Re: [PATCH 07/23] drm: omapdrm: crtc: switch pending variable to atomic bitset

Laurent Pinchart
2016-03-28T19:18:27Z

Hi Sebastian,

Thank you for the patch.

On Tuesday 08 Mar 2016 17:39:39 Sebastian Reichel wrote:
> Having the pending variable available as atomic bit helps
> with the later addition of manually updated display support.
>
> Signed-off-by: Sebastian Reichel <sre@kernel.org>
> ---
> drivers/gpu/drm/omapdrm/omap_crtc.c | 31 +++++++++++++++++--------------
> 1 file changed, 17 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/gpu/drm/omapdrm/omap_crtc.c
> b/drivers/gpu/drm/omapdrm/omap_crtc.c index 2ed0754ed19e..5ef27664bcfa
> 100644
> --- a/drivers/gpu/drm/omapdrm/omap_crtc.c
> +++ b/drivers/gpu/drm/omapdrm/omap_crtc.c
> @@ -28,6 +28,11 @@
>
> #define to_omap_crtc(x) container_of(x, struct omap_crtc, base)
>
> +enum omap_crtc_state {

I find that using an enum and calling it omap_crtc_state is confusing, it
seems to imply that the CRTC state will be one of the enumerated values, while
the values are actually non-exclusive bits in a bitmask.

> + crtc_enabled = 0,

You don't seem to be using this bit in this patch, you can define it in patch
08/23.

> + crtc_pending = 1

Please name this OMAP_CRTC_STATE_PENDING.

> +};

Please add a short description of each bit in a comment above the enum.

> +
> struct omap_crtc {
> struct drm_crtc base;
>
> @@ -49,7 +54,7 @@ struct omap_crtc {
>
> bool ignore_digit_sync_lost;
>
> - bool pending;
> + unsigned long state;
> wait_queue_head_t pending_wait;
> };
>
> @@ -81,7 +86,7 @@ int omap_crtc_wait_pending(struct drm_crtc *crtc)
> struct omap_crtc *omap_crtc = to_omap_crtc(crtc);
>
> return wait_event_timeout(omap_crtc->pending_wait,
> - !omap_crtc->pending,
> + !test_bit(crtc_pending, &omap_crtc->state),
> msecs_to_jiffies(50));
> }
>
> @@ -311,10 +316,8 @@ static void omap_crtc_vblank_irq(struct omap_drm_irq
> *irq, uint32_t irqstatus)
>
> __omap_irq_unregister(dev, &omap_crtc->vblank_irq);
>
> - rmb();
> - WARN_ON(!omap_crtc->pending);
> - omap_crtc->pending = false;
> - wmb();
> + if (!test_and_clear_bit(crtc_pending, &omap_crtc->state))
> + dev_warn(dev->dev, "pending bit was not set in vblank irq");

Documentation/atomic_ops.txt states that the atomic bit ops must be atomic and
include memory barriers, but I'm confused by the ARM implementation.

The constant bit number version ____atomic_test_and_clear_bit() uses
raw_local_irq_save() and raw_low_irq_restore() for synchronization, which
expand to arch_local_irq_save() and arch_local_irq_restore(). On ARMv6 and
newer, the functions are defined as

static inline unsigned long arch_local_irq_save(void)
{
unsigned long flags;

asm volatile(
" mrs %0, " IRQMASK_REG_NAME_R " @
arch_local_irq_save\n"
" cpsid i"
: "=r" (flags) : : "memory", "cc");
return flags;
}

and

static inline void arch_local_irq_restore(unsigned long flags)
{
asm volatile(
" msr " IRQMASK_REG_NAME_W ", %0 @
local_irq_restore"
:
: "r" (flags)
: "memory", "cc");
}

I'm probably missing something obvious, but I don't see the memory barriers
:-/

> /* wake up userspace */
> omap_crtc_complete_page_flip(&omap_crtc->base);
> @@ -351,13 +354,12 @@ static bool omap_crtc_mode_fixup(struct drm_crtc
> *crtc, static void omap_crtc_enable(struct drm_crtc *crtc)
> {
> struct omap_crtc *omap_crtc = to_omap_crtc(crtc);
> + struct drm_device *dev = crtc->dev;
>
> DBG("%s", omap_crtc->name);
>
> - rmb();
> - WARN_ON(omap_crtc->pending);
> - omap_crtc->pending = true;
> - wmb();
> + if (test_and_set_bit(crtc_pending, &omap_crtc->state))
> + dev_warn(dev->dev, "crtc enable while pending bit set!");
>
> omap_irq_register(crtc->dev, &omap_crtc->vblank_irq);
>
> @@ -397,6 +399,7 @@ static void omap_crtc_atomic_flush(struct drm_crtc
> *crtc, struct drm_crtc_state *old_crtc_state) {
> struct omap_crtc *omap_crtc = to_omap_crtc(crtc);
> + struct drm_device *dev = crtc->dev;
>
> WARN_ON(omap_crtc->vblank_irq.registered);
>
> @@ -404,10 +407,8 @@ static void omap_crtc_atomic_flush(struct drm_crtc
> *crtc,
>
> DBG("%s: GO", omap_crtc->name);
>
> - rmb();
> - WARN_ON(omap_crtc->pending);
> - omap_crtc->pending = true;
> - wmb();
> + if (test_and_set_bit(crtc_pending, &omap_crtc->state))
> + dev_warn(dev->dev, "atomic flush while pending bit set!");
>
> dispc_mgr_go(omap_crtc->channel);
> omap_irq_register(crtc->dev, &omap_crtc->vblank_irq);
> @@ -509,6 +510,8 @@ struct drm_crtc *omap_crtc_init(struct drm_device *dev,
>
> init_waitqueue_head(&omap_crtc->pending_wait);
>
> + omap_crtc->state = 0;
> +
> omap_crtc->channel = channel;
> omap_crtc->name = channel_names[channel];

--
Regards,

Laurent Pinchart