linux-kernel - Re: [PATCH 1/4] drivers/bus: Added Freescale Management Complex APIs

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Message-ID: <20140916142806.f255250be8df08c56241580f@freescale.com>
Date:	Tue, 16 Sep 2014 14:28:06 -0500
From:	Kim Phillips <kim.phillips@...escale.com>
To:	Scott Wood <scottwood@...escale.com>
CC:	"J. German Rivera" <German.Rivera@...escale.com>,
	<gregkh@...uxfoundation.org>, <arnd@...db.de>,
	<linux-kernel@...r.kernel.org>, <stuart.yoder@...escale.com>,
	<agraf@...e.de>, <linuxppc-release@...ux.freescale.net>
Subject: Re: [PATCH 1/4] drivers/bus: Added Freescale Management Complex
 APIs

On Mon, 15 Sep 2014 23:31:21 -0500
Scott Wood <scottwood@...escale.com> wrote:

> On Mon, 2014-09-15 at 18:44 -0500, Kim Phillips wrote:
> > On Thu, 11 Sep 2014 12:34:21 -0500
> > "J. German Rivera" <German.Rivera@...escale.com> wrote:
> > 
> > > +int mc_get_version(struct fsl_mc_io *mc_io, struct mc_version *mc_ver_info)
> > > +{
> > > +	struct mc_command cmd = { 0 };
> > 
> > we can save some cycles if this initialization is not absolutely
> > necessary: is it?  i.e., does the h/w actually look at the params
> > section when doing a get_version?  not sure to what other commands
> > this comment would apply to...at least get_container_id, but maybe
> > more - all of them?
> 
> Do you really want to open that can of worms, much less to speed up
> something that doesn't look performance critical?  Have fun debugging it
> if it turns out the hardware does look at something you didn't
> initialize.

point taken.

> > > diff --git a/drivers/bus/fsl-mc/fsl_mc_sys.c b/drivers/bus/fsl-mc/fsl_mc_sys.c
> > 
> > > +/**
> > > + * Map an MC portal in the kernel virtual address space
> > > + */
> > > +static int map_mc_portal(phys_addr_t mc_portal_phys_addr,
> > > +			 uint32_t mc_portal_size,
> > > +			 void __iomem **new_mc_portal_virt_addr)
> > > +{
> > > +	void __iomem *mc_portal_virt_addr = NULL;
> > > +	struct resource *res = NULL;
> > > +	int error = -EINVAL;
> > > +
> > > +	res =
> > > +	    request_mem_region(mc_portal_phys_addr, mc_portal_size,
> > > +			       "mc_portal");
> > > +	if (res == NULL) {
> > > +		pr_err("request_mem_region() failed for MC portal %#llx\n",
> > > +		       mc_portal_phys_addr);
> > > +		error = -EBUSY;
> > > +		goto error;
> > > +	}
> > > +
> > > +	mc_portal_virt_addr = ioremap_nocache(mc_portal_phys_addr,
> > > +					      mc_portal_size);
> > > +	if (mc_portal_virt_addr == NULL) {
> > > +		pr_err("ioremap_nocache() failed for MC portal %#llx\n",
> > > +		       mc_portal_phys_addr);
> > > +		error = -EFAULT;
> > > +		goto error;
> > > +	}
> > > +
> > > +	*new_mc_portal_virt_addr = mc_portal_virt_addr;
> > > +	return 0;
> > > +error:
> > > +	if (mc_portal_virt_addr != NULL)
> > > +		iounmap(mc_portal_virt_addr);
> > > +
> > > +	if (res != NULL)
> > > +		release_mem_region(mc_portal_phys_addr, mc_portal_size);
> > > +
> > > +	return error;
> > > +}
> > 
> > unnecessary initializations, bad error codes (both should be
> > -ENOMEM),
> 
> Why should the first one be -ENOMEM?  It's not allocating memory, but
> rather reserving I/O space.

I was going with what most of the drivers are already doing, but I
see EFAULT is 'Bad address', which, you're right, is probably more
appropriate.

> >  unnecessarily complicated error path, plus a simpler
> > implementation can be made if fn can return the mapped address, like
> > so:
> > 
> > static void __iomem *map_mc_portal(phys_addr_t mc_portal_phys_addr,
> >                                    uint32_t mc_portal_size)
> > { 
> >         struct resource *res;
> >         void __iomem *mapped_addr;
> > 
> >         res = request_mem_region(mc_portal_phys_addr, mc_portal_size, 
> >                                  "mc_portal");
> >         if (!res) 
> >                 return NULL;
> > 
> >         mapped_addr = ioremap_nocache(mc_portal_phys_addr,
> >                                       mc_portal_size);
> >         if (!mapped_addr)
> >                 release_mem_region(mc_portal_phys_addr, mc_portal_size);
> > 
> >         return mapped_addr;
> > }
> > 
> > the callsite can return -ENOMEM to its caller if returned NULL.
> 
> -ENOMEM would only be appropriate for one of these errors.

in that case, ERR_PTR() can be used to return the specific error.

> > > +#define ioread64(_p)	    readq(_p)
> > > +#define iowrite64(_v, _p)   writeq(_v, _p)
> > 
> > these definitions have names that are too generic to belong in a FSL
> > h/w header: conflicts will be introduced once the existing
> > io{read,write}32 functions get promoted.  Either use readq/writeq
> > directly, or, if you can justify it, patch a more generic io.h.
> > 
> > Also, is there a reason the 'relaxed' versions of the i/o accessors
> > aren't being used?
> 
> Raw accessors should only be used in performance critical sections where
> it's worth the effort to implement and verify manual synchronization.
> My understanding is that the entire management complex is related to
> setup, not on the I/O fast path.

ok.

Thanks,

Kim
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