linux-kernel - Re: [PATCH v3 03/17] powerpc: Add PFO support to the VIO bus

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Message-ID: <20120510190835.GB12304@linux.vnet.ibm.com>
Date:	Thu, 10 May 2012 14:08:35 -0500
From:	Robert Jennings <rcj@...ux.vnet.ibm.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Kent Yoder <key@...ux.vnet.ibm.com>, linux-kernel@...r.kernel.org,
	linux-crypto@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v3 03/17] powerpc: Add PFO support to the VIO bus

* Benjamin Herrenschmidt (benh@...nel.crashing.org) wrote:
> Hrm... I don't like that much:
> 
> > +	if (op->timeout)
> > +		deadline = jiffies + msecs_to_jiffies(op->timeout);
> > +
> > +	while (true) {
> > +		hret = plpar_hcall_norets(H_COP, op->flags,
> > +				vdev->resource_id,
> > +				op->in, op->inlen, op->out,
> > +				op->outlen, op->csbcpb);
> > +
> > +		if (hret == H_SUCCESS ||
> > +		    (hret != H_NOT_ENOUGH_RESOURCES &&
> > +		     hret != H_BUSY && hret != H_RESOURCE) ||
> > +		    (op->timeout && time_after(deadline, jiffies)))
> > +			break;
> > +
> > +		dev_dbg(dev, "%s: hcall ret(%ld), retrying.\n", __func__, hret);
> > +	}
> > +
> 
> Is this meant to be called in atomic context ? If not, maybe it should
> at the very least do a cond_resched() ?
> 
> Else, what about ceding the processor ? Or at the very least reducing
> the thread priority for a bit ?
> 
> Shouldn't we also enforce to always have a timeout ? IE. Something like
> 30s or so if nothing specified to avoid having the kernel just hard
> lock...
> 
> In general I don't like that sort of synchronous code, I'd rather return
> the busy status up the chain which gives a chance to the caller to take
> more appropriate measures depending on what it's doing, but that really
> depends what you use that synchronous call for. I suppose if it's for
> configuration type operations, it's ok...

This function is called in atomic context, it is used by PFO-type device
drivers to perform operations with the nest accelerator unit (like
crypto acceleration).

Having the timeout and retries in this function is the wrong thing to do.
We'll resubmit this without the loop and the caller will be responsible for
retrying the operations.

I would rather have the caller cede the processor or alter thread
priority where appropriate than doing that in this function.  I don't
think this should be done in this crypto driver.

--Rob Jennings

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