Re: [PATCH 1/2] pci: Block config access during BIST (resend)

[Paul Mackerras]

Grant Grundler writes:

> On Fri, Sep 02, 2005 at 06:56:35PM -0500, Brian King wrote:
> > Andrew Morton wrote:
> > >Brian King <brking@xxxxxxxxxx> wrote:
> > >
> > >>+void pci_block_user_cfg_access(struct pci_dev *dev)
> > >>+{
> > >>+	unsigned long flags;
> > >>+
> > >>+	pci_save_state(dev);
> > >>+	spin_lock_irqsave(&pci_lock, flags);
> > >>+	dev->block_ucfg_access = 1;
> > >>+	spin_unlock_irqrestore(&pci_lock, flags);
> > >
> > >
> > >Are you sure the locking in here is meaningful?  All it will really do is
> > >give you a couple of barriers.
> > 
> > Actually, it is meaningful. It synchronizes the blocking of pci config 
> > accesses with other pci config accesses that may be going on when this 
> > function is called. Without the locking, the API cannot guarantee that 
> > no further user initiated PCI config accesses will be initiated after 
> > this function is called.
> 
> I don't have the impression you understood what Andrew wrote.
> dev->block_ucfg_access = 1 is essentially an atomic operation.
> AFAIK, Use of the pci_lock doesn't solve any race conditions that mb()
> wouldn't solve.

Think about it.  Taking the lock ensures that we don't do the
assignment (dev->block_ucfg_access = 1) while any other cpu has the
pci_lock.  In other words, the reason for taking the lock is so that
we wait until nobody else is doing an access, not to make others wait.

> If you had:
> 	spin_lock_irqsave(&pci_lock, flags);
> 	pci_save_state(dev);
> 	dev->block_ucfg_access = 1;
> 	spin_unlock_irqrestore(&pci_lock, flags);
> 
> Then I could buy your arguement since the flag now implies
> we need to atomically save state and set the flag.

That's probably a good thing to do to.

Paul.
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