usb: gadget: f_fastboot: make EP buffer size configurable

Introduce CONFIG_USB_FUNCTION_FASTBOOT_EP_BUFFER_SIZE to allow
customization of the endpoint buffer size used for fastboot transfers.

The EP_BUFFER_SIZE must always be an integral multiple of the maxpacket
size (64, 512, or 1024 bytes depending on USB speed), as controllers
like DWC3 expect bulk OUT requests to be divisible by maxpacket size.

On DWC3 controllers operating in SuperSpeed mode, increasing the
endpoint buffer size from the default value significantly improves
download throughput - from approximately 50 MB/s to over 170 MB/s.
This is particularly beneficial for flashing large images during
development or production.

The configurable buffer size allows board maintainers to tune the
trade-off between memory usage and transfer performance based on
their specific requirements and available resources.

Signed-off-by: Anton Burticica <mouse@ya.ru>

Author: ya-mouse

drivers/fastboot/Kconfig

@@ -26,6 +26,18 @@ config USB_FUNCTION_FASTBOOT
 	help
 	  This enables the USB part of the fastboot gadget.
 
+config USB_FUNCTION_FASTBOOT_EP_BUFFER_SIZE
+	hex "USB fastboot endpoint buffer size"
+	depends on USB_FUNCTION_FASTBOOT
+	default 0x1000
+	help
+	  The buffer size used for USB endpoint transfers in fastboot.
+	  This must always be an integral multiple of the maxpacket size
+	  (64 or 512 or 1024), otherwise transfers may fail on certain
+	  controllers like DWC3 that expect bulk OUT requests to be
+	  divisible by maxpacket size.
+	  The default value is 0x1000 (4096 bytes).
+
 config UDP_FUNCTION_FASTBOOT
 	depends on NET
 	select FASTBOOT

drivers/usb/gadget/f_fastboot.c

@@ -31,7 +31,7 @@
 #define RX_ENDPOINT_MAXIMUM_PACKET_SIZE_1_1  (0x0040)
 #define TX_ENDPOINT_MAXIMUM_PACKET_SIZE      (0x0040)
 
-#define EP_BUFFER_SIZE			4096
+#define EP_BUFFER_SIZE			(CONFIG_USB_FUNCTION_FASTBOOT_EP_BUFFER_SIZE)
 /*
  * EP_BUFFER_SIZE must always be an integral multiple of maxpacket size
  * (64 or 512 or 1024), else we break on certain controllers like DWC3