Re: [PATCH 2/8] cxl/mem: Introduce 'struct cxl_regs'

["Williams, Dan J" <dan.j.williams@intel.com>]

On Thu, 2021-03-25 at 08:15 +0000, Christoph Hellwig wrote:
> On Wed, Mar 24, 2021 at 02:30:35PM -0700, Dan Williams wrote:
> > In preparation for common register mapping facility, introduce a
> > generic
> > container, 'struct cxl_regs', for CXL device register and later
> > component register block base pointers. Some CXL device types
> > implement
> > both.
> 
> The code looks like complete gibberish to me with a struct of a union of
> a struct declaring members in a macro.  This needs a much more detailed
> explanation and rationale.

Ok, let me know if this makes the rationale clearer and lets this move
ahead. Ben has some follow-on patches that double-down on this
organization, so we want to get this question answered first.

Ben, I kept your reviewed-by, but if the new changelog does not look
quite right feel free to holler.

---

From a5618df78d28a9be6a75d4c1b1cd4e50f6e43297 Mon Sep 17 00:00:00 2001
From: Dan Williams <dan.j.williams@intel.com>
Date: Tue, 9 Feb 2021 00:05:22 -0800
Subject: [PATCH] cxl/mem: Introduce 'struct cxl_regs' for "composable" CXL
 devices

CXL MMIO register blocks are organized by device type and capabilities.
There are Component registers, Device registers (yes, an ambiguous
name), and Memory Device registers (a specific extension of Device
registers).

It is possible for a given device instance (endpoint or port) to
implement register sets from multiple of the above categories.

The driver code that enumerates and maps the registers is type specific
so it is useful to have a dedicated type and helpers for each block
type.

At the same time, once the registers are mapped the origin type does not
matter. It is overly pedantic to reference the register block type in
code that is using the registers.

In preparation for the endpoint driver to incorporate Component registers
into its MMIO operations reorganize the registers to allow typed
enumeration + mapping, but anonymous usage. With the end state of
'struct cxl_regs' to be:

struct cxl_regs {
	union {
		struct {
			CXL_DEVICE_REGS();
		};
		struct cxl_device_regs device_regs;
	};
	union {
		struct {
			CXL_COMPONENT_REGS();
		};
		struct cxl_component_regs component_regs;
	};
};

With this arrangement the driver can share component init code with
ports, but when using the registers it can directly reference the
component register block type by name without the 'component_regs'
prefix.

So, map + enumerate can be shared across drivers of different CXL
classes e.g.:

void cxl_setup_device_regs(struct device *dev, void __iomem *base,
			   struct cxl_device_regs *regs);

void cxl_setup_component_regs(struct device *dev, void __iomem *base,
			      struct cxl_component_regs *regs);

...while inline usage in the driver need not indicate where the
registers came from:

readl(cxlm->regs.mbox + MBOX_OFFSET);
readl(cxlm->regs.hdm + HDM_OFFSET);

...instead of:

readl(cxlm->regs.device_regs.mbox + MBOX_OFFSET);
readl(cxlm->regs.component_regs.hdm + HDM_OFFSET);

This complexity of the definition in .h yields improvement in code
readability in .c while maintaining type-safety for organization of
setup code. It prepares the implementation to maintain organization in
the face of CXL devices that compose register interfaces consisting of
multiple types.

Reviewed-by: Ben Widawsky <ben.widawsky@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
 drivers/cxl/cxl.h | 33 +++++++++++++++++++++++++++++++++
 drivers/cxl/mem.c | 44 ++++++++++++++++++++++++--------------------
 drivers/cxl/mem.h | 13 +++++--------
 3 files changed, 62 insertions(+), 28 deletions(-)

diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
index 2e3bdacb32e7..37325e504fb7 100644
--- a/drivers/cxl/cxl.h
+++ b/drivers/cxl/cxl.h
@@ -34,5 +34,38 @@
 #define CXLDEV_MBOX_BG_CMD_STATUS_OFFSET 0x18
 #define CXLDEV_MBOX_PAYLOAD_OFFSET 0x20
 
