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

[Lorenzo Bianconi <lorenzo@kernel.org>]

[-- Attachment #1: Type: text/plain, Size: 2931 bytes --]

> 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
> 

Sorry you are right, I misread the code, your patch is correct. Since as you
pointed out REG_QUEUE_CLOSE_CFG() is actually never set at the moment and the
default register value is 0, I would repost this patch for net-next as soon as
it is opened (this will avoid merge conflicts).

Regards,
Lorenzo

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 228 bytes --]