Re: [PATCH] staging: axis-fifo: fix maximum TX packet length check

[Ovidiu Panait <ovidiu.panait.oss@gmail.com>]

On 8/17/25 2:12 PM, Greg KH wrote:
> On Sat, Aug 02, 2025 at 11:59:43PM +0300, Ovidiu Panait wrote:
>> Since commit 2ca34b508774 ("staging: axis-fifo: Correct handling of
>> tx_fifo_depth for size validation"), write() operations with packets
>> larger than 'tx_fifo_depth - 4' words are no longer rejected with -EINVAL.
>>
>> Fortunately, the packets are not actually getting transmitted to hardware,
>> otherwise they would be raising a 'Transmit Packet Overrun Error'
>> interrupt, which requires a reset of the TX circuit to recover from.
>>
>> Instead, the request times out inside wait_event_interruptible_timeout()
>> and always returns -EAGAIN, since the wake up condition can never be true
>> for these packets. But still, they unnecessarily block other tasks from
>> writing to the FIFO and the EAGAIN return code signals userspace to retry
>> the write() call, even though it will always fail and time out.
>>
>> According to the AXI4-Stream FIFO reference manual (PG080), the maximum
>> valid packet length is 'tx_fifo_depth - 4' words, so attempting to send
>> larger packets is invalid and should not be happening in the first place:
>>
>>> The maximum packet that can be transmitted is limited by the size of
>>> the FIFO, which is (C_TX_FIFO_DEPTH–4)*(data interface width/8) bytes.
>>
>> Therefore, bring back the old behavior and outright reject packets larger
>> than 'tx_fifo_depth - 4' with -EINVAL. Add a comment to explain why the
>> check is necessary. The dev_err() message was removed to avoid cluttering
>> the dmesg log if an invalid packet is received from userspace.
>>
>> Fixes: 2ca34b508774 ("staging: axis-fifo: Correct handling of tx_fifo_depth for size validation")
>> Signed-off-by: Ovidiu Panait <ovidiu.panait.oss@gmail.com>
> 
> Any specific reason you didn't cc: the staging list?  Or add cc: stable
> to this commit?
> 

It was a copy-paste error. I'll resend this with the cc: stable tag
included.

Thanks,
Ovidiu

> thanks,
> 
> greg k-h