[PATCH v2] dmaengine: mpc512x: fix dead empty check in mpc_dma_prep_slave_sg()

[Maoyi Xie <maoyixie.tju@gmail.com>]

mpc_dma_prep_slave_sg() reads mchan->free with list_first_entry()
and then tests the returned pointer against NULL. list_first_entry()
never returns NULL. On an empty free list it returns
container_of(&mchan->free, struct mpc_dma_desc, node), an aliased
pointer derived from the list head. The recovery path (drop lock,
scan completed list, return NULL) is dead code.

Use list_first_entry_or_null() so the empty case returns NULL and
the existing recovery path runs as intended.

The same shape has been cleaned up elsewhere, for example in
commit fbb8bc408027 ("net: qed: Remove redundant NULL checks after list_first_entry()"),
commit c708d3fad421 ("crypto: atmel - use list_first_entry_or_null to simplify find_dev"),
and commit 10379171f346 ("ksmbd: use list_first_entry_or_null for opinfo_get_list()").
This site was missed by those cleanups.

Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Maoyi Xie <maoyixie.tju@gmail.com>
---
v2: Trim two paragraphs from the commit message per Frank Li's
    nit on v1. No code change. Carry forward Frank's Reviewed-by.
    Drop the rz-dmac patch from v1: Geert pointed out that
    Claudiu's "[PATCH v5 09/17] dmaengine: sh: rz-dmac: Use
    virt-dma APIs for channel descriptor processing" rewrites
    rz_dmac_chan_get_residue() through vchan_find_desc() and
    removes ld_active, which supersedes the fix.
    https://lore.kernel.org/r/20260512121219.216159-10-claudiu.beznea.uj@bp.renesas.com
v1: https://lore.kernel.org/r/20260521144755.3476353-2-maoyixie.tju@gmail.com

 drivers/dma/mpc512x_dma.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/dma/mpc512x_dma.c b/drivers/dma/mpc512x_dma.c
index 0adc8e01057e..f5934136efc4 100644
--- a/drivers/dma/mpc512x_dma.c
+++ b/drivers/dma/mpc512x_dma.c
@@ -706,8 +706,8 @@ mpc_dma_prep_slave_sg(struct dma_chan *chan, struct scatterlist *sgl,
 	for_each_sg(sgl, sg, sg_len, i) {
 		spin_lock_irqsave(&mchan->lock, iflags);
 
-		mdesc = list_first_entry(&mchan->free,
-						struct mpc_dma_desc, node);
+		mdesc = list_first_entry_or_null(&mchan->free,
+						 struct mpc_dma_desc, node);
 		if (!mdesc) {
 			spin_unlock_irqrestore(&mchan->lock, iflags);
 			/* Try to free completed descriptors */
-- 
2.34.1