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

[Ivan Vecera]

13. srpna 2026 22:38:55 SELČ, Vadim Fedorenko <vadim.fedorenko@xxxxxxxxx> napsal:
>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?
>

No idea 
I will drop it...

Thanks,
Ivan