[PATCH] drm/drm_fb_helper: fix fbdev with sparc64

[Dave Airlie]

On Tue, 14 Jul 2020 at 18:56, Thomas Zimmermann <tzimmermann at suse.de> wrote:
>
> Hi
>
> Am 14.07.20 um 10:41 schrieb Daniel Vetter:
> > On Tue, Jul 14, 2020 at 08:41:58AM +0200, Thomas Zimmermann wrote:
> >> Hi
> >>
> >> Am 13.07.20 um 18:21 schrieb Daniel Vetter:
> >>> On Fri, Jul 10, 2020 at 08:28:16AM +0200, Thomas Zimmermann wrote:
> >>>> Hi
> >>>>
> >>>> Am 09.07.20 um 21:30 schrieb Sam Ravnborg:
> >>>>> Mark reported that sparc64 would panic while booting using qemu.
> >>>>> Mark bisected this to a patch that introduced generic fbdev emulation to
> >>>>> the bochs DRM driver.
> >>>>> Mark pointed out that a similar bug was fixed before where
> >>>>> the sys helpers was replaced by cfb helpers.
> >>>>>
> >>>>> The culprint here is that the framebuffer reside in IO memory which
> >>>>> requires SPARC ASI_PHYS (physical) loads and stores.
> >>>>>
> >>>>> The current bohcs DRM driver uses a shadow buffer.
> >>>>> So all copying to the framebuffer happens in
> >>>>> drm_fb_helper_dirty_blit_real().
> >>>>>
> >>>>> The fix is to replace the memcpy with memcpy_toio() from io.h.
> >>>>>
> >>>>> memcpy_toio() uses writeb() where the original fbdev code
> >>>>> used sbus_memcpy_toio(). The latter uses sbus_writeb().
> >>>>>
> >>>>> The difference between writeb() and sbus_memcpy_toio() is
> >>>>> that writeb() writes bytes in little-endian, where sbus_writeb() writes
> >>>>> bytes in big-endian. As endian does not matter for byte writes they are
> >>>>> the same. So we can safely use memcpy_toio() here.
> >>>>>
> >>>>> For many architectures memcpy_toio() is a simple memcpy().
> >>>>> One sideeffect that is unknow is if this has any impact on other
> >>>>> architectures.
> >>>>> So far the analysis tells that this change is OK for other arch's.
> >>>>> but testing would be good.
> >>>>>
> >>>>> Signed-off-by: Sam Ravnborg <sam at ravnborg.org>
> >>>>> Reported-by: Mark Cave-Ayland <mark.cave-ayland at ilande.co.uk>
> >>>>> Tested-by: Mark Cave-Ayland <mark.cave-ayland at ilande.co.uk>
> >>>>> Cc: Mark Cave-Ayland <mark.cave-ayland at ilande.co.uk>
> >>>>> Cc: Thomas Zimmermann <tzimmermann at suse.de>
> >>>>> Cc: Gerd Hoffmann <kraxel at redhat.com>
> >>>>> Cc: "David S. Miller" <davem at davemloft.net>
> >>>>> Cc: sparclinux at vger.kernel.org
> >>>>
> >>>> So this actually is a problem in practice. Do you know how userspace
> >>>> handles this?
> >>>>
> >>>> For this patch
> >>>>
> >>>> Acked-by: Thomas Zimmermann <tzimmermann at suse.de>
> >>>>
> >>>> but I'd like to have someone with more architecture expertise ack this
> >>>> as well.
> >>>>
> >>>> Best regards
> >>>> Thomas
> >>>>
> >>>>> ---
> >>>>>  drivers/gpu/drm/drm_fb_helper.c | 2 +-
> >>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>>
> >>>>> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> >>>>> index 5609e164805f..4d05b0ab1592 100644
> >>>>> --- a/drivers/gpu/drm/drm_fb_helper.c
> >>>>> +++ b/drivers/gpu/drm/drm_fb_helper.c
> >>>>> @@ -399,7 +399,7 @@ static void drm_fb_helper_dirty_blit_real(struct drm_fb_helper *fb_helper,
> >>>>>   unsigned int y;
> >>>>>
> >>>>>   for (y = clip->y1; y < clip->y2; y++) {
> >>>>> -         memcpy(dst, src, len);
> >>>>> +         memcpy_toio(dst, src, len);
> >>>
> >>> I don't think we can do this unconditionally, there's fbdev-helper drivers
> >>> using shmem helpers, and for shmem memcpy_toio is wrong. We need a switch
> >>> to fix this properly I think.
> >>
> >> I once has a patch set for this problem, but it didn't make it. [1]
> >>
> >> Buffers can move between I/O and system memory, so a simple flag would
> >> not work. I'd propose this
> >>
> >> bool drm_gem_is_iomem(struct drm_gem_object *obj)
> >> {
> >>      if (obj->funcs && obj->funcs->is_iomem)
> >>              return obj->funcs->is_iomem(obj);
> >>      return false;
> >> }
> >>
> >> Most GEM implmentations wouldn't bother, but VRAM helpers could set the
> >> is_iomem function and return the current state. Fbdev helpers can then
> >> pick the correct memcpy_*() function.
> >
> > Hm wasn't the (long term at least) idea to add the is_iomem flag to the
> > vmap functions? is_iomem is kinda only well-defined if there's a vmap of
> > the buffer around (which also pins it), or in general when the buffer is
> > pinned. Outside of that an ->is_iomem function doesn't make much sense.
>
> Oh. From how I understood the original discussion, you shoot down the
> idea because sparse would not support it well?
>
> The other idea was to add an additional vmap_iomem() helper that returns
> an__iomem pointer. Can we try that?
>
Did we get anywhere with this yet?

Dave.