RE: [Linuxarm] Re: [PATCH for-next 00/32] spin lock usage optimization for SCSI drivers

[Finn Thain]

On Tue, 23 Feb 2021, Song Bao Hua (Barry Song) wrote:

> > 
> > Regarding m68k, your analysis overlooks the timing issue. E.g. patch 
> > 11/32 could be a problem because removing the irqsave would allow PDMA 
> > transfers to be interrupted. Aside from the timing issues, I agree 
> > with your analysis above regarding m68k.
> 
> You mentioned you need realtime so you want an interrupt to be able to 
> preempt another one.

That's not what I said. But for the sake of discussion, yes, I do know 
people who run Linux on ARM hardware (if Android vendor kernels can be 
called "Linux") and who would benefit from realtime support on those 
devices.

> Now you said you want an interrupt not to be preempted as it will make a 
> timing issue.

mac_esp deliberately constrains segment sizes so that it can harmlessly 
disable interrupts for the duration of the transfer.

Maybe the irqsave in this driver is over-cautious. Who knows? The PDMA 
timing problem relates to SCSI bus signalling and the tolerance of real-
world SCSI devices to same. The other problem is that the PDMA logic 
circuit is undocumented hardware. So there may be further timing 
requirements lurking there. Therefore, patch 11/32 is too risky.

> If this PDMA transfer will have some problem when it is preempted, I 
> believe we need some enhanced ways to handle this, otherwise, once we 
> enable preempt_rt or threaded_irq, it will get the timing issue. so here 
> it needs a clear comment and IRQF_NO_THREAD if this is the case.
> 

People who require fast response times cannot expect random drivers or 
platforms to meet such requirements. I fear you may be asking too much 
from Mac Quadra machines.

> > 
> > With regard to other architectures and platforms, in specific cases, 
> > e.g. where there's never more than one IRQ involved, then I could 
> > agree that your assumptions probably hold and an irqsave would be 
> > probably redundant.
> > 
> > When you find a redundant irqsave, to actually patch it would bring a 
> > risk of regression with little or no reward. It's not my place to veto 
> > this entire patch series on that basis but IMO this kind of churn is 
> > misguided.
> 
> Nope.
> 
> I would say the real misguidance is that the code adds one lock while it 
> doesn't need the lock. Easily we can add redundant locks or exaggerate 
> the coverage range of locks, but the smarter way is that people add 
> locks only when they really need the lock by considering concurrency and 
> realtime performance.
> 

You appear to be debating a strawman. No-one is advocating excessive 
locking in new code.

> Thanks
> Barry
>