[PATCHv2] dmaengine: ste_dma40: turn d40_base phy_chans into a flexible array

[PATCHv2] dmaengine: ste_dma40: turn d40_base phy_chans into a flexible array

[Rosen Penev]

Convert the separately-offset phy_chans pointer to a C99 flexible array
member at the end of struct d40_base, and switch the allocation to
struct_size(). The log_chans and memcpy_chans slots continue to live
in the same allocation immediately after phy_chans, indexed via
base->log_chans. This removes the hand-rolled pointer fixup that
recomputed phy_chans from base + ALIGN(sizeof(struct d40_base), 4).

The ALIGN(sizeof(struct d40_base), 4) requirement is met implicitly by the
C compiler when using a flexible array member. With struct d40_chan
phy_chans[] as the last member, the C standard guarantees
sizeof(struct d40_base) includes trailing padding to satisfy the alignment
of the flexible array element type (struct d40_chan). Since struct d40_chan
contains members like spinlock_t, pointers, and struct dma_chan — all with
alignment ≥ 4 — the compiler ensures sizeof(struct d40_base) is already a
multiple of _Alignof(struct d40_chan) >= 4. The struct_size() macro then
computes sizeof(struct d40_base) + sizeof(struct d40_chan) * num_phy_chans,
so phy_chans[0] lands at a properly aligned offset without needing the manual
ALIGN.

Assisted-by: Claude:Opus-4.7
Signed-off-by: Rosen Penev <rosenp@gmail.com>
---
 v2: added ALIGN description
 drivers/dma/ste_dma40.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/drivers/dma/ste_dma40.c b/drivers/dma/ste_dma40.c
index 9b803c0aec25..d3e3c4cd43f1 100644
--- a/drivers/dma/ste_dma40.c
+++ b/drivers/dma/ste_dma40.c
@@ -602,7 +602,6 @@ struct d40_base {
 	struct dma_device		  dma_both;
 	struct dma_device		  dma_slave;
 	struct dma_device		  dma_memcpy;
-	struct d40_chan			 *phy_chans;
 	struct d40_chan			 *log_chans;
 	struct d40_chan			**lookup_log_chans;
 	struct d40_chan			**lookup_phy_chans;
@@ -621,6 +620,7 @@ struct d40_base {
 	u32				 *regs_interrupt;
 	u16				  gcc_pwr_off_mask;
 	struct d40_gen_dmac		  gen_dmac;
+	struct d40_chan			 phy_chans[];
 };

 static struct device *chan2dev(struct d40_chan *d40c)
@@ -3128,6 +3128,7 @@ static int __init d40_hw_detect_init(struct platform_device *pdev,
 	struct clk *clk;
 	void __iomem *virtbase;
 	struct d40_base *base;
+	size_t alloc_size;
 	int num_log_chans;
 	int num_phy_chans;
 	int num_memcpy_chans;
@@ -3197,10 +3198,9 @@ static int __init d40_hw_detect_init(struct platform_device *pdev,
 		 "hardware rev: %d with %d physical and %d logical channels\n",
 		 rev, num_phy_chans, num_log_chans);

-	base = devm_kzalloc(dev,
-		ALIGN(sizeof(struct d40_base), 4) +
-		(num_phy_chans + num_log_chans + num_memcpy_chans) *
-		sizeof(struct d40_chan), GFP_KERNEL);
+	alloc_size = struct_size(base, phy_chans, num_phy_chans);
+	alloc_size += sizeof(*base->log_chans) * (num_log_chans + num_memcpy_chans);
+	base = devm_kzalloc(dev, alloc_size, GFP_KERNEL);

 	if (!base)
 		return -ENOMEM;
@@ -3213,7 +3213,6 @@ static int __init d40_hw_detect_init(struct platform_device *pdev,
 	base->virtbase = virtbase;
 	base->plat_data = plat_data;
 	base->dev = dev;
-	base->phy_chans = ((void *)base) + ALIGN(sizeof(struct d40_base), 4);
 	base->log_chans = &base->phy_chans[num_phy_chans];

 	if (base->plat_data->num_of_phy_chans == 14) {
--
2.54.0