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Date: Wed, 20 Jul 2022 10:29:42 +0200
From: Christian König <christian.koenig@....com>
To: Dmitry Osipenko <dmitry.osipenko@...labora.com>,
David Airlie <airlied@...ux.ie>,
Gerd Hoffmann <kraxel@...hat.com>,
Gurchetan Singh <gurchetansingh@...omium.org>,
Chia-I Wu <olvaffe@...il.com>, Daniel Vetter <daniel@...ll.ch>,
Daniel Almeida <daniel.almeida@...labora.com>,
Gert Wollny <gert.wollny@...labora.com>,
Gustavo Padovan <gustavo.padovan@...labora.com>,
Daniel Stone <daniel@...ishbar.org>,
Tomeu Vizoso <tomeu.vizoso@...labora.com>,
Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
Maxime Ripard <mripard@...nel.org>,
Thomas Zimmermann <tzimmermann@...e.de>,
Rob Clark <robdclark@...il.com>,
Sumit Semwal <sumit.semwal@...aro.org>,
"Pan, Xinhui" <Xinhui.Pan@....com>,
Thierry Reding <thierry.reding@...il.com>,
Tomasz Figa <tfiga@...omium.org>,
Marek Szyprowski <m.szyprowski@...sung.com>,
Mauro Carvalho Chehab <mchehab@...nel.org>,
Alex Deucher <alexander.deucher@....com>,
Jani Nikula <jani.nikula@...ux.intel.com>,
Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
Rodrigo Vivi <rodrigo.vivi@...el.com>,
Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
Thomas Hellström <thomas_os@...pmail.org>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
Dmitry Osipenko <digetx@...il.com>,
linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
amd-gfx@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
kernel@...labora.com, virtualization@...ts.linux-foundation.org,
spice-devel@...ts.freedesktop.org, linux-rdma@...r.kernel.org,
linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH v1 4/6] dma-buf: Acquire wait-wound context on attachment
Am 19.07.22 um 22:05 schrieb Dmitry Osipenko:
> On 7/15/22 09:59, Dmitry Osipenko wrote:
>> On 7/15/22 09:50, Christian König wrote:
>>> Am 15.07.22 um 02:52 schrieb Dmitry Osipenko:
>>>> Intel i915 GPU driver uses wait-wound mutex to lock multiple GEMs on the
>>>> attachment to the i915 dma-buf. In order to let all drivers utilize
>>>> shared
>>>> wait-wound context during attachment in a general way, make dma-buf
>>>> core to
>>>> acquire the ww context internally for the attachment operation and update
>>>> i915 driver to use the importer's ww context instead of the internal one.
>>>>
>>>> From now on all dma-buf exporters shall use the importer's ww context
>>>> for
>>>> the attachment operation.
>>>>
>>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>
>>>> ---
>>>> drivers/dma-buf/dma-buf.c | 8 +++++-
>>>> drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c | 2 +-
>>>> .../gpu/drm/i915/gem/i915_gem_execbuffer.c | 2 +-
>>>> drivers/gpu/drm/i915/gem/i915_gem_object.h | 6 ++---
>>>> drivers/gpu/drm/i915/i915_gem_evict.c | 2 +-
>>>> drivers/gpu/drm/i915/i915_gem_ww.c | 26 +++++++++++++++----
>>>> drivers/gpu/drm/i915/i915_gem_ww.h | 15 +++++++++--
>>>> 7 files changed, 47 insertions(+), 14 deletions(-)
>>>>
>>>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
>>>> index 0ee588276534..37545ecb845a 100644
>>>> --- a/drivers/dma-buf/dma-buf.c
>>>> +++ b/drivers/dma-buf/dma-buf.c
>>>> @@ -807,6 +807,8 @@ static struct sg_table * __map_dma_buf(struct
>>>> dma_buf_attachment *attach,
>>>> * Optionally this calls &dma_buf_ops.attach to allow
>>>> device-specific attach
>>>> * functionality.
>>>> *
>>>> + * Exporters shall use ww_ctx acquired by this function.
>>>> + *
>>>> * Returns:
>>>> *
>>>> * A pointer to newly created &dma_buf_attachment on success, or a
>>>> negative
>>>> @@ -822,6 +824,7 @@ dma_buf_dynamic_attach_unlocked(struct dma_buf
>>>> *dmabuf, struct device *dev,
>>>> void *importer_priv)
>>>> {
>>>> struct dma_buf_attachment *attach;
>>>> + struct ww_acquire_ctx ww_ctx;
>>>> int ret;
>>>> if (WARN_ON(!dmabuf || !dev))
>>>> @@ -841,7 +844,8 @@ dma_buf_dynamic_attach_unlocked(struct dma_buf
>>>> *dmabuf, struct device *dev,
>>>> attach->importer_ops = importer_ops;
>>>> attach->importer_priv = importer_priv;
>>>> - dma_resv_lock(dmabuf->resv, NULL);
>>>> + ww_acquire_init(&ww_ctx, &reservation_ww_class);
>>>> + dma_resv_lock(dmabuf->resv, &ww_ctx);
>>> That won't work like this. The core property of a WW context is that you
>>> need to unwind all the locks and re-quire them with the contended one
>>> first.
>>>
>>> When you statically lock the imported one here you can't do that any more.
>> You're right. I felt that something is missing here, but couldn't
>> notice. I'll think more about this and enable
>> CONFIG_DEBUG_WW_MUTEX_SLOWPATH. Thank you!
>>
> Christian, do you think we could make an excuse for the attach()
> callback and make the exporter responsible for taking the resv lock? It
> will be inconsistent with the rest of the callbacks, where importer
> takes the lock, but it will be the simplest and least invasive solution.
> It's very messy to do a cross-driver ww locking, I don't think it's the
> right approach.
So to summarize the following calls will require that the caller hold
the resv lock:
1. dma_buf_pin()/dma_buf_unpin()
2. dma_buf_map_attachment()/dma_buf_unmap_attachment()
3. dma_buf_vmap()/dma_buf_vunmap()
4. dma_buf_move_notify()
The following calls require that caller does not held the resv lock:
1. dma_buf_attach()/dma_buf_dynamic_attach()/dma_buf_detach()
2. dma_buf_export()/dma_buf_fd()
3. dma_buf_get()/dma_buf_put()
4. dma_buf_begin_cpu_access()/dma_buf_end_cpu_access()
If that's correct than that would work for me as well, but we should
probably document this.
Or let me ask the other way around: What calls exactly do you need to
change to solve your original issue? That was vmap/vunmap, wasn't it? If
yes then let's concentrate on those for the moment.
Regards,
Christian.
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