[RFC PATCH 7/9] cxl/mem: Implement polled mode mailbox

[Ben Widawsky <ben.widawsky@intel.com>]

Create a function to handle sending a command, optionally with a
payload, to the memory device, polling on a result, and then optionally
copying out the payload. The algorithm for doing this come straight out
of the CXL 2.0 specification.

Primary mailboxes are capable of generating an interrupt when submitting
a command in the background. That implementation is saved for a later
time.

Secondary mailboxes aren't implemented at this time.

WARNING: This is untested with actual timeouts occurring.

Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
---
 drivers/cxl/cxl.h |  16 +++++++
 drivers/cxl/mem.c | 107 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 123 insertions(+)

diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
index 482fc9cdc890..f49ab80f68bd 100644
--- a/drivers/cxl/cxl.h
+++ b/drivers/cxl/cxl.h
@@ -21,8 +21,12 @@
 #define CXLDEV_MB_CTRL 0x04
 #define   CXLDEV_MB_CTRL_DOORBELL BIT(0)
 #define CXLDEV_MB_CMD 0x08
+#define   CXLDEV_MB_CMD_PAYLOAD_LENGTH_SHIFT 16
 #define CXLDEV_MB_STATUS 0x10
+#define   CXLDEV_MB_STATUS_RET_CODE_SHIFT 32
+#define   CXLDEV_MB_STATUS_RET_CODE_MASK 0xffff
 #define CXLDEV_MB_BG_CMD_STATUS 0x18
+#define CXLDEV_MB_PAYLOAD 0x20
 
 /* Memory Device */
 #define CXLMDEV_STATUS 0
@@ -114,4 +118,16 @@ static inline u64 __cxl_raw_read_reg64(struct cxl_mem *cxlm, u32 reg)
 
 	return readq(reg_addr + reg);
 }
+
+static inline void cxl_mbox_payload_fill(struct cxl_mem *cxlm, u8 *input,
+					    unsigned int length)
+{
+	memcpy_toio(cxlm->mbox.regs + CXLDEV_MB_PAYLOAD, input, length);
+}
+
+static inline void cxl_mbox_payload_drain(struct cxl_mem *cxlm,
+					     u8 *output, unsigned int length)
+{
+	memcpy_fromio(output, cxlm->mbox.regs + CXLDEV_MB_PAYLOAD, length);
+}
 #endif /* __CXL_H__ */
diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
index 9fd2d1daa534..08913360d500 100644
--- a/drivers/cxl/mem.c
+++ b/drivers/cxl/mem.c
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0-only
 // Copyright(c) 2020 Intel Corporation. All rights reserved.
+#include <linux/sched/clock.h>
 #include <linux/module.h>
 #include <linux/pci.h>
 #include <linux/io.h>
@@ -7,6 +8,112 @@
 #include "pci.h"
 #include "cxl.h"
 
+struct mbox_cmd {
+	u16 cmd;
+	u8 *payload;
+	size_t payload_size;
+	u16 return_code;
+};
+
+static int cxldev_wait_for_doorbell(struct cxl_mem *cxlm)
+{
+	u64 start, now;
+	int cpu, ret, timeout = 2000000000;
+
+	start = local_clock();
+	preempt_disable();
+	cpu = smp_processor_id();
+	for (;;) {
+		now = local_clock();
+		preempt_enable();
+		if ((cxl_read_mbox_reg32(cxlm, CXLDEV_MB_CTRL) &
+		     CXLDEV_MB_CTRL_DOORBELL) == 0) {
+			ret = 0;
+			break;
+		}
+
+		if (now - start >= timeout) {
+			ret = -ETIMEDOUT;
+			break;
+		}
+
+		cpu_relax();
+		preempt_disable();
+		if (unlikely(cpu != smp_processor_id())) {
+			timeout -= (now - start);
+			cpu = smp_processor_id();
+			start = local_clock();
+		}
+	}
+
+	return ret;
+}
+
+/*
+ * Returns 0 if the doorbell transaction was successful from a protocol level.
+ * Caller should check the return code in @mbox_cmd to make sure it succeeded.
+ */
+static int __maybe_unused cxl_mem_mbox_send_cmd(struct cxl_mem *cxlm, struct mbox_cmd *mbox_cmd)
+{
+	u64 cmd, status;
+	int rc;
+
+	lockdep_assert_held(&cxlm->mbox_lock);
+
+	/*
+	 * Here are the steps from 8.2.8.4 of the CXL 2.0 spec.
+	 *   1. Caller reads MB Control Register to verify doorbell is clear
+	 *   2. Caller writes Command Register
+	 *   3. Caller writes Command Payload Registers if input payload is non-empty
+	 *   4. Caller writes MB Control Register to set doorbell
+	 *   5. Caller either polls for doorbell to be clear or waits for interrupt if configured
+	 *   6. Caller reads MB Status Register to fetch Return code
+	 *   7. If command successful, Caller reads Command Register to get Payload Length
+	 *   8. If output payload is non-empty, host reads Command Payload Registers
+	 */
+
+	cmd = mbox_cmd->cmd;
+	if (mbox_cmd->payload_size) {
+		/* #3 */
+		cmd |= mbox_cmd->payload_size
+		       << CXLDEV_MB_CMD_PAYLOAD_LENGTH_SHIFT;
+		cxl_mbox_payload_fill(cxlm, mbox_cmd->payload, mbox_cmd->payload_size);
+	}
+
+	/* #2 */
+	cxl_write_mbox_reg64(cxlm, CXLDEV_MB_CMD, cmd);
+
+	/* #4 */
+	cxl_write_mbox_reg32(cxlm, CXLDEV_MB_CTRL, CXLDEV_MB_CTRL_DOORBELL);
+
+	/* #5 */
+	rc = cxldev_wait_for_doorbell(cxlm);
+	if (rc == -ETIMEDOUT) {
+		dev_warn(&cxlm->pdev->dev, "Mailbox command timed out\n");
+		return rc;
+	}
+
+	/* #6 */
+	status = cxl_read_mbox_reg64(cxlm, CXLDEV_MB_STATUS);
+	cmd = cxl_read_mbox_reg64(cxlm, CXLDEV_MB_CMD);
+
+	mbox_cmd->return_code = (status >> CXLDEV_MB_STATUS_RET_CODE_SHIFT) &
+				CXLDEV_MB_STATUS_RET_CODE_MASK;
+
+	/* There was a problem, let the caller deal with it */
+	if (mbox_cmd->return_code != 0)
+		return 0;
+
+	/* #7 */
+	mbox_cmd->payload_size = cmd >> CXLDEV_MB_CMD_PAYLOAD_LENGTH_SHIFT;
+
+	/* #8 */
+	if (mbox_cmd->payload_size)
+		cxl_mbox_payload_drain(cxlm, mbox_cmd->payload, mbox_cmd->payload_size);
+
+	return 0;
+}
+
 static int cxl_mem_mbox_get(struct cxl_mem *cxlm)
 {
 	u64 md_status;
-- 
2.29.2