Re: [PATCH v2 2/2] x86/mm/pat, drivers/media/ivtv: move pat warn and replace WARN() with pr_warn()

Ingo Molnar
2015-06-29T06:55:05Z

* Andy Walls <andy@silverblocksystems.net> wrote:

> On Fri, 2015-06-26 at 10:45 +0200, Ingo Molnar wrote:
> > * Luis R. Rodriguez <mcgrof@suse.com> wrote:
> >
> > > On Thu, Jun 25, 2015 at 08:51:47AM +0200, Ingo Molnar wrote:
> > > >
> > > > * Luis R. Rodriguez <mcgrof@do-not-panic.com> wrote:
> > > >
> > > > > From: "Luis R. Rodriguez" <mcgrof@suse.com>
> > > > >
> > > > > On built-in kernels this warning will always splat as this is part
> > > > > of the module init. Fix that by shifting the PAT requirement check
> > > > > out under the code that does the "quasi-probe" for the device. This
> > > > > device driver relies on an existing driver to find its own devices,
> > > > > it looks for that device driver and its own found devices, then
> > > > > uses driver_for_each_device() to try to see if it can probe each of
> > > > > those devices as a frambuffer device with ivtvfb_init_card(). We
> > > > > tuck the PAT requiremenet check then on the ivtvfb_init_card()
> > > > > call making the check at least require an ivtv device present
> > > > > before complaining.
> > > > >
> > > > > Reported-by: Fengguang Wu <fengguang.wu@intel.com> [0-day test robot]
> > > > > Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
> > > > > ---
> > > > > drivers/media/pci/ivtv/ivtvfb.c | 15 +++++++++------
> > > > > 1 file changed, 9 insertions(+), 6 deletions(-)
> > > > >
> > > > > diff --git a/drivers/media/pci/ivtv/ivtvfb.c b/drivers/media/pci/ivtv/ivtvfb.c
> > > > > index 4cb365d..8b95eef 100644
> > > > > --- a/drivers/media/pci/ivtv/ivtvfb.c
> > > > > +++ b/drivers/media/pci/ivtv/ivtvfb.c
> > > > > @@ -38,6 +38,8 @@
> > > > > Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
> > > > > */
> > > > >
> > > > > +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> > > > > +
> > > > > #include <linux/module.h>
> > > > > #include <linux/kernel.h>
> > > > > #include <linux/fb.h>
> > > > > @@ -1171,6 +1173,13 @@ static int ivtvfb_init_card(struct ivtv *itv)
> > > > > {
> > > > > int rc;
> > > > >
> > > > > +#ifdef CONFIG_X86_64
> > > > > + if (pat_enabled()) {
> > > > > + pr_warn("ivtvfb needs PAT disabled, boot with nopat kernel parameter\n");
> > > > > + return -ENODEV;
> > > > > + }
> > > > > +#endif
> > > > > +
> > > > > if (itv->osd_info) {
> > > > > IVTVFB_ERR("Card %d already initialised\n", ivtvfb_card_id);
> > > > > return -EBUSY;
> > > >
> > > > Same argument as for ipath: why not make arch_phys_wc_add() fail on PAT and
> > > > return -1, and check it in arch_phys_wc_del()?
> > >
> > > The arch_phys_wc_add() is a no-op for PAT systems but for PAT to work we need
> > > not only need to add this in where we replace the MTRR call but we also need to
> > > convert ioremap_nocache() calls to ioremap_wc() but only if things were split up
> > > already.
> >
>
> Hi Ingo,
>
> > We don't need to do that: for such legacy drivers we can fall back to UC just
> > fine, and inform the user that by booting with 'nopat' the old behavior will be
> > back...
>
> This is really a "user experience" decision.
>
> IMO anyone who is still using ivtvfb and an old conventional PCI PVR-350 to
> render, at SDTV resolution, an X Desktop display or video playback on a
> television screen, isn't going to give a hoot about modern things like PAT. The
> user will simply want the framebuffer performance they are accustomed to having
> with their system. UC will probably yield unsatisfactory performance for an
> ivtvfb framebuffer.
>
> With that in mind, I would think it better to obviously and clearly disable the
> ivtvfb framebuffer module with PAT enabled, so the user will check the log and
> read the steps needed to obtain acceptable performance.
>
> Maybe that's me just wanting to head off the "poor ivtvfb performance with
> latest kernel" bug reports.
>
> Whatever the decision, my stock response to bug reports related to this will
> always be "What do the logs say?".

So what if that frame buffer is their only (working) frame buffer? A slow
framebuffer is still much better at giving people logs to look at than a
non-working one.

Thanks,

Ingo