linux-kernel - Re: [PATCH 04/12] dma-buf: add optional invalidate

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Message-ID: <0611f62c-2b81-b85f-a8d9-69c3daf0c635@amd.com>
Date:   Thu, 18 Apr 2019 08:28:51 +0000
From:   "Koenig, Christian" <Christian.Koenig@....com>
To:     "sumit.semwal@...aro.org" <sumit.semwal@...aro.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "amd-gfx@...ts.freedesktop.org" <amd-gfx@...ts.freedesktop.org>
Subject: Re: [PATCH 04/12] dma-buf