Re: [Intel-wired-lan] [PATCH iwl-net v1] igc: fix race condition in TX timestamp read for register 0

[Jacob Keller <jacob.e.keller@intel.com>]

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On 9/18/2025 1:47 PM, Vadim Fedorenko wrote:
> On 18/09/2025 19:38, Chwee-Lin Choong wrote:
>> The current HW bug workaround checks the TXTT_0 ready bit first,
>> then reads LOW -> HIGH -> LOW from register 0 to detect if a
>> timestamp was captured.
>>
>> This sequence has a race: if a new timestamp is latched after
>> reading the TXTT mask but before the first LOW read, both old
>> and new timestamp match, causing the driver to drop a valid
>> timestamp.
>>
>> Fix by reading the LOW register first, then the TXTT mask,
>> so a newly latched timestamp will always be detected.
>>
>> This fix also prevents TX unit hangs observed under heavy
>> timestamping load.
>>
>> Fixes: c789ad7cbebc ("igc: Work around HW bug causing missing timestamps")
>> Suggested-by: Avi Shalev <avi.shalev@intel.com>
>> Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
>> Signed-off-by: Chwee-Lin Choong <chwee.lin.choong@intel.com>
>> ---
>>   drivers/net/ethernet/intel/igc/igc_ptp.c | 10 ++++++++--
>>   1 file changed, 8 insertions(+), 2 deletions(-)
>>
> 
> [...]
> 
>>   		 * timestamp was captured, we can read the "high"
>>   		 * register again.
>>   		 */
> 
> This comment begins with 'read the "high" register (to latch a new 
> timestamp)' ...
> 
>> -		u32 txstmpl_old, txstmpl_new;
>> +		u32 txstmpl_new;
>>   
>> -		txstmpl_old = rd32(IGC_TXSTMPL);
>>   		rd32(IGC_TXSTMPH);
>>   		txstmpl_new = rd32(IGC_TXSTMPL);
> 
> and a couple of lines later in this function you have
> 
> 		regval = txstmpl_new;
> 		regval |= (u64)rd32(IGC_TXSTMPH) << 32;
> 
> According to the comment above, the value in the register will be
> latched after reading IGC_TXSTMPH. As there will be no read of "low"
> part of the register, it will stay latched with old value until the
> next call to the same function. Could it be the reason of unit hangs?
> 
> It looks like the value of previous read of IGC_TXSTMPH should be stored
> and used to construct new timestamp, right?
> 

I wouldn't trust the comment, but instead double check the data sheets.
Unfortunately, I don't seem to have a copy of the igc hardware data
sheet handy :(

Thanks,
Jake


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