device_select: add shortcut for MESA_VK_DEVICE_SELECT_FORCE_DEFAULT_DEVICE

MESA_VK_DEVICE_SELECT_FORCE_DEFAULT_DEVICE=1 is useful but it's tedious
to type. This commit extends slightly DRI_PRIME/MESA_VK_DEVICE_SELECT
syntax support: if "!" is appended at the end then only the selected GPU
will be made visible to the application.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24750>

docs/envvars.rst

@@ -395,7 +395,10 @@ Core Mesa environment variables
 
    - ``DRI_PRIME=N``: selects the Nth non-default GPU (N > 0).
    - ``DRI_PRIME=pci-0000_02_00_0``: selects the GPU connected to this PCIe bus
-   - ``DRI_PRIME=vendor_id:device_id``: selects the first GPU matching these ids
+   - ``DRI_PRIME=vendor_id:device_id``: selects the first GPU matching these ids.
+
+   For Vulkan it's possible to append ``!``, in which case only the selected GPU
+   will be exposed to the application (eg: DRI_PRIME=1!).
 
    .. note::
 
@@ -801,6 +804,8 @@ Vulkan mesa device select layer environment variables
    when set to "vid:did" number from PCI device. That PCI device is
    selected as default. The default device is returned as the first
    device in vkEnumeratePhysicalDevices API.
+   Using "vid:did!" will have the same effect as using the
+   ``MESA_VK_DEVICE_SELECT_FORCE_DEFAULT_DEVICE`` variable.
 
 .. envvar:: MESA_VK_DEVICE_SELECT_FORCE_DEFAULT_DEVICE
 

src/vulkan/device-select-layer/device_select_layer.c

@@ -284,6 +284,15 @@ static int device_select_find_dri_prime_tag_default(struct device_pci_info *pci_
                                                     const char *dri_prime)
 {
    int default_idx = -1;
+
+   /* Drop the trailing '!' if present. */
+   int ref = strlen("pci-xxxx_yy_zz_w");
+   int n = strlen(dri_prime);
+   if (n < ref)
+      return default_idx;
+   if (n == ref + 1 && dri_prime[n - 1] == '!')
+      n--;
+
    for (unsigned i = 0; i < device_count; ++i) {
       char *tag = NULL;
       if (asprintf(&tag, "pci-%04x_%02x_%02x_%1u",
@@ -291,7 +300,7 @@ static int device_select_find_dri_prime_tag_default(struct device_pci_info *pci_
                    pci_infos[i].bus_info.bus,
                    pci_infos[i].bus_info.dev,
                    pci_infos[i].bus_info.func) >= 0) {
-         if (strcmp(dri_prime, tag) == 0)
+         if (strncmp(dri_prime, tag, n) == 0)
             default_idx = i;
       }
       free(tag);
@@ -419,10 +428,16 @@ static bool should_debug_device_selection() {
       debug_get_bool_option("DRI_PRIME_DEBUG", false);
 }
 
+static bool ends_with_exclamation_mark(const char *str) {
+   size_t n = strlen(str);
+   return n > 1 && str[n - 1] == '!';
+}
+
 static uint32_t get_default_device(const struct instance_info *info,
                                    const char *selection,
                                    uint32_t physical_device_count,
-                                   VkPhysicalDevice *pPhysicalDevices)
+                                   VkPhysicalDevice *pPhysicalDevices,
+                                   bool *expose_only_one_dev)
 {
    int default_idx = -1;
    const char *dri_prime = getenv("DRI_PRIME");
@@ -447,12 +462,18 @@ static uint32_t get_default_device(const struct instance_info *info,
 
    if (selection)
       default_idx = device_select_find_explicit_default(pci_infos, physical_device_count, selection);
+   if (default_idx != -1) {
+      *expose_only_one_dev = ends_with_exclamation_mark(selection);
+   }
 
