From 19ddadf3ff61059a42eb7885e2be6dce434ba875 Mon Sep 17 00:00:00 2001
From: Jonathan Bennett <jbennett@incomsystems.biz>
Date: Mon, 20 Jul 2026 02:58:58 -0500
Subject: [PATCH] Check for the SPI bufsize, and throw a useful error when set
 too small (#11072)

---
 src/platform/portduino/PortduinoGlue.cpp | 43 ++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/src/platform/portduino/PortduinoGlue.cpp b/src/platform/portduino/PortduinoGlue.cpp
index c8e27eb8d..750396769 100644
--- a/src/platform/portduino/PortduinoGlue.cpp
+++ b/src/platform/portduino/PortduinoGlue.cpp
@@ -120,6 +120,44 @@ static error_t parse_opt(int key, char *arg, struct argp_state *state)
     return 0;
 }
 
+// A kernel SPI transfer is capped by the spidev module's `bufsiz` parameter (4096 by default).
+// LovyanGFX pushes the framebuffer in large chunks, so a display bigger than that budget fails
+// deep inside the driver with a bare -EMSGSIZE. Check up front so the user gets told what to fix.
+static void checkSpidevBufsiz()
+{
+    if (portduino_config.display_spi_dev == "" || portduino_config.displayWidth == 0 || portduino_config.displayHeight == 0) {
+        return;
+    }
+    switch (portduino_config.displayPanel) {
+    case no_screen:
+    case x11:
+    case fb:
+    case hub75:
+        return; // not driven over spidev
+    default:
+        break;
+    }
+
+    const long required = (long)portduino_config.displayWidth * portduino_config.displayHeight / 2 * 3;
+
+    std::ifstream bufsizFile("/sys/module/spidev/parameters/bufsiz");
+    long bufsiz = 0;
+    if (!bufsizFile.is_open() || !(bufsizFile >> bufsiz)) {
+        // spidev may be built into the kernel without exposing the parameter; nothing to check.
+        return;
+    }
+
+    if (bufsiz < required) {
+        std::cerr << "SPI display " << portduino_config.displayWidth << "x" << portduino_config.displayHeight
+                  << " needs a spidev buffer of at least " << required << " bytes, but "
+                  << "/sys/module/spidev/parameters/bufsiz is " << bufsiz << "." << std::endl;
+        std::cerr << "Add 'spidev.bufsiz=" << required << "' to your kernel command line "
+                  << "(/boot/firmware/cmdline.txt on Raspberry Pi OS) and reboot." << std::endl;
+        std::cerr << "Or echo that value into /etc/modprobe.d/spidev.conf and reload the spidev module" << std::endl;
+        exit(EXIT_FAILURE);
+    }
+}
+
 void portduinoCustomInit()
 {
     static struct argp_option options[] = {{"port", 'p', "PORT", 0, "The TCP port to use."},
@@ -559,6 +597,11 @@ void portduinoSetup()
         }
     }
 
+    // if we have s SPI display, check /sys/module/spidev/parameters/bufsiz
+    // It needs to be at least width * height / 2 * 3
+    // fail with a more useful error message.
+    checkSpidevBufsiz();
+
     // if we're using a usermode driver, we need to initialize it here, to get a serial number back for mac address
     uint8_t dmac[6] = {0};
     if (portduino_config.lora_spi_dev == "ch341") {