[Skiboot] [RFC PATCH linux] powerpc/powernv: Add support for OPAL_DEBUG_READ/WRITE

[Oliver O'Halloran]

On Fri, Sep 18, 2020 at 2:35 AM Cédric Le Goater <clg at kaod.org> wrote:
>
> Signed-off-by: Cédric Le Goater <clg at kaod.org>
> ---
> diff --git a/arch/powerpc/platforms/powernv/opal-debug.c b/arch/powerpc/platforms/powernv/opal-debug.c
> new file mode 100644
> index 000000000000..59fe9ea310a1
> --- /dev/null
> +++ b/arch/powerpc/platforms/powernv/opal-debug.c
> @@ -0,0 +1,124 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * PowerNV OPAL debugfs interface
> + *
> + * Copyright 2020 IBM Corp.
> + */
> +
> +#define pr_fmt(fmt)     "opal-stat: " fmt
> +
> +#include <asm/io.h>
> +#include <asm/opal.h>
> +#include <asm/debugfs.h>
> +#include <linux/debugfs.h>
> +#include <linux/seq_file.h>
> +#include <linux/of.h>
> +#include <linux/of_address.h>
> +#include <linux/of_platform.h>
> +#include <linux/types.h>
> +#include <linux/slab.h>
> +
> +static int opal_debug_show(struct seq_file *m, void *p)
> +{
> +       u64 id = (u64) m->private;
> +       int size = PAGE_SIZE;

If we're going to do this then we really need to be able to support
arbitrarily large outputs IMO. That would probably require us to
support seeking on the OPAL side since we can't spend forever inside
of OPAL with interrupts off just printing stuff. That said, it'd be
good if we could avoid over complicating all this.

Maybe we can get away with allocating a single large buffer at boot
and having all the opal_debug_read() calls use that under a mutex. It
probably wouldn't help with the interrupt problem, but maybe it
doesn't matter. Printing text is reasonably fast provided you aren't
actually doing IO so it might not matter.

> +       char *buf;
> +       int rc;
> +
> +       buf = kzalloc(size, GFP_KERNEL);
> +       if (!buf)
> +               return -ENOMEM;
> +
> +       rc = opal_debug_read(id, __pa(buf), size);
> +       if (rc < 0) {
> +               rc = opal_error_code(rc);
> +       } else {
> +               seq_puts(m, buf);
> +               rc = 0;
> +       }
> +
> +       kfree(buf);
> +       return rc;
> +}
> +
> +static int opal_debug_open(struct inode *inode, struct file *file)
> +{
> +       return single_open(file, opal_debug_show, inode->i_private);
> +}
> +
> +static ssize_t opal_debug_do_write(struct file *file, const char __user *user_buf,
> +                              size_t count, loff_t *ppos)
> +{
> +       u64 id = (u64) file_inode(file)->i_private;
> +       char buf[8] = { 0 };
> +       char *data;
> +       size_t size;
> +       int rc;
> +
> +       size = min(sizeof(buf) - 1, count);
> +       if (copy_from_user(buf, user_buf, size))
> +               return -EFAULT;
> +       data = strim(buf);
> +
> +       rc = opal_debug_write(id, __pa(data), size);
> +       if (rc < 0)
> +               return opal_error_code(rc);
> +
> +       return count;
> +}
> +
> +static const struct file_operations opal_debug_ops = {
> +       .open           = opal_debug_open,
> +       .write          = opal_debug_do_write,
> +       .read           = seq_read,
> +       .llseek         = no_llseek,
> +       .release        = single_release,
> +};
> +
> +static struct dentry *debug_dir;
> +
> +static int opal_debug_probe(struct platform_device *pdev)
> +{
> +       struct device_node *node = pdev->dev.of_node;
> +       u32 reg;
> +       const char *label;
> +       const char *name;
> +       u32 chip_id;
> +
> +       if (of_property_read_u32(node, "reg", &reg)) {
> +               pr_warn("OPAL: 'reg' property not found\n");
> +               return -ENODEV;
> +       }
> +
> +       if (of_property_read_string(node, "label", &label))
> +               label = node->name;
> +
> +       if (!of_property_read_u32(node, "ibm,chip-id", &chip_id))
> +               name = kasprintf(GFP_KERNEL, "%s-%x", label, chip_id);
> +       else
> +               name = kasprintf(GFP_KERNEL, "%s", label);
> +
> +       if (!debug_dir)
> +               debug_dir = debugfs_create_dir("opal-debug",
> +                                              powerpc_debugfs_root);
> +
> +       debugfs_create_file(name, 0644, debug_dir, (void *) (u64) reg,
> +                           &opal_debug_ops);

I think having all these in one directory is going to get a bit
unwieldy. Each PHB has 13 IODA tables and I'd like to have a seperate
debugfs file for each. Each chip has 6x PHBs so we'd have 13 * 6 * 2
(156) files for PHB debug alone in the same directory. Maybe add an
extra string to the DT which specifies a subdirectory name?

That said, for the PHBs we already have a debugfs directory for each
of them at /sys/kernel/debug/PCI<xxxx>/. It might be nice if we could
insert these opal debug files under the existing PHB debugfs directory
rather than having a seperate set of files elsewhere. If we're going
to do that we might need to stop building platform devices and just
have a helper built into the powernv platform which parses the
ibm,opal-debug node and inserts debugfs files. Having the helper set
OF_POPULATED so it doesn't get re-added might also work.

Oliver