linux-kernel - Re: [PATCH 2/8] drm/rockchip: Get rid of some unnecessary code

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Date:   Sun, 18 Sep 2016 13:01:53 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Mark yao <mark.yao@...k-chips.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Heiko Stuebner <heiko@...ech.de>,
        David Airlie <airlied@...ux.ie>,
        Sean Paul <seanpaul@...omium.org>,
        Daniel Kurtz <djkurtz@...omium.org>
Subject: Re: [PATCH 2/8] drm/rockchip: Get rid of some unnecessary code

Hi Mark,

On Sun, Sep 18, 2016 at 10:50 AM, Mark yao <mark.yao@...k-chips.com> wrote:
> On 2016年09月14日 20:54, Tomasz Figa wrote:
>>
>> Current code implements prepare_fb and cleanup_fb callbacks only to
>> grab/release fb references, which is already done by atomic framework
>> when creating/destryoing plane state. Let's remove these
>> unused bits.
>>
>> Signed-off-by: Tomasz Figa <tfiga@...omium.org>
>> ---
>>   drivers/gpu/drm/rockchip/rockchip_drm_vop.c | 18 ------------------
>>   1 file changed, 18 deletions(-)
>
>
> Hi Tomasz
>
> I think we can't get rid of the prepare_fb and cleanup_fb

I think I have to disagree. Please see below for detailed explanation.

>
> see the reason:
> commit 44d0237a26395ac94160cf23f32769013b365590
> Author: Mark Yao <mark.yao@...k-chips.com>
> Date:   Fri Apr 29 11:37:20 2016 +0800
>
>     drm/rockchip: vop: fix iommu crash with async atomic
>
>     After async atomic_commit callback, drm_atomic_clean_old_fb will
>     clean all old fb, but because async, the old fb may be also on
>     the vop hardware, dma will access the old fb buffer, clean old
>     fb will cause iommu page fault.

I think the above is not quite right. Atomic plane state holds a
reference to its fb and old state is not supposed to be destroyed
until the flip completes.

Indeed current rockchip_atomic_commit implementation has following
order of calls: rockchip_atomic_wait_for_complete(),
drm_atomic_helper_cleanup_planes(), drm_atomic_state_free(). This
means that .cleanup_fb() is called from
drm_atomic_helper_cleanup_planes() just before drm_atomic_state_free()
will release references by destroying old plane states. Note that both
are called already after rockchip_atomic_wait_for_complete(), so it
should be already safe to free the old fbs.

So the above fix doesn't really do anything, possibly just covers the
race condition of the original wait for vblank function by delaying
drm_atomic_state_free() a bit.

Moreover, the whole series has been thoroughly tested in Chrome OS 4.4
kernel, including async commits. (There is still a possibility some
newer upstream changes slightly modified the semantics, but I couldn't
find such difference. Actually one of the advantages of atomic helpers
was to avoid manually refcounting the fbs from the driver.)

Best regards,
Tomasz