Re: [PATCH 6/8] scsi: ufs: Make ufshcd_poll() complain about unsupported arguments

[Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>]

On Thu, Jun 20, 2024 at 01:13:10PM -0700, Bart Van Assche wrote:
> On 6/19/24 12:32 AM, Manivannan Sadhasivam wrote:
> > On Mon, Jun 17, 2024 at 02:07:45PM -0700, Bart Van Assche wrote:
> > > The ufshcd_poll() implementation does not support queue_num ==
> > > UFSHCD_POLL_FROM_INTERRUPT_CONTEXT in MCQ mode. Hence complain
> > > if queue_num == UFSHCD_POLL_FROM_INTERRUPT_CONTEXT in MCQ mode.
> > > 
> > > Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> > > ---
> > >   drivers/ufs/core/ufshcd.c | 1 +
> > >   1 file changed, 1 insertion(+)
> > > 
> > > diff --git a/drivers/ufs/core/ufshcd.c b/drivers/ufs/core/ufshcd.c
> > > index 7761ccca2115..db374a788140 100644
> > > --- a/drivers/ufs/core/ufshcd.c
> > > +++ b/drivers/ufs/core/ufshcd.c
> > > @@ -5562,6 +5562,7 @@ static int ufshcd_poll(struct Scsi_Host *shost, unsigned int queue_num)
> > >   	struct ufs_hw_queue *hwq;
> > >   	if (is_mcq_enabled(hba)) {
> > > +		WARN_ON_ONCE(queue_num == UFSHCD_POLL_FROM_INTERRUPT_CONTEXT);
> > 
> > So what does the user has to do if this WARN_ON gets triggered? Can't we use
> > dev_err()/dev_warn() and return instead if the intention is to just report the
> > error or warning.
> > 
> > I know that UFS code has WARN_ON sprinkled all over, but those should be audited
> > at some point and also the new additions.
> > 
> > Also, this is a bug fix as it essentially fixes array out of the bounds issue.
> > So should have a fixes tag and CC stable list for backporting.
> 
> No, this is not a bug fix. There is only one caller that passes the value
> UFSHCD_POLL_FROM_INTERRUPT_CONTEXT as the 'queue_num' argument and it is a code
> path that supports legacy mode (single queue mode). Since the above WARN_ON_ONCE()
> is in an MCQ code path, it will never be triggered. The above WARN_ON_ONCE() can
> be seen as a form of documentation and also as defensive programming. I think
> using WARN_ON_ONCE() to document which code paths will never be triggered is fine.
> 

Why should we insert a warning in a dead code? WARN_ON* makes sense if a certain
condition is never expected to happen, but if that happens then most likely
something wrong happened seriously so the users should be warned.

But here I don't see a possibility to get this triggered at all. Please correct
me if I'm wrong.

- Mani

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