blob: 3d6bd0baa56dec3e45e84638719c8842986a935c [file] [log] [blame]
* The Guest console driver
* 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.
/*M:002 The console can be flooded: while the Guest is processing input the
* Host can send more. Buffering in the Host could alleviate this, but it is a
* difficult problem in general. :*/
/* Copyright (C) 2006 Rusty Russell, IBM Corporation
* (at your option) any later version.
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* GNU General Public License for more details.
#include <linux/err.h>
#include <linux/init.h>
#include <linux/lguest_bus.h>
#include <asm/paravirt.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: 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(, "lguest") != 0)
return 0;
return hvc_instantiate(0, 0, &lguest_cons);
/*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,
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,
.probe = lguestcons_probe,
/* The standard init function */
static int __init hvc_lguest_init(void)
return register_lguest_driver(&lguestcons_drv);