Virtio console driver

This is an hvc-based virtio console driver.  It's suboptimal becuase
hvc expects to have raw access to interrupts and virtio doesn't assume
that, so it currently polls.

There are two solutions: expose hvc's "kick" interface, or wean off hvc.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

3 files changed, 230 insertions, 0 deletions

diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
index 65491103e0fb..bf18d757b876 100644
--- a/drivers/char/Kconfig
+++ b/drivers/char/Kconfig
@@ -613,6 +613,10 @@ config HVC_XEN
         help
           Xen virtual console device driver
 
+config VIRTIO_CONSOLE
+        bool
+        select HVC_DRIVER
+
 config HVCS
         tristate "IBM Hypervisor Virtual Console Server support"
         depends on PPC_PSERIES
diff --git a/drivers/char/Makefile b/drivers/char/Makefile
index c78ff26647ee..057c8bbd7723 100644
--- a/drivers/char/Makefile
+++ b/drivers/char/Makefile
@@ -50,6 +50,7 @@ obj-$(CONFIG_HVC_RTAS)		+= hvc_rtas.o
 obj-$(CONFIG_HVC_BEAT)          += hvc_beat.o
 obj-$(CONFIG_HVC_DRIVER)        += hvc_console.o
 obj-$(CONFIG_HVC_XEN)           += hvc_xen.o
+obj-$(CONFIG_VIRTIO_CONSOLE)    += virtio_console.o
 obj-$(CONFIG_RAW_DRIVER)        += raw.o
 obj-$(CONFIG_SGI_SNSC)          += snsc.o snsc_event.o
 obj-$(CONFIG_MSPEC)             += mspec.o
diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
new file mode 100644
index 000000000000..100e8a201e3a
--- /dev/null
+++ b/drivers/char/virtio_console.c
@@ -0,0 +1,225 @@
+/*D:300
+ * The Guest console driver
+ *
+ * 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, 2007 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
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ */
+#include <linux/err.h>
+#include <linux/init.h>
+#include <linux/virtio.h>
+#include <linux/virtio_console.h>
+#include "hvc_console.h"
+
+/*D:340 These represent our input and output console queues, and the virtio
+ * operations for them. */
+static struct virtqueue *in_vq, *out_vq;
+static struct virtio_device *vdev;
+
+/* This is our input buffer, and how much data is left in it. */
+static unsigned int in_len;
+static char *in, *inbuf;
+
+/* The operations for our console. */
+static struct hv_ops virtio_cons;
+
+/*D:310 The put_chars() callback is pretty straightforward.
+ *
+ * We turn the characters into a scatter-gather list, add it to the output
+ * queue and then kick the Host.  Then we sit here waiting for it to finish:
+ * inefficient in theory, but in practice implementations will do it
+ * immediately (lguest's Launcher does). */
+static int put_chars(u32 vtermno, const char *buf, int count)
+{
+        struct scatterlist sg[1];
+        unsigned int len;
+
+        /* This is a convenient routine to initialize a single-elem sg list */
+        sg_init_one(sg, buf, count);
+
+        /* add_buf wants a token to identify this buffer: we hand it any
+         * non-NULL pointer, since there's only ever one buffer. */
+        if (out_vq->vq_ops->add_buf(out_vq, sg, 1, 0, (void *)1) == 0) {
+                /* Tell Host to go! */
+                out_vq->vq_ops->kick(out_vq);
+                /* Chill out until it's done with the buffer. */
+                while (!out_vq->vq_ops->get_buf(out_vq, &len))
+                        cpu_relax();
+        }
+
+        /* We're expected to return the amount of data we wrote: all of it. */
+        return count;
+}
+
+/* Create a scatter-gather list representing our input buffer and put it in the
+ * queue. */
+static void add_inbuf(void)
+{
+        struct scatterlist sg[1];
+        sg_init_one(sg, inbuf, PAGE_SIZE);
+
+        /* We should always be able to add one buffer to an empty queue. */
+        if (in_vq->vq_ops->add_buf(in_vq, sg, 0, 1, inbuf) != 0)
+                BUG();
+        in_vq->vq_ops->kick(in_vq);
+}
+
+/*D:350 get_chars() is the callback from the hvc_console infrastructure when
+ * an interrupt is received.
+ *
+ * Most of the code deals with the fact that the hvc_console() infrastructure
+ * only asks us for 16 bytes at a time.  We keep in_offset and in_used fields
+ * for partially-filled buffers. */
+static int get_chars(u32 vtermno, char *buf, int count)
+{
+        /* If we don't have an input queue yet, we can't get input. */
+        BUG_ON(!in_vq);
+
+        /* No buffer?  Try to get one. */
+        if (!in_len) {
+                in = in_vq->vq_ops->get_buf(in_vq, &in_len);
+                if (!in)
+                        return 0;
+        }
+
+        /* You want more than we have to give?  Well, try wanting less! */
+        if (in_len < count)
+                count = in_len;
+
+        /* Copy across to their buffer and increment offset. */
+        memcpy(buf, in, count);
+        in += count;
+        in_len -= count;
+
+        /* Finished?  Re-register buffer so Host will use it again. */
+        if (in_len == 0)
+                add_inbuf();
+
+        return count;
+}
+/*:*/
+
+/*D:320 Console drivers are initialized very early so boot messages can go out,
+ * so we do things slightly differently from the generic virtio initialization
+ * of the net and block drivers.
+ *
+ * At this stage, the console is output-only.  It's too early to set up a
+ * virtqueue, so we let the drivers do some boutique early-output thing. */
+int __init virtio_cons_early_init(int (*put_chars)(u32, const char *, int))
+{
+        virtio_cons.put_chars = put_chars;
+        return hvc_instantiate(0, 0, &virtio_cons);
+}
+
+/*D:370 Once we're further in boot, we get probed like any other virtio device.
+ * At this stage we set up the output virtqueue.
+ *
+ * To set up and manage our virtual console, we call hvc_alloc().  Since we
+ * never remove the console device we never need this pointer again.
+ *
+ * Finally we put our input buffer in the input queue, ready to receive. */
+static int virtcons_probe(struct virtio_device *dev)
+{
+        int err;
+        struct hvc_struct *hvc;
+
+        vdev = dev;
+
+        /* This is the scratch page we use to receive console input */
+        inbuf = kmalloc(PAGE_SIZE, GFP_KERNEL);
+        if (!inbuf) {
+                err = -ENOMEM;
+                goto fail;
+        }
+
+        /* Find the input queue. */
+        /* FIXME: This is why we want to wean off hvc: we do nothing
+         * when input comes in. */
+        in_vq = vdev->config->find_vq(vdev, NULL);
+        if (IS_ERR(in_vq)) {
+                err = PTR_ERR(in_vq);
+                goto free;
+        }
+
+        out_vq = vdev->config->find_vq(vdev, NULL);
+        if (IS_ERR(out_vq)) {
+                err = PTR_ERR(out_vq);
+                goto free_in_vq;
+        }
+
+        /* Start using the new console output. */
+        virtio_cons.get_chars = get_chars;
+        virtio_cons.put_chars = put_chars;
+
+        /* The first argument of hvc_alloc() is the virtual console number, so
+         * we use zero.  The second argument is the interrupt number; we
+         * currently leave this as zero: it would be better not to use the
+         * hvc mechanism and fix this (FIXME!).
+         *
+         * 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 can do any size, so we put PAGE_SIZE here. */
+        hvc = hvc_alloc(0, 0, &virtio_cons, PAGE_SIZE);
+        if (IS_ERR(hvc)) {
+                err = PTR_ERR(hvc);
+                goto free_out_vq;
+        }
+
+        /* Register the input buffer the first time. */
+        add_inbuf();
+        return 0;
+
+free_out_vq:
+        vdev->config->del_vq(out_vq);
+free_in_vq:
+        vdev->config->del_vq(in_vq);
+free:
+        kfree(inbuf);
+fail:
+        return err;
+}
+
+static struct virtio_device_id id_table[] = {
+        { VIRTIO_ID_CONSOLE, VIRTIO_DEV_ANY_ID },
+        { 0 },
+};
+
+static struct virtio_driver virtio_console = {
+        .driver.name =  KBUILD_MODNAME,
+        .driver.owner = THIS_MODULE,
+        .id_table =     id_table,
+        .probe =        virtcons_probe,
+};
+
+static int __init init(void)
+{
+        return register_virtio_driver(&virtio_console);
+}
+module_init(init);
+
+MODULE_DEVICE_TABLE(virtio, id_table);
+MODULE_DESCRIPTION("Virtio console driver");
+MODULE_LICENSE("GPL");