Re: [PATCH net-next 2/2] net: macb: configure ENST registers for all queues

[Théo Lebrun]

On Mon Jul 27, 2026 at 7:32 AM CEST, Karumanchi, Vineeth wrote:
> On 7/24/2026 11:36 PM, Théo Lebrun wrote:
>> On Fri Jul 24, 2026 at 6:32 AM CEST, Vineeth Karumanchi wrote:
>>> The initial "tc" command was not overwriting the reset value of ENST
>>> registers if only a subset of queues were configured, leading to an
>>> invalid setup. To fix this, configure all queues unconditionally.
>>> Unconfigured queues are zero-initialized via kcalloc(), ensuring a
>>> complete and consistent configuration.
>> But if a subset of queues are configured, the remaining queues don't
>> have their ENST support enabled thanks to ENST_CONTROL and their ENST
>> per-queue register values aren't read? Or HW is broken and reads ENST
>> registers anyway? Or something else I misunderstood?
>>
>> What reset value of ENST regs you observed that caused you trouble?
>> That info could make it into the commit message.
> //
> Yes, this is a confirmed hardware issue. We raised it with Cadence, and 
> they have
> acknowledged the problem. The reset value of the |enst_on_time_qX| 
> registers is *0x0001FFFF*.

ACK. 0x1FFFF translates to the max value of on_time. Important also to
note that off_time reset value is 0x0 according to the manual. This
combination probably explains why queues feel free to emit whenever
they want.

> During ENST initialization, packet interleaving was observed when only a
> subset of the available queues was configured, while the remaining 
> queues—whose
> corresponding |enst_on_time_qX| registers still contained non-zero 
> values—were left disabled in |ENST_CONTROL|.
>
> For example, in a configuration where only two of the four queues are 
> enabled,
> some packets from *Q0* getting transmitted during the *Q1* time slot, 
> and vice versa.

This description isn't enough to fully show there is a bug.
 - You say "only two of the four queues are enabled" but did you mean
   "queue enabled" or "EnST is enabled on that queue"?
 - EnST enabled doesn't mean the timeslots don't overlap, this is one
   allowed config.

Maybe it's not a bug? What behavior would you expect when EnST is
enabled on some queues only? When should queues without EnST active
transmit their frames?

Please be exhaustive in your future commit message; thanks!

> Furthermore, once the hardware enters this state, it does not recover 
> even after a
> complete ENST queue reconfiguration is performed.

---

As you contribute to EnST support, you might be interested in a bug I
just noticed in the enst_ns_to_hw_units() implementation. It doesn't do
rounding properly.

Eg ns=100 speed_mbps=100 => 2 units of time but that is 2*80ns=160ns,
whereas one unit of time (80ns) would have been a better choice. This
example is the worst case scenario (the closer we are to 1 unit of
time, the worst the bug is).

---

PS: please note your email formatting is off because of hard wrapping.
It's well visible on lore. Email is still readable but less than optimal.

https://lore.kernel.org/netdev/3924babc-9755-46ce-97b2-bd6dfce6f6c6@xxxxxxx/

Thanks,

--
Théo Lebrun, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com