[PATCH RFC 1/1] drm/radeon: fix bad DMA from INTERRUPT_CNTL2

[Alex Deucher]

On Wed, Nov 13, 2019 at 9:53 PM Sam Bobroff <sbobroff at linux.ibm.com> wrote:
>
> Currently, binding the radeon driver to a Radeon FirePro (PCI
> 1002:68f2) that has been passed through to a guest on a Power8 machine
> causes a bad DMA read which causes the device to be frozen, leading to
> the driver failing to bind and other problems.
>
> The bad DMA is to the address written into the INTERRUPT_CNTL2
> register during r600_irq_init(), and it can be fixed by substituting a
> valid (dummy) DMA address.

The patch is correct.  INTERRUPT_CNTL2 takes a bus address not a GPU
MC address.  Care to update si.c and cik.c as well?

Thanks,

Alex

> ---
> Hello,
>
> I've been tracking down a bug, described above, and I've been able to hack
> in a workaround but I could use some help understanding how to fix it
> properly.
>
> What seems to be happening is that when the first CRTC is enabled (when
> evergreen_irq_set() writes to GRPH_INT_CONTROL) the device immediately
> performs a DMA read from the address that's been programmed into
> INTERRUPT_CNTL2 by r600_irq_init().
>
> That address isn't a valid DMA address, so it triggers the problem. The
> address comes from rdev->ih.gpu_addr, which seems to be a 'linear GPU
> address', calculated from the size of the card's VRAM. It definitely hasn't
> come from a DMA mapping operation.
>
> Based on the nearby comment, "set dummy read address to ring address", I
> tried substituting a valid dummy DMA address (using the address mapped to
> the 'dummy page' used by the GART) and it does prevent the problem.
> However, I don't know what that register is supposed to do or what
> information the device might be communicating via that DMA, so presumably
> it breaks something.
>
> Note: I have tried loading the patched driver with "test=1" and all the
> self tests succeed.
>
> Could anyone offer some insight into this problem?
> What does that register do? What kind of address is it expecting?
> What might be a good way of fixing it?
>
> Thanks in advance,
> Sam.
>
>  drivers/gpu/drm/radeon/r600.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/radeon/r600.c b/drivers/gpu/drm/radeon/r600.c
> index e937cc01910d..033bc466a862 100644
> --- a/drivers/gpu/drm/radeon/r600.c
> +++ b/drivers/gpu/drm/radeon/r600.c
> @@ -3696,8 +3696,8 @@ int r600_irq_init(struct radeon_device *rdev)
>         }
>
>         /* setup interrupt control */
> -       /* set dummy read address to ring address */
> -       WREG32(INTERRUPT_CNTL2, rdev->ih.gpu_addr >> 8);
> +       /* set dummy read address to dummy page address */
> +       WREG32(INTERRUPT_CNTL2, rdev->dummy_page.addr >> 8);
>         interrupt_cntl = RREG32(INTERRUPT_CNTL);
>         /* IH_DUMMY_RD_OVERRIDE=0 - dummy read disabled with msi, enabled without msi
>          * IH_DUMMY_RD_OVERRIDE=1 - dummy read controlled by IH_DUMMY_RD_EN
> --
> 2.22.0.216.g00a2a96fc9
>
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