Re: [PATCH net-next v7 2/3] dpll: zl3073x: add channel ToD, phase step and TIE operations

[Vadim Fedorenko <vadim.fedorenko@linux.dev>]

On 12/08/2026 12:00, Ivan Vecera wrote:
> On 8/12/26 12:04 PM, Vadim Fedorenko wrote:
>> On 12/08/2026 07:57, Ivan Vecera wrote:
>>> Sashiko findings. Replies inline.
>>>
>>>  > Could a transient hardware error bring down the system here?
>>>  >
>>>  > If an I2C/SPI bus glitch causes the device to return 0xFF, the SEM
>>>  > bit will be set and the CMD field will hit this default case. On
>>>  > systems with panic_on_warn, using WARN_ON for validating external
>>>  > hardware states turns recoverable bus errors into fatal kernel 
>>> panics.
>>>
>>> The SEM-first check already handles the most common bus glitch (0x00
>>> return). For 0xFF: the CMD field is only written by the driver, never
>>> by firmware, so an unknown CMD with SEM set indicates either a bus
>>> error or firmware misbehavior that warrants attention. The switch
>>> structure with WARN_ON in the default case was requested by Vadim
>>> in his v4 review. Systems that enable panic_on_warn accept this
>>> trade-off.
>>>
>>>  > Will this sleep-based polling loop destroy the timestamp's precision?
>>>  >
>>>  > Should the postts be captured immediately after the trigger command
>>>  > in zl3073x_chan_tod_ctrl() instead?
>>>
>>> The hardware latches the ToD value when it processes the command,
>>> which completes when the semaphore clears. The post-timestamp must
>>> be taken after the semaphore clears to guarantee the window contains
>>> the actual latch event. Moving it before the wait would risk the
>>> timestamp window not containing the latch moment.
>>>
>>>  > Could this loop exhaust its retries and return -EBUSY prematurely?
>>>  >
>>>  > The loop spins without an explicit wait [...] On fast SPI/I2C buses,
>>>  > it will execute all 20 reads in a few milliseconds
>>>
>>> Testing on I2C at both 100 kHz and 400 kHz bus speeds shows that a
>>> single iteration of the loop body (two ToD reads, each involving a
>>> ready-wait, command write, second ready-wait and data reads) takes
>>> approximately 17-19 ms regardless of bus speed. The iteration time
>>> is dominated by the device's internal processing, not bus transfer
>>> time. With 20 retries the budget is 340-380 ms, well beyond the
>>> 20 ms margin window.
>>>
>>>  > Is it safe to use WARN_ON to validate user-controlled input?
>>>  >
>>>  > Since delta_ns originates from the clock_adjtime syscall's tx.offset
>>>  > (via the adjphase PTP callback) [...]
>>>
>>> The PTP core already validates the input via getmaxphase, which
>>> returns NSEC_PER_SEC - 1, rejecting values with magnitude >=
>>> NSEC_PER_SEC before the driver callback is invoked. The WARN_ON is
>>> a defensive check for a condition that should never be reached
>>> through normal code paths, not user input validation.
>>
>> AFAIR, the general rule is not to write defensive code in kernel if 
>> you know that the core has already validated inputs.
> 
> Yes, but this low-level helper zl3073x_chan_tie_write() is called from
> multiple places and current code-paths are OK. But in future, if another
> caller will be introduced or existing code will be refactored this WARN
> immediately detects potential bug.
> 
> The same is also valid for WARN in zl3073x_chan_tod_ready_wait()... new
> TOD command starts to be used but someone forget to update this function
> accordingly.

That's a little bit weak and goes against "trust internal APIs". It's
currently called from adjtime and adjphase callbacks, what do you expect
to have in the future?