    if (default_idx == -1 && dri_prime && dri_prime_as_int == 0) {
       /* Try DRI_PRIME=vendor_id:device_id */
       default_idx = device_select_find_explicit_default(pci_infos, physical_device_count, dri_prime);
-      if (debug && default_idx != -1)
+      if (default_idx != -1) {
-         fprintf(stderr, "device-select: device_select_find_explicit_default selected %i\n", default_idx);
+         if (debug)
+            fprintf(stderr, "device-select: device_select_find_explicit_default selected %i\n", default_idx);
+         *expose_only_one_dev = ends_with_exclamation_mark(dri_prime);
+      }
 
       if (default_idx == -1) {
          /* Try DRI_PRIME=pci-xxxx_yy_zz_w */
@@ -461,8 +482,11 @@ static uint32_t get_default_device(const struct instance_info *info,
          else
             default_idx = device_select_find_dri_prime_tag_default(pci_infos, physical_device_count, dri_prime);
 
-         if (debug && default_idx != -1)
+         if (default_idx != -1) {
-            fprintf(stderr, "device-select: device_select_find_dri_prime_tag_default selected %i\n", default_idx);
+            if (debug)
+               fprintf(stderr, "device-select: device_select_find_dri_prime_tag_default selected %i\n", default_idx);
+            *expose_only_one_dev = ends_with_exclamation_mark(dri_prime);
+         }
       }
    }
    if (default_idx == -1 && info->has_wayland) {
@@ -506,8 +530,11 @@ static uint32_t get_default_device(const struct instance_info *info,
    if (dri_prime_as_int > 0 && physical_device_count > (cpu_count + 1)) {
       if (default_idx == 0 || default_idx == 1) {
          default_idx = find_non_cpu_skip(pci_infos, physical_device_count, default_idx, dri_prime_as_int);
-         if (debug && default_idx != -1)
+         if (default_idx != -1) {
-            fprintf(stderr, "device-select: find_non_cpu_skip selected %i\n", default_idx);
+            if (debug)
+               fprintf(stderr, "device-select: find_non_cpu_skip selected %i\n", default_idx);
+            *expose_only_one_dev = ends_with_exclamation_mark(dri_prime);
+         }
       }
    }
    free(pci_infos);
@@ -522,6 +549,7 @@ static VkResult device_select_EnumeratePhysicalDevices(VkInstance instance,
    uint32_t physical_device_count = 0;
    uint32_t selected_physical_device_count = 0;
    const char* selection = getenv("MESA_VK_DEVICE_SELECT");
+   bool expose_only_one_dev = false;
    VkResult result = info->EnumeratePhysicalDevices(instance, &physical_device_count, NULL);
    VK_OUTARRAY_MAKE_TYPED(VkPhysicalDevice, out, pPhysicalDevices, pPhysicalDeviceCount);
    if (result != VK_SUCCESS)
@@ -563,7 +591,8 @@ static VkResult device_select_EnumeratePhysicalDevices(VkInstance instance,
          exit(0);
    }
 
-   unsigned selected_index = get_default_device(info, selection, physical_device_count, physical_devices);
+   unsigned selected_index = get_default_device(info, selection, physical_device_count,
+                                                physical_devices, &expose_only_one_dev);
    selected_physical_device_count = physical_device_count;
    selected_physical_devices[0] = physical_devices[selected_index];
    for (unsigned i = 0; i < physical_device_count - 1; ++i) {
@@ -580,6 +609,9 @@ static VkResult device_select_EnumeratePhysicalDevices(VkInstance instance,
    /* do not give multiple device option to app if force default device */
    const char *force_default_device = getenv("MESA_VK_DEVICE_SELECT_FORCE_DEFAULT_DEVICE");
    if (force_default_device && !strcmp(force_default_device, "1") && selected_physical_device_count != 0)
+      expose_only_one_dev = true;
+
+   if (expose_only_one_dev)
       selected_physical_device_count = 1;
 
    for (unsigned i = 0; i < selected_physical_device_count; i++) {