From 881dd65a69c841870a941d54f6701837a5837bb6 Mon Sep 17 00:00:00 2001
From: Dom Cobley <popcornmix@gmail.com>
Date: Fri, 2 Oct 2026 11:03:45 +0100
Subject: [PATCH] vclog: Report when gpu_mem is too low for logs

With gpu_mem=16 on Pi 0-4 the cut-down firmware provides no log buffer,
so /proc/device-tree/chosen/log is absent and vclog failed with an
unhelpful "Could not determine logs location" error.

When the logs can't be found, query the VC memory size via the mailbox
and, if it is 16MB or less, report that gpu_mem must be increased.

Also make die() accept printf-style arguments.
---
 vclog/vclog.c | 47 ++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 44 insertions(+), 3 deletions(-)

diff --git a/vclog/vclog.c b/vclog/vclog.c
index 34590fd..c7c996b 100644
--- a/vclog/vclog.c
+++ b/vclog/vclog.c
@@ -11,6 +11,7 @@
 #include <fcntl.h>
 #include <getopt.h>
 #include <netinet/in.h>
+#include <stdarg.h>
 #include <stdbool.h>
 #include <stdint.h>
 #include <stdio.h>
@@ -75,6 +76,9 @@ enum
 #define ARRAY_SIZE(_a) (sizeof(_a)/sizeof(_a[0]))
 
 #define FBIODMACOPY _IOW('z', 0x22, struct fb_dmacopy)
+#define IOCTL_MBOX_PROPERTY _IOWR(100, 0, char *)
+#define MBOX_TAG_GET_VC_MEMORY 0x00010006
+#define MIN_GPU_MEM_FOR_LOGS (16 * 1024 * 1024)
 
 #ifdef DEBUG
 #define DLOG(...) printf(__VA_ARGS__)
@@ -104,12 +108,13 @@ static int dma_fd = -1;
 static char *vc_map = MAP_FAILED;
 
 static bool find_logs(uint32_t *logs_start, uint32_t *logs_size);
+static bool get_gpu_mem_size(uint32_t *size);
 static bool prepare_vc_mapping(uint32_t vc_start, uint32_t vc_size);
 static void destroy_vc_mapping(void);
 static void read_vc_mem(uint32_t vc_addr, uint32_t size, void *dest);
 static uint32_t log_copy_wrap(const char *log_buffer, uint32_t log_size,
                               uint32_t offset, uint32_t len, char *dest);
-static void die(const char *msg);
+static void die(const char *fmt, ...) __attribute__((format(printf, 1, 2), noreturn));
 
 int32_t main(int32_t argc, char *argv[])
 {
@@ -143,7 +148,14 @@ int32_t main(int32_t argc, char *argv[])
     
     // find the address and size of the logs in VC memory
     if (!find_logs(&logs_start_vc, &logs_size))
+    {
+        uint32_t gpu_mem;
+
+        if (get_gpu_mem_size(&gpu_mem) && gpu_mem <= MIN_GPU_MEM_FOR_LOGS)
+            die("Logs are not available with gpu_mem=%u. "
+                "Increase gpu_mem in config.txt", gpu_mem >> 20);
         die("Could not determine logs location from Device Tree");
+    }
 
     if (!prepare_vc_mapping(logs_start_vc, logs_size))
         die("Cannot access logs");
@@ -355,6 +367,29 @@ static bool find_logs(uint32_t *logs_start, uint32_t *logs_size)
     return ret;
 }
 
+static bool get_gpu_mem_size(uint32_t *size)
+{
+    uint32_t buf[8] = {
+        sizeof(buf), 0,
+        MBOX_TAG_GET_VC_MEMORY, 8, 0, 0, 0,
+        0
+    };
+    int fd;
+    int err;
+
+    fd = open("/dev/vcio", 0);
+    if (fd < 0)
+        return false;
+
+    err = ioctl(fd, IOCTL_MBOX_PROPERTY, buf);
+    close(fd);
+    if (err < 0 || buf[1] != 0x80000000)
+        return false;
+
+    *size = buf[6];
+    return true;
+}
+
 static bool prepare_vc_mapping(uint32_t vc_start, uint32_t vc_size)
 {
     const char *dma_filenames[] = { "/dev/vc-mem", "/dev/fb0" };
@@ -498,8 +533,14 @@ static uint32_t log_copy_wrap(const char *log_buffer, uint32_t log_size,
     }
 }
 
-static void die(const char *msg)
+static void die(const char *fmt, ...)
 {
-    fprintf(stderr, "Fatal error: %s\n", msg);
+    va_list ap;
+
+    va_start(ap, fmt);
+    fprintf(stderr, "Fatal error: ");
+    vfprintf(stderr, fmt, ap);
+    fprintf(stderr, "\n");
+    va_end(ap);
     exit(EXIT_FAILURE);
 }