitable for all UART
+ * implementations, so only do so if an affected platform is detected in
+ * parse_spcr().
+ */
+static bool qdf2400_e44 = false;
+
+static struct vendor_data vendor_qdt_qdf2400_e44 = {
+	.reg_offset		= pl011_std_offsets,
+	.fr_busy		= UART011_FR_TXFE,
+	.fr_dsr			= UART01x_FR_DSR,
+	.fr_cts			= UART01x_FR_CTS,
+	.fr_ri			= UART011_FR_RI,
+	.inv_fr			= UART011_FR_TXFE,
+	.access_32b		= true,
+	.oversampling		= false,
+	.dma_threshold		= false,
+	.cts_event_workaround	= false,
+	.always_enabled		= true,
+	.fixed_options		= true,
+};
+
 static u16 pl011_st_offsets[REG_ARRAY_SIZE] = {
 	[REG_DR] = UART01x_DR,
 	[REG_ST_DMAWM] = ST_UART011_DMAWM,
@@ -1518,7 +1543,8 @@ static unsigned int pl011_tx_empty(struct uart_port *port)
 {
 	struct uart_amba_port *uap =
 	    container_of(port, struct uart_amba_port, port);
-	unsigned int status = pl011_read(uap, REG_FR);
+	/* Allow feature register bits to be inverted to work around errata */
+	unsigned int status = pl011_read(uap, REG_FR) ^ uap->vendor->inv_fr;
 	return status & (uap->vendor->fr_busy | UART01x_FR_TXFF) ?
 							0 : TIOCSER_TEMT;
 }
@@ -2215,10 +2241,12 @@ pl011_console_write(struct console *co, const char *s, unsigned int count)
 	uart_console_write(&uap->port, s, count, pl011_console_putchar);
 
 	/*
-	 *	Finally, wait for transmitter to become empty
-	 *	and restore the TCR
+	 *	Finally, wait for transmitter to become empty and restore the
+	 *	TCR. Allow feature register bits to be inverted to work around
+	 *	errata.
 	 */
-	while (pl011_read(uap, REG_FR) & uap->vendor->fr_busy)
+	while ((pl011_read(uap, REG_FR) ^ uap->vendor->inv_fr)
+						& uap->vendor->fr_busy)
 		cpu_relax();
 	if (!uap->vendor->always_enabled)
 		pl011_write(old_cr, uap, REG_CR);
@@ -2340,8 +2368,13 @@ static int __init pl011_console_match(struct console *co, char *name, int idx,
 	resource_size_t addr;
 	int i;
 
-	if (strcmp(name, "pl011") != 0)
+	if (strcmp(name, "qdf2400_e44") == 0) {
+		if (!qdf2400_e44)
+			pr_info("UART: Working around QDF2400 SoC erratum 44");
+		qdf2400_e44 = true;
+	} else if (strcmp(name, "pl011") != 0) {
 		return -ENODEV;
+	}
 
 	if (uart_parse_earlycon(options, &iotype, &addr, &options))
 		return -ENODEV;
@@ -2383,6 +2416,25 @@ static struct console amba_console = {
 
 #define AMBA_CONSOLE	(&amba_console)
 
+static void qdf2400_e44_putc(struct uart_port *port, int c)
+{
+	while (readl(port->membase + UART01x_FR) & UART01x_FR_TXFF)
+		cpu_relax();
+	if (port->iotype == UPIO_MEM32)
+		writel(c, port->membase + UART01x_DR);
+	else
+		writeb(c, port->membase + UART01x_DR);
+	while (!(readl(port->membase + UART01x_FR) & UART011_FR_TXFE))
+		cpu_relax();
+}
+
+static void qdf2400_e44_early_write(struct console *con, const char *s, unsigned n)
+{
+	struct earlycon_device *dev = con->data;
+
+	uart_console_write(&dev->port, s, n, qdf2400_e44_putc);
+}
+
 static void pl011_putc(struct uart_port *port, int c)
 {
 	while (readl(port->membase + UART01x_FR) & UART01x_FR_TXFF)
@@ -2408,7 +2460,8 @@ static int __init pl011_early_console_setup(struct earlycon_device *device,
 	if (!device->port.membase)
 		return -ENODEV;
 
-	device->con->write = pl011_early_write;
+	device->con->write = qdf2400_e44 ?
+		qdf2400_e44_early_write : pl011_early_write;
 	return 0;
 }
 OF_EARLYCON_DECLARE(pl011, "arm,pl011", pl011_early_console_setup);
@@ -2645,7 +2698,7 @@ static int sbsa_uart_probe(struct platform_device *pdev)
 	uap->port.irq	= ret;
 
 	uap->reg_offset	= vendor_sbsa.reg_offset;
-	uap->vendor	= &vendor_sbsa;
+	uap->vendor	= qdf2400_e44 ? &vendor_qdt_qdf2400_e44 : &vendor_sbsa;
 	uap->fifosize	= 32;
 	uap->port.iotype = vendor_sbsa.access_32b ? UPIO_MEM32 : UPIO_MEM;
 	uap->port.ops	= &sbsa_uart_pops;
-- 
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm
Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora
Forum, a Linux Foundation Collaborative Project.