firmware: google: memconsole: Adapt to new coreboot ring buffer format

The upstream coreboot implementation of memconsole was enhanced from a
single-boot console to a persistent ring buffer
(https://review.coreboot.org/#/c/18301). This patch changes the kernel
memconsole driver to be able to read the new format in all cases.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/firmware/google/memconsole-coreboot.c b/drivers/firmware/google/memconsole-coreboot.c
index d48a80c3042dbdfb9c1a773e9228fdb67633d0cf..7d39f4ef5d9e47c72645a29d09d9a490e92ff3de 100644 (file)
--- a/drivers/firmware/google/memconsole-coreboot.c
+++ b/drivers/firmware/google/memconsole-coreboot.c
@@ -26,19 +26,50 @@
 
 /* CBMEM firmware console log descriptor. */
 struct cbmem_cons {
-       u32 buffer_size;
-       u32 buffer_cursor;
-       u8  buffer_body[0];
+       u32 size;
+       u32 cursor;
+       u8  body[0];
 } __packed;
 
+#define CURSOR_MASK ((1 << 28) - 1)
+#define OVERFLOW (1 << 31)
+
 static struct cbmem_cons __iomem *cbmem_console;
 
+/*
+ * 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)
 {
-       return memory_read_from_buffer(buf, count, &pos,
-                                      cbmem_console->buffer_body,
-                                      min(cbmem_console->buffer_cursor,
-                                          cbmem_console->buffer_size));
+       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_coreboot_init(phys_addr_t physaddr)
@@ -51,7 +82,7 @@ static int memconsole_coreboot_init(phys_addr_t physaddr)
                return -ENOMEM;
 
        cbmem_console = memremap(physaddr,
-                                tmp_cbmc->buffer_size + sizeof(*cbmem_console),
+                                tmp_cbmc->size + sizeof(*cbmem_console),
                                 MEMREMAP_WB);
        memunmap(tmp_cbmc);