mtd: lpddr: fix excessive stack usage with clang

Building lpddr2_nvm with clang can result in a giant stack usage
in one function:

drivers/mtd/lpddr/lpddr2_nvm.c:399:12: error: stack frame size of 1144 bytes in function 'lpddr2_nvm_probe' [-Werror,-Wframe-larger-than=]

The problem is that clang decides to build a copy of the mtd_info
structure on the stack and then do a memcpy() into the actual version. It
shouldn't really do it that way, but it's not strictly a bug either.

As a workaround, use a static const version of the structure to assign
most of the members upfront and then only set the few members that
require runtime knowledge at probe time.

Fixes: 96ba9dd ("mtd: lpddr: add driver for LPDDR2-NVM PCM memories")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200505140136.263461-1-arnd@arndb.de

Author: arndb

drivers/mtd/lpddr/lpddr2_nvm.c

@@ -393,6 +393,17 @@ static int lpddr2_nvm_lock(struct mtd_info *mtd, loff_t start_add,
 	return lpddr2_nvm_do_block_op(mtd, start_add, len, LPDDR2_NVM_LOCK);
 }
 
+static const struct mtd_info lpddr2_nvm_mtd_info = {
+	.type		= MTD_RAM,
+	.writesize	= 1,
+	.flags		= (MTD_CAP_NVRAM | MTD_POWERUP_LOCK),
+	._read		= lpddr2_nvm_read,
+	._write		= lpddr2_nvm_write,
+	._erase		= lpddr2_nvm_erase,
+	._unlock	= lpddr2_nvm_unlock,
+	._lock		= lpddr2_nvm_lock,
+};
+
 /*
  * lpddr2_nvm driver probe method
  */
@@ -433,6 +444,7 @@ static int lpddr2_nvm_probe(struct platform_device *pdev)
 		.pfow_base	= OW_BASE_ADDRESS,
 		.fldrv_priv	= pcm_data,
 	};
+
 	if (IS_ERR(map->virt))
 		return PTR_ERR(map->virt);
 
@@ -444,22 +456,13 @@ static int lpddr2_nvm_probe(struct platform_device *pdev)
 		return PTR_ERR(pcm_data->ctl_regs);
 
 	/* Populate mtd_info data structure */
-	*mtd = (struct mtd_info) {
-		.dev		= { .parent = &pdev->dev },
-		.name		= pdev->dev.init_name,
-		.type		= MTD_RAM,
-		.priv		= map,
-		.size		= resource_size(add_range),
-		.erasesize	= ERASE_BLOCKSIZE * pcm_data->bus_width,
-		.writesize	= 1,
-		.writebufsize	= WRITE_BUFFSIZE * pcm_data->bus_width,
-		.flags		= (MTD_CAP_NVRAM | MTD_POWERUP_LOCK),
-		._read		= lpddr2_nvm_read,
-		._write		= lpddr2_nvm_write,
-		._erase		= lpddr2_nvm_erase,
-		._unlock	= lpddr2_nvm_unlock,
-		._lock		= lpddr2_nvm_lock,
-	};
+	*mtd = lpddr2_nvm_mtd_info;
+	mtd->dev.parent		= &pdev->dev;
+	mtd->name		= pdev->dev.init_name;
+	mtd->priv		= map;
+	mtd->size		= resource_size(add_range);
+	mtd->erasesize		= ERASE_BLOCKSIZE * pcm_data->bus_width;
+	mtd->writebufsize	= WRITE_BUFFSIZE * pcm_data->bus_width;
 
 	/* Verify the presence of the device looking for PFOW string */
 	if (!lpddr2_nvm_pfow_present(map)) {