+/* See note for 'struct cxl_regs' for the rationale of this organization */
+#define CXL_DEVICE_REGS() \
+	void __iomem *status; \
+	void __iomem *mbox; \
+	void __iomem *memdev
+
+/**
+ * struct cxl_device_regs - Common container of CXL Device register
+ * 			    block base pointers
+ * @status: CXL 2.0 8.2.8.3 Device Status Registers
+ * @mbox: CXL 2.0 8.2.8.4 Mailbox Registers
+ * @memdev: CXL 2.0 8.2.8.5 Memory Device Registers
+ */
+struct cxl_device_regs {
+	CXL_DEVICE_REGS();
+};
+
+/*
+ * Note, the anonymous union organization allows for per
+ * register-block-type helper routines, without requiring block-type
+ * agnostic code to include the prefix. I.e.
+ * cxl_setup_device_regs(&cxlm->regs.dev) vs readl(cxlm->regs.mbox).
+ * The specificity reads naturally from left-to-right.
+ */
+struct cxl_regs {
+	union {
+		struct {
+			CXL_DEVICE_REGS();
+		};
+		struct cxl_device_regs device_regs;
+	};
+};
+
 extern struct bus_type cxl_bus_type;
 #endif /* __CXL_H__ */
diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
index 45871ef65152..6951243d128e 100644
--- a/drivers/cxl/mem.c
+++ b/drivers/cxl/mem.c
@@ -31,7 +31,7 @@
  */
 
 #define cxl_doorbell_busy(cxlm)                                                \
-	(readl((cxlm)->mbox_regs + CXLDEV_MBOX_CTRL_OFFSET) &                  \
+	(readl((cxlm)->regs.mbox + CXLDEV_MBOX_CTRL_OFFSET) &                  \
 	 CXLDEV_MBOX_CTRL_DOORBELL)
 
 /* CXL 2.0 - 8.2.8.4 */
@@ -271,7 +271,7 @@ static void cxl_mem_mbox_timeout(struct cxl_mem *cxlm,
 static int __cxl_mem_mbox_send_cmd(struct cxl_mem *cxlm,
 				   struct mbox_cmd *mbox_cmd)
 {
-	void __iomem *payload = cxlm->mbox_regs + CXLDEV_MBOX_PAYLOAD_OFFSET;
+	void __iomem *payload = cxlm->regs.mbox + CXLDEV_MBOX_PAYLOAD_OFFSET;
 	u64 cmd_reg, status_reg;
 	size_t out_len;
 	int rc;
@@ -314,12 +314,12 @@ static int __cxl_mem_mbox_send_cmd(struct cxl_mem *cxlm,
 	}
 
 	/* #2, #3 */
-	writeq(cmd_reg, cxlm->mbox_regs + CXLDEV_MBOX_CMD_OFFSET);
+	writeq(cmd_reg, cxlm->regs.mbox + CXLDEV_MBOX_CMD_OFFSET);
 
 	/* #4 */
 	dev_dbg(&cxlm->pdev->dev, "Sending command\n");
 	writel(CXLDEV_MBOX_CTRL_DOORBELL,
-	       cxlm->mbox_regs + CXLDEV_MBOX_CTRL_OFFSET);
+	       cxlm->regs.mbox + CXLDEV_MBOX_CTRL_OFFSET);
 
 	/* #5 */
 	rc = cxl_mem_wait_for_doorbell(cxlm);
@@ -329,7 +329,7 @@ static int __cxl_mem_mbox_send_cmd(struct cxl_mem *cxlm,
 	}
 
