Re: [PATCH 15/16] drm/etnaviv: use ktime_t for timeouts

[Arnd Bergmann <arnd@arndb.de>]

On Sat, Nov 9, 2019 at 12:03 AM Lucas Stach <l.stach@pengutronix.de> wrote:
>
> Am Freitag, den 08.11.2019, 22:32 +0100 schrieb Arnd Bergmann:
> > struct timespec is being removed from the kernel because it often leads
> > to code that is not y2038-safe.
> >
> > In the etnaviv driver, monotonic timestamps are used, which do not suffer
> > from overflow, but using ktime_t still leads to better code overall.
> >
> > The conversion is straightforward for the most part, except for
> > etnaviv_timeout_to_jiffies(), which needs to handle arguments larger
> > than MAX_JIFFY_OFFSET on 32-bit architectures.
> >
> > Signed-off-by: Arnd Bergmann <arnd@arndb.de>

> > @@ -368,7 +366,7 @@ static int etnaviv_ioctl_wait_fence(struct drm_device *dev, void *data,
> >               return -ENXIO;
> >
> >       if (args->flags & ETNA_WAIT_NONBLOCK)
> > -             timeout = NULL;
> > +             timeout = ktime_set(0, 0);
>
> This is a change in behavior, as far as I can see. After this change
> the called internal function is not able to differentiate between a
> NONBLOCK call and a blocking call with 0 timeout. The difference being
> that on a busy object the NONBLOCK call will return -EBUSY, while a
> blocking call will return -ETIMEDOUT.

Ah, good point. I created this patch a long time ago (cherry-picked it out
of an older branch I had done), so I don't remember how I concluded this
was a safe conversion, of if I missed that difference originally.

> But then CLOCK_MONOTONIC starts at 0 and should not never wrap, right?

Yes, that is correct.

> If that's the case then we should never encounter a genuine 0 timeout
> and this change would be okay.

That's quite likely, I'd say any program passing {0,0} as a timeout without
ETNA_WAIT_NONBLOCK is already broken, but if we leave it like that,
it would be best to describe the reasoning in the changelog.

Should I change the changelog, or change the patch to restore the
current behavior instead?

I guess I could fold the change below into my patch to make it transparent
to the application again.

      Arnd

diff --git a/drivers/gpu/drm/etnaviv/etnaviv_drv.c
b/drivers/gpu/drm/etnaviv/etnaviv_drv.c
index 1250c5e06329..162cedfb7f72 100644
--- a/drivers/gpu/drm/etnaviv/etnaviv_drv.c
+++ b/drivers/gpu/drm/etnaviv/etnaviv_drv.c
@@ -354,6 +354,7 @@ static int etnaviv_ioctl_wait_fence(struct
drm_device *dev, void *data,
        ktime_t timeout = ktime_set(args->timeout.tv_sec,
                                    args->timeout.tv_nsec);
        struct etnaviv_gpu *gpu;
+       int ret;

        if (args->flags & ~(ETNA_WAIT_NONBLOCK))
                return -EINVAL;
@@ -365,8 +366,12 @@ static int etnaviv_ioctl_wait_fence(struct
drm_device *dev, void *data,
        if (!gpu)
                return -ENXIO;

-       if (args->flags & ETNA_WAIT_NONBLOCK)
-               timeout = ktime_set(0, 0);
+       if (args->flags & ETNA_WAIT_NONBLOCK) {
+               ret = etnaviv_gpu_wait_fence_interruptible(gpu, args->fence,
+                                                          ktime_set(0, 0));
+
+               return (ret == -ETIMEDOUT) ? -EBUSY : ret;
+       }

        return etnaviv_gpu_wait_fence_interruptible(gpu, args->fence,
                                                    timeout);
@@ -421,10 +426,13 @@ static int etnaviv_ioctl_gem_wait(struct
drm_device *dev, void *data,
        if (!obj)
                return -ENOENT;

-       if (args->flags & ETNA_WAIT_NONBLOCK)
-               timeout = ktime_set(0, 0);
-
-       ret = etnaviv_gem_wait_bo(gpu, obj, timeout);
+       if (args->flags & ETNA_WAIT_NONBLOCK) {
+               ret = etnaviv_gem_wait_bo(gpu, obj, ktime_set(0, 0));
+               if (ret == -ETIMEDOUT)
+                       ret = -EBUSY;
+       } else {
+               ret = etnaviv_gem_wait_bo(gpu, obj, timeout);
+       }

        drm_gem_object_put_unlocked(obj);

diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gpu.c
b/drivers/gpu/drm/etnaviv/etnaviv_gpu.c
index e42b1c4d902c..fa6986c5a5fe 100644
--- a/drivers/gpu/drm/etnaviv/etnaviv_gpu.c
+++ b/drivers/gpu/drm/etnaviv/etnaviv_gpu.c
@@ -1135,6 +1135,7 @@ int etnaviv_gpu_wait_fence_interruptible(struct
etnaviv_gpu *gpu,
        u32 id, ktime_t timeout)
 {
        struct dma_fence *fence;
+       unsigned long remaining;
        int ret;

        /*
@@ -1151,12 +1152,12 @@ int
etnaviv_gpu_wait_fence_interruptible(struct etnaviv_gpu *gpu,
        if (!fence)
                return 0;

-       if (!timeout) {
-               /* No timeout was requested: just test for completion */
-               ret = dma_fence_is_signaled(fence) ? 0 : -EBUSY;
+       if (!timeout ||
+           (remaining = etnaviv_timeout_to_jiffies(timeout)) == 0) {
+               /* No timeout was requested, or timeout is already expired,
+                * just test for completion */
+               ret = dma_fence_is_signaled(fence) ? 0 : -ETIMEDOUT;
        } else {
-               unsigned long remaining = etnaviv_timeout_to_jiffies(timeout);
-
                ret = dma_fence_wait_timeout(fence, true, remaining);
                if (ret == 0)
                        ret = -ETIMEDOUT;
@@ -1185,7 +1186,7 @@ int etnaviv_gpu_wait_obj_inactive(struct etnaviv_gpu *gpu,
        long ret;

        if (!timeout)
-               return !is_active(etnaviv_obj) ? 0 : -EBUSY;
+               return !is_active(etnaviv_obj) ? 0 : -ETIMEDOUT;

        remaining = etnaviv_timeout_to_jiffies(timeout);