[PATCH 6.12.y 2/2] ALSA: scarlett2: Allow flash writes ending at segment boundary

[Sasha Levin <sashal@kernel.org>]

From: Cássio Gabriel <cassiogabrielcontato@gmail.com>

[ Upstream commit a69b677e47a80319ce148d61cc29a2b57006e78d ]

scarlett2_hwdep_write() rejects writes when offset + count is greater than
or equal to the selected flash segment size. That incorrectly treats a
write ending exactly at the end of the segment as out of space, although
the last byte written is still within the segment.

Split invalid argument checks from the segment-space check, keep
zero-length writes as no-ops, and compare count against the remaining
segment size. This permits exact-end writes and avoids relying on
offset + count before deciding whether the request is in bounds.

Fixes: 1abfbd3c9527 ("ALSA: scarlett2: Add support for uploading new firmware")
Cc: stable@vger.kernel.org
Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
Link: https://patch.msgid.link/20260519-alsa-scarlett2-flash-write-boundary-v1-1-b550480e92da@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 sound/usb/mixer_scarlett2.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/sound/usb/mixer_scarlett2.c b/sound/usb/mixer_scarlett2.c
index 564a9b04a443a..568bb0d4ebbd1 100644
--- a/sound/usb/mixer_scarlett2.c
+++ b/sound/usb/mixer_scarlett2.c
@@ -9522,12 +9522,15 @@ static long scarlett2_hwdep_write(struct snd_hwdep *hw,
 	flash_size = private->flash_segment_blocks[segment_id] *
 		     SCARLETT2_FLASH_BLOCK_SIZE;
 
-	if (count < 0 || *offset < 0 || *offset + count >= flash_size)
-		return -ENOSPC;
+	if (count < 0 || *offset < 0)
+		return -EINVAL;
 
 	if (!count)
 		return 0;
 
+	if (*offset >= flash_size || count > flash_size - *offset)
+		return -ENOSPC;
+
 	/* Limit the *req size to SCARLETT2_FLASH_RW_MAX */
 	if (count > max_data_size)
 		count = max_data_size;
-- 
2.53.0