[partition] Document how the configurable EFI size is coordinated

src/modules/partition/Config.cpp

@@ -235,16 +235,18 @@ fillGSConfigurationEFI( Calamares::GlobalStorage* gs, const QVariantMap& configu
     if ( configurationMap.contains( "efiSystemPartitionSize" ) )
     {
         const QString sizeString = CalamaresUtils::getString( configurationMap, "efiSystemPartitionSize" );
-        CalamaresUtils::Partition::PartitionSize part_size = CalamaresUtils::Partition::PartitionSize( sizeString );
+        CalamaresUtils::Partition::PartitionSize part_size
-        if ( part_size.isValid() )
+            = CalamaresUtils::Partition::PartitionSize( sizeString );
+        if (part_size.isValid())
         {
-            gs->insert( "efiSystemPartitionSize", sizeString );
+            // Insert once as string, once as a size-in-bytes;
-            gs->insert( "efiSystemPartitionSize_i", part_size.toBytes() );
+            // changes to these keys should be synchronized with PartUtils.cpp
+            gs->insert( "efiSystemPartitionSize",  sizeString );
+            gs->insert( "efiSystemPartitionSize_i", part_size.toBytes());
 
-            if ( part_size.toBytes() != PartUtils::efiFilesystemMinimumSize() )
+            if (part_size.toBytes() != PartUtils::efiFilesystemMinimumSize())
             {
-                cWarning() << "EFI partition size" << sizeString << "has been adjusted to"
+                cWarning() << "EFI partition size" << sizeString << "has been adjusted to" << PartUtils::efiFilesystemMinimumSize() << "bytes";
-                           << PartUtils::efiFilesystemMinimumSize() << "bytes";
             }
         }
         else

src/modules/partition/core/PartUtils.cpp

@@ -527,6 +527,9 @@ efiFilesystemMinimumSize()
     using CalamaresUtils::Units::operator""_MiB;
 
     auto uefisys_part_sizeB = 300_MiB;
+
+    // The default can be overridden; the key used here comes
+    // from the partition module Config.cpp
     auto* gs = Calamares::JobQueue::instance()->globalStorage();
     if ( gs->contains( "efiSystemPartitionSize_i" ) )
     {

src/modules/partition/core/PartUtils.h

@@ -100,6 +100,12 @@ bool isEfiFilesystemSuitableSize( const Partition* candidate );
  * and EFI documentation (and not a little discussion in Calamares
  * issues about what works, what is effective, and what is mandated
  * by the standard and how all of those are different).
+ *
+ * This can be configured through the `partition.conf` file,
+ * key *efiSystemPartitionSize*, which will then apply to both
+ * automatic partitioning **and** the warning for manual partitioning.
+ *
+ * A minimum of 32MiB (which is bonkers-small) is enforced.
  */
 size_t efiFilesystemMinimumSize();
 

src/modules/partition/partition.conf

@@ -10,6 +10,12 @@ efiSystemPartition:     "/boot/efi"
 
 # This optional setting specifies the size of the EFI system partition.
 # If nothing is specified, the default size of 300MiB will be used.
+#
+# This size applies both to automatic partitioning and the checks
+# during manual partitioning. A minimum of 32MiB is enforced,
+# 300MiB is the default, M is treated as MiB, and if you really want
+# one-million (10^6) bytes, use MB.
+#
 # efiSystemPartitionSize:     300M
 
 # This optional setting specifies the name of the EFI system partition (see