mtd: clear cache_state to avoid writing to bad blocks repeatedly

The function call process is as follows:
	mtd_blktrans_work()
	  while (1)
	    do_blktrans_request()
	      mtdblock_writesect()
	        do_cached_write()
	          write_cached_data() /*if cache_state is STATE_DIRTY*/
	            erase_write()

write_cached_data() returns failure without modifying cache_state
and cache_offset. So when do_cached_write() is called again,
write_cached_data() will be called again to perform erase_write()
on the same cache_offset.

But if this cache_offset points to a bad block, erase_write() will
always return -EIO. Writing to this mtdblk is equivalent to losing
the current data, and repeatedly writing to the bad block.

Repeatedly writing a bad block has no real benefits,
but brings some negative effects:
1 Lost subsequent data
2 Loss of flash device life
3 erase_write() bad blocks are very time-consuming. For example:
	the function do_erase_oneblock() in chips/cfi_cmdset_0020.c or
	chips/cfi_cmdset_0002.c may take more than 20 seconds to return

Therefore, when erase_write() returns -EIO in write_cached_data(),
clear cache_state to avoid writing to bad blocks repeatedly.

Signed-off-by: Xiaoming Ni <nixiaoming@huawei.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/mtdblock.c

@@ -89,8 +89,6 @@ static int write_cached_data (struct mtdblk_dev *mtdblk)
 
 	ret = erase_write (mtd, mtdblk->cache_offset,
 			   mtdblk->cache_size, mtdblk->cache_data);
-	if (ret)
-		return ret;
 
 	/*
 	 * Here we could arguably set the cache state to STATE_CLEAN.
@@ -98,9 +96,14 @@ static int write_cached_data (struct mtdblk_dev *mtdblk)
 	 * be notified if this content is altered on the flash by other
 	 * means.  Let's declare it empty and leave buffering tasks to
 	 * the buffer cache instead.
+	 *
+	 * If this cache_offset points to a bad block, data cannot be
+	 * written to the device. Clear cache_state to avoid writing to
+	 * bad blocks repeatedly.
 	 */
-	mtdblk->cache_state = STATE_EMPTY;
-	return 0;
+	if (ret == 0 || ret == -EIO)
+		mtdblk->cache_state = STATE_EMPTY;
+	return ret;
 }