lguest: documentation III: Drivers

Documentation: The Drivers

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 74 insertions, 3 deletions

diff --git a/drivers/char/hvc_lguest.c b/drivers/char/hvc_lguest.c
index e7b889e404a..1de8967cce0 100644
--- a/drivers/char/hvc_lguest.c
+++ b/drivers/char/hvc_lguest.c
@@ -1,6 +1,19 @@
-/* Simple console for lguest.
+/*D:300
+ * The Guest console driver
  *
- * Copyright (C) 2006 Rusty Russell, IBM Corporation
+ * This is a trivial console driver: we use lguest's DMA mechanism to send
+ * bytes out, and register a DMA buffer to receive bytes in.  It is assumed to
+ * be present and available from the very beginning of boot.
+ *
+ * Writing console drivers is one of the few remaining Dark Arts in Linux.
+ * Fortunately for us, the path of virtual consoles has been well-trodden by
+ * the PowerPC folks, who wrote "hvc_console.c" to generically support any
+ * virtual console.  We use that infrastructure which only requires us to write
+ * the basic put_chars and get_chars functions and call the right register
+ * functions.
+ :*/
+
+/* Copyright (C) 2006 Rusty Russell, IBM Corporation
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -21,49 +34,81 @@
 #include <linux/lguest_bus.h>
 #include "hvc_console.h"
 
+/*D:340 This is our single console input buffer, with associated "struct
+ * lguest_dma" referring to it.  Note the 0-terminated length array, and the
+ * use of physical address for the buffer itself. */
 static char inbuf[256];
 static struct lguest_dma cons_input = { .used_len = 0,
 					.addr[0] = __pa(inbuf),
 					.len[0] = sizeof(inbuf),
 					.len[1] = 0 };
 
+/*D:310 The put_chars() callback is pretty straightforward.
+ *
+ * First we put the pointer and length in a "struct lguest_dma": we only have
+ * one pointer, so we set the second length to 0.  Then we use SEND_DMA to send
+ * the data to (Host) buffers attached to the console key.  Usually a device's
+ * key is a physical address within the device's memory, but because the
+ * console device doesn't have any associated physical memory, we use the
+ * LGUEST_CONSOLE_DMA_KEY constant (aka 0). */
 static int put_chars(u32 vtermno, const char *buf, int count)
 {
 	struct lguest_dma dma;
 
-	/* FIXME: what if it's over a page boundary? */
+	/* FIXME: DMA buffers in a "struct lguest_dma" are not allowed
+	 * to go over page boundaries.  This never seems to happen,
+	 * but if it did we'd need to fix this code. */
 	dma.len[0] = count;
 	dma.len[1] = 0;
 	dma.addr[0] = __pa(buf);
 
 	lguest_send_dma(LGUEST_CONSOLE_DMA_KEY, &dma);
+	/* We're expected to return the amount of data we wrote: all of it. */
 	return count;
 }
 
+/*D:350 get_chars() is the callback from the hvc_console infrastructure when
+ * an interrupt is received.
+ *
+ * Firstly we see if our buffer has been filled: if not, we return.  The rest
+ * of the code deals with the fact that the hvc_console() infrastructure only
+ * asks us for 16 bytes at a time.  We keep a "cons_offset" variable for
+ * partially-read buffers. */
 static int get_chars(u32 vtermno, char *buf, int count)
 {
 	static int cons_offset;
 
+	/* Nothing left to see here... */
 	if (!cons_input.used_len)
 		return 0;
 
+	/* You want more than we have to give?  Well, try wanting less! */
 	if (cons_input.used_len - cons_offset < count)
 		count = cons_input.used_len - cons_offset;
 
+	/* Copy across to their buffer and increment offset. */
 	memcpy(buf, inbuf + cons_offset, count);
 	cons_offset += count;
+
+	/* Finished?  Zero offset, and reset cons_input so Host will use it
+	 * again. */
 	if (cons_offset == cons_input.used_len) {
 		cons_offset = 0;
 		cons_input.used_len = 0;
 	}
 	return count;
 }
+/*:*/
 
 static struct hv_ops lguest_cons = {
 	.get_chars = get_chars,
 	.put_chars = put_chars,
 };
 
+/*D:320 Console drivers are initialized very early so boot messages can go
+ * out.  At this stage, the console is output-only.  Our driver checks we're a
+ * Guest, and if so hands hvc_instantiate() the console number (0), priority
+ * (0), and the struct hv_ops containing the put_chars() function. */
 static int __init cons_init(void)
 {
 	if (strcmp(paravirt_ops.name, "lguest") != 0)
@@ -73,21 +118,46 @@ static int __init cons_init(void)
 }
 console_initcall(cons_init);
 
+/*D:370 To set up and manage our virtual console, we call hvc_alloc() and
+ * stash the result in the private pointer of the "struct lguest_device".
+ * Since we never remove the console device we never need this pointer again,
+ * but using ->private is considered good form, and you never know who's going
+ * to copy your driver.
+ *
+ * Once the console is set up, we bind our input buffer ready for input. */
 static int lguestcons_probe(struct lguest_device *lgdev)
 {
 	int err;
 
+	/* The first argument of hvc_alloc() is the virtual console number, so
+	 * we use zero.  The second argument is the interrupt number.
+	 *
+	 * The third argument is a "struct hv_ops" containing the put_chars()
+	 * and get_chars() pointers.  The final argument is the output buffer
+	 * size: we use 256 and expect the Host to have room for us to send
+	 * that much. */
 	lgdev->private = hvc_alloc(0, lgdev_irq(lgdev), &lguest_cons, 256);
 	if (IS_ERR(lgdev->private))
 		return PTR_ERR(lgdev->private);
 
+	/* We bind a single DMA buffer at key LGUEST_CONSOLE_DMA_KEY.
+	 * "cons_input" is that statically-initialized global DMA buffer we saw
+	 * above, and we also give the interrupt we want. */
 	err = lguest_bind_dma(LGUEST_CONSOLE_DMA_KEY, &cons_input, 1,
 			      lgdev_irq(lgdev));
 	if (err)
 		printk("lguest console: failed to bind buffer.\n");
 	return err;
 }
+/* Note the use of lgdev_irq() for the interrupt number.  We tell hvc_alloc()
+ * to expect input when this interrupt is triggered, and then tell
+ * lguest_bind_dma() that is the interrupt to send us when input comes in. */
 
+/*D:360 From now on the console driver follows standard Guest driver form:
+ * register_lguest_driver() registers the device type and probe function, and
+ * the probe function sets up the device.
+ *
+ * The standard "struct lguest_driver": */
 static struct lguest_driver lguestcons_drv = {
 	.name = "lguestcons",
 	.owner = THIS_MODULE,
@@ -95,6 +165,7 @@ static struct lguest_driver lguestcons_drv = {
 	.probe = lguestcons_probe,
 };
 
+/* The standard init function */
 static int __init hvc_lguest_init(void)
 {
 	return register_lguest_driver(&lguestcons_drv);