linux-kernel - ftrace use of pci_resource_to

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Message-ID: <20160504191713.GA3117@localhost>
Date:	Wed, 4 May 2016 14:17:13 -0500
From:	Bjorn Helgaas <helgaas@...nel.org>
To:	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Pekka Paalanen <ppaalanen@...il.com>
Cc:	linux-kernel@...r.kernel.org, Yinghai Lu <yinghai@...nel.org>
Subject: ftrace use of pci_resource_to_user()

138295373ccf ("ftrace: mmiotrace update, #2") added this use of
pci_resource_to_user():

  +static int mmio_print_pcidev(struct trace_seq *s, const struct pci_dev *dev)
  +{
  ...
  +       /*
  +        * XXX: is pci_resource_to_user() appropriate, since we are
  +        * supposed to interpret the __ioremap() phys_addr argument based on
  +        * these printed values?
  +        */
  +       for (i = 0; i < 7; i++) {
  +               pci_resource_to_user(dev, i, &dev->resource[i], &start, &end);
  +               ret += trace_seq_printf(s, " %llx",
  +                       (unsigned long long)(start |
  +                       (dev->resource[i].flags & PCI_REGION_FLAG_MASK)));
  +       }

I think it was a mistake to use pci_resource_to_user() here because it
adds unnecessary arch dependencies in whatever consumes this output.

On most arches, pci_resource_to_user() is a no-op and the result is
normal resource addresses, i.e., CPU physical addresses that match
things in /proc/iomem and /sys/devices/pci.../resource.

On microblaze, mips, powerpc, and sparc, the result of
pci_resource_to_user() is something else, usually a PCI bus address (a
raw BAR value).  These values are only useful for using mmap on
files like /proc/bus/pci/....

I don't know what, if anything, consumes this output.  If things parse
it, we shouldn't break them.  But those things likely would need
special cases for microblaze, mips, powerpc, and sparc.

If it's only for human consumption, I think we should consider
removing the use of pci_resource_to_user() and printing
dev->resource[i].start instead.

Bjorn