 	/* #6 */
-	status_reg = readq(cxlm->mbox_regs + CXLDEV_MBOX_STATUS_OFFSET);
+	status_reg = readq(cxlm->regs.mbox + CXLDEV_MBOX_STATUS_OFFSET);
 	mbox_cmd->return_code =
 		FIELD_GET(CXLDEV_MBOX_STATUS_RET_CODE_MASK, status_reg);
 
@@ -339,7 +339,7 @@ static int __cxl_mem_mbox_send_cmd(struct cxl_mem *cxlm,
 	}
 
 	/* #7 */
-	cmd_reg = readq(cxlm->mbox_regs + CXLDEV_MBOX_CMD_OFFSET);
+	cmd_reg = readq(cxlm->regs.mbox + CXLDEV_MBOX_CMD_OFFSET);
 	out_len = FIELD_GET(CXLDEV_MBOX_CMD_PAYLOAD_LENGTH_MASK, cmd_reg);
 
 	/* #8 */
@@ -400,7 +400,7 @@ static int cxl_mem_mbox_get(struct cxl_mem *cxlm)
 		goto out;
 	}
 
-	md_status = readq(cxlm->memdev_regs + CXLMDEV_STATUS_OFFSET);
+	md_status = readq(cxlm->regs.memdev + CXLMDEV_STATUS_OFFSET);
 	if (!(md_status & CXLMDEV_MBOX_IF_READY && CXLMDEV_READY(md_status))) {
 		dev_err(dev, "mbox: reported doorbell ready, but not mbox ready\n");
 		rc = -EBUSY;
@@ -868,7 +868,7 @@ static int cxl_mem_setup_regs(struct cxl_mem *cxlm)
 	int cap, cap_count;
 	u64 cap_array;
 
-	cap_array = readq(cxlm->regs + CXLDEV_CAP_ARRAY_OFFSET);
+	cap_array = readq(cxlm->base + CXLDEV_CAP_ARRAY_OFFSET);
 	if (FIELD_GET(CXLDEV_CAP_ARRAY_ID_MASK, cap_array) !=
 	    CXLDEV_CAP_ARRAY_CAP_ID)
 		return -ENODEV;
@@ -881,25 +881,25 @@ static int cxl_mem_setup_regs(struct cxl_mem *cxlm)
 		u16 cap_id;
 
 		cap_id = FIELD_GET(CXLDEV_CAP_HDR_CAP_ID_MASK,
-				   readl(cxlm->regs + cap * 0x10));
-		offset = readl(cxlm->regs + cap * 0x10 + 0x4);
-		register_block = cxlm->regs + offset;
+				   readl(cxlm->base + cap * 0x10));
+		offset = readl(cxlm->base + cap * 0x10 + 0x4);
+		register_block = cxlm->base + offset;
 
 		switch (cap_id) {
 		case CXLDEV_CAP_CAP_ID_DEVICE_STATUS:
 			dev_dbg(dev, "found Status capability (0x%x)\n", offset);
-			cxlm->status_regs = register_block;
+			cxlm->regs.status = register_block;
 			break;
 		case CXLDEV_CAP_CAP_ID_PRIMARY_MAILBOX:
 			dev_dbg(dev, "found Mailbox capability (0x%x)\n", offset);
-			cxlm->mbox_regs = register_block;
+			cxlm->regs.mbox = register_block;
 			break;
 		case CXLDEV_CAP_CAP_ID_SECONDARY_MAILBOX:
 			dev_dbg(dev, "found Secondary Mailbox capability (0x%x)\n", offset);
 			break;
 		case CXLDEV_CAP_CAP_ID_MEMDEV:
 			dev_dbg(dev, "found Memory Device capability (0x%x)\n", offset);
-			cxlm->memdev_regs = register_block;
+			cxlm->regs.memdev = register_block;
 			break;
 		default:
 			dev_dbg(dev, "Unknown cap ID: %d (0x%x)\n", cap_id, offset);
@@ -907,11 +907,11 @@ static int cxl_mem_setup_regs(struct cxl_mem *cxlm)
 		}
 	}
 
-	if (!cxlm->status_regs || !cxlm->mbox_regs || !cxlm->memdev_regs) {
+	if (!cxlm->regs.status || !cxlm->regs.mbox || !cxlm->regs.memdev) {
 		dev_err(dev, "registers not found: %s%s%s\n",
-			!cxlm->status_regs ? "status " : "",
-			!cxlm->mbox_regs ? "mbox " : "",
-			!cxlm->memdev_regs ? "memdev" : "");
+			!cxlm->regs.status ? "status " : "",
+			!cxlm->regs.mbox ? "mbox " : "",
+			!cxlm->regs.memdev ? "memdev" : "");
 		return -ENXIO;
 	}
 
@@ -920,7 +920,7 @@ static int cxl_mem_setup_regs(struct cxl_mem *cxlm)
 
 static int cxl_mem_setup_mailbox(struct cxl_mem *cxlm)
 {
-	const int cap = readl(cxlm->mbox_regs + CXLDEV_MBOX_CAPS_OFFSET);
+	const int cap = readl(cxlm->regs.mbox + CXLDEV_MBOX_CAPS_OFFSET);
 
 	cxlm->payload_size =
 		1 << FIELD_GET(CXLDEV_MBOX_CAP_PAYLOAD_SIZE_MASK, cap);
@@ -980,7 +980,7 @@ static struct cxl_mem *cxl_mem_create(struct pci_dev *pdev, u32 reg_lo,
 
 	mutex_init(&cxlm->mbox_mutex);
 	cxlm->pdev = pdev;
-	cxlm->regs = regs + offset;
+	cxlm->base = regs + offset;
 	cxlm->enabled_cmds =
 		devm_kmalloc_array(dev, BITS_TO_LONGS(cxl_cmd_count),
 				   sizeof(unsigned long),
@@ -1495,6 +1495,10 @@ static __init int cxl_mem_init(void)
 	dev_t devt;
 	int rc;
 
+	/* Double check the anonymous union trickery in struct cxl_regs */
+	BUILD_BUG_ON(offsetof(struct cxl_regs, memdev) !=
+		     offsetof(struct cxl_regs, device_regs.memdev));
+
 	rc = alloc_chrdev_region(&devt, 0, CXL_MEM_MAX_DEVS, "cxl");
 	if (rc)
 		return rc;
diff --git a/drivers/cxl/mem.h b/drivers/cxl/mem.h
index daa9aba0e218..c247cf9c71af 100644
--- a/drivers/cxl/mem.h
+++ b/drivers/cxl/mem.h
@@ -53,10 +53,9 @@ struct cxl_memdev {
 /**
  * struct cxl_mem - A CXL memory device
  * @pdev: The PCI device associated with this CXL device.
- * @regs: IO mappings to the device's MMIO
- * @status_regs: CXL 2.0 8.2.8.3 Device Status Registers
- * @mbox_regs: CXL 2.0 8.2.8.4 Mailbox Registers
- * @memdev_regs: CXL 2.0 8.2.8.5 Memory Device Registers
+ * @base: IO mappings to the device's MMIO
+ * @cxlmd: Logical memory device chardev / interface
+ * @regs: Parsed register blocks
  * @payload_size: Size of space for payload
  *                (CXL 2.0 8.2.8.4.3 Mailbox Capabilities Register)
  * @mbox_mutex: Mutex to synchronize mailbox access.
@@ -67,12 +66,10 @@ struct cxl_memdev {
  */
 struct cxl_mem {
 	struct pci_dev *pdev;
-	void __iomem *regs;
+	void __iomem *base;
 	struct cxl_memdev *cxlmd;
 
-	void __iomem *status_regs;
-	void __iomem *mbox_regs;
-	void __iomem *memdev_regs;
+	struct cxl_regs regs;
 
 	size_t payload_size;
 	struct mutex mbox_mutex; /* Protects device mailbox and firmware */
-- 
2.26.2