Re: [PATCH net] net: airoha: Fix TX scheduler queue mask loop upper bound

[Wayen Yan <win847@gmail.com>]

On Tue, Jun 17, 2026, Lorenzo Bianconi wrote:
> Even if the current codebase supports just AIROHA_NUM_QOS_CHANNEL (4), the hw
> exposes 32 hw QoS channels (AIROHA_NUM_TX_RING). Here we are just clearing the
> configuration, so I guess the current implementation is correct.

Hi Lorenzo,

You are right that there is no functional impact, and I agree this
should not go to net. Let me explain the register layout I was worried
about, and you can decide whether it is worth a net-next cleanup or
should just be dropped.

The two macros are:

	REG_QUEUE_CLOSE_CFG(_n)             = 0x00a0 + ((_n) & 0xfc)
	TXQ_DISABLE_CHAN_QUEUE_MASK(_n, _m) = BIT((_m) + (((_n) & 0x3) << 3))

REG_QUEUE_CLOSE_CFG() masks the channel with 0xfc, and the bit macro
folds the channel with & 0x3 (mod 4) shifted by 3. So one 32-bit
register holds 4 channels x 8 queues, 8 queue bits per channel:

	channel 0 -> reg 0x00a0, bits  0..7
	channel 1 -> reg 0x00a0, bits  8..15
	channel 2 -> reg 0x00a0, bits 16..23
	channel 3 -> reg 0x00a0, bits 24..31
	channel 4 -> reg 0x00a4, bits  0..7
	...

In airoha_qdma_set_chan_tx_sched() the loop variable 'i' is passed as
the *queue* argument _m, not as a channel:

	for (i = 0; i < AIROHA_NUM_TX_RING; i++)   // i = 0..31
		airoha_qdma_clear(qdma, REG_QUEUE_CLOSE_CFG(channel),
				  TXQ_DISABLE_CHAN_QUEUE_MASK(channel, i));

Since each channel only has AIROHA_NUM_QOS_QUEUES (8) queues, the correct
logic is to clear the 8 queue bits belonging to 'channel'. With i running
up to 31 the BIT() shift instead walks past those 8 bits and into the bit
ranges of the other channels folded into the same register. For channel 0
the accumulated mask becomes 0xffffffff, i.e. it touches channels 1..3 as
well.

This is harmless today only because REG_QUEUE_CLOSE_CFG is written
exclusively here, via airoha_qdma_clear() (RMW clear), and the register
resets to 0 and is never set anywhere -- so clearing extra bits is a
no-op. Functionally the current code is fine, as you say.

The point is just the loop-bound semantics: 'i' is a per-channel queue
index, so the bound should be AIROHA_NUM_QOS_QUEUES (8), not
AIROHA_NUM_TX_RING (32). The two happen to be related (32 == 4 channels *
8 queues) but mean different things.

Since there is no functional change, feel free to drop this if you would
rather not carry a cosmetic patch. If you think the clarity is worth it I
can resend against net-next without the Fixes tag.

Thanks,
Wayen