text: // SPDX-License-Identifier: GPL-2.0-only
  /*
   * memconsole-coreboot.c
   *
   * Memory based BIOS console accessed through coreboot table.
   *
   * Copyright 2017 Google Inc.
   */
  
  #include <linux/device.h>
  #include <linux/io.h>
  #include <linux/kernel.h>
  #include <linux/module.h>
  
  #include "memconsole.h"
  #include "coreboot_table.h"
  
  #define CB_TAG_CBMEM_CONSOLE	0x17
  
  /* CBMEM firmware console log descriptor. */
  struct cbmem_cons {
  	u32 size_dont_access_after_boot;
  	u32 cursor;
  	u8  body[];
  } __packed;
  
  #define CURSOR_MASK ((1 << 28) - 1)
  #define OVERFLOW (1 << 31)
  
  static struct cbmem_cons *cbmem_console;
  static u32 cbmem_console_size;
  
  /*
   * The cbmem_console structure is read again on every access because it may
   * change at any time if runtime firmware logs new messages. This may rarely
   * lead to race conditions where the firmware overwrites the beginning of the
   * ring buffer with more lines after we have already read |cursor|. It should be
   * rare and harmless enough that we don't spend extra effort working around it.
   */
  static ssize_t memconsole_coreboot_read(char *buf, loff_t pos, size_t count)
  {
  	u32 cursor = cbmem_console->cursor & CURSOR_MASK;
  	u32 flags = cbmem_console->cursor & ~CURSOR_MASK;
  	u32 size = cbmem_console_size;
  	struct seg {	/* describes ring buffer segments in logical order */
  		u32 phys;	/* physical offset from start of mem buffer */
  		u32 len;	/* length of segment */
  	} seg[2] = { {0}, {0} };
  	size_t done = 0;
  	int i;
  
  	if (flags & OVERFLOW) {
  		if (cursor > size)	/* Shouldn't really happen, but... */
  			cursor = 0;
  		seg[0] = (struct seg){.phys = cursor, .len = size - cursor};
  		seg[1] = (struct seg){.phys = 0, .len = cursor};
  	} else {
  		seg[0] = (struct seg){.phys = 0, .len = min(cursor, size)};
  	}
  
  	for (i = 0; i < ARRAY_SIZE(seg) && count > done; i++) {
  		done += memory_read_from_buffer(buf + done, count - done, &pos,
  			cbmem_console->body + seg[i].phys, seg[i].len);
  		pos -= seg[i].len;
  	}
  	return done;
  }
  
  static int memconsole_probe(struct coreboot_device *dev)
  {
  	struct cbmem_cons *tmp_cbmc;
  
  	tmp_cbmc = memremap(dev->cbmem_ref.cbmem_addr,
  			    sizeof(*tmp_cbmc), MEMREMAP_WB);
  
  	if (!tmp_cbmc)
  		return -ENOMEM;
  
  	/* Read size only once to prevent overrun attack through /dev/mem. */
  	cbmem_console_size = tmp_cbmc->size_dont_access_after_boot;
  	cbmem_console = devm_memremap(&dev->dev, dev->cbmem_ref.cbmem_addr,
  				 cbmem_console_size + sizeof(*cbmem_console),
  				 MEMREMAP_WB);
  	memunmap(tmp_cbmc);
  
  	if (IS_ERR(cbmem_console))
  		return PTR_ERR(cbmem_console);
  
  	memconsole_setup(memconsole_coreboot_read);
  
  	return memconsole_sysfs_init();
  }
  
  static int memconsole_remove(struct coreboot_device *dev)
  {
  	memconsole_exit();
  
  	return 0;
  }
  
  static struct coreboot_driver memconsole_driver = {
  	.probe = memconsole_probe,
  	.remove = memconsole_remove,
  	.drv = {
  		.name = "memconsole",
  	},
  	.tag = CB_TAG_CBMEM_CONSOLE,
  };
  module_coreboot_driver(memconsole_driver);
  
  MODULE_AUTHOR("Google, Inc.");
  MODULE_LICENSE("GPL");
  