Re: [PATCH v1] scsi: mpt3sas: Use lo_hi_writeq() helper

[James Bottomley <jejb@linux.vnet.ibm.com>]

On Wed, 2018-02-14 at 21:48 +0200, Andy Shevchenko wrote:
> On Wed, 2018-02-14 at 11:40 -0800, James Bottomley wrote:
> > 
> > On Wed, 2018-02-14 at 20:10 +0200, Andy Shevchenko wrote:
> > > 
> > > Since we have a writeq() for 32-bit architectures as provided by
> > > IO
> > > non-atomic helpers, there is no need to open code it.
> > > 
> > > Moreover sparse complains about this:
> > > 
> > > drivers/scsi/mpt3sas/mpt3sas_base.c:2975:16: expected unsigned
> > > long
> > > long val
> > > drivers/scsi/mpt3sas/mpt3sas_base.c:2975:16: got restricted
> > > __le64
> > > <noident>
> > > 
> > > Fixing this by replacing custom writeq() with one provided by
> > > io-64-nonatomic-lo-hi.h header.
> > > 
> 
> > 
> > > 
> > > -#if defined(writeq) && defined(CONFIG_64BIT)
> > > -static inline void
> > > -_base_writeq(__u64 b, volatile void __iomem *addr, spinlock_t
> > > *writeq_lock)
> > > -{
> > > -	writeq(cpu_to_le64(b), addr);
> > > -}
> > > -#else
> > >  static inline void
> > >  _base_writeq(__u64 b, volatile void __iomem *addr, spinlock_t
> > > *writeq_lock)
> > >  {
> > > @@ -2985,11 +2979,9 @@ _base_writeq(__u64 b, volatile void
> > > __iomem
> > > *addr, spinlock_t *writeq_lock)
> > >  	__u64 data_out = cpu_to_le64(b);
> > >  
> > >  	spin_lock_irqsave(writeq_lock, flags);
> > > -	writel((u32)(data_out), addr);
> > > -	writel((u32)(data_out >> 32), (addr + 4));
> > > +	writeq(data_out, addr);
> > >  	spin_unlock_irqrestore(writeq_lock, flags);
> > >  }
> > > -#endif
> > 
> > This would rather defeat the purpose of the original code, I think.
> 
> James, TBH, I don't see any value of that lock. What it's protecting
> from? simultaneous thread doing writeq()? But this is pointless if at
> the same time you will have writel() to the device.

The lock prevents two threads doing an interleaved write to this
specific address which could be caused by two threads writing to the
same address.  If I remember correctly the firmware hangs in that case.

> For my opinion perhaps complete removal of the custom writeq() and
> replacing it by just writeq() with enabled non-atomic helpers will do
> the job.
> 
> The code is very old, and I have no idea why it's done this (strange)
> way.

The write seems to trigger starting the engine on the HBA if you look
at the code, which is why it must be written completely and in order.
 It's equivalent to a mailbox post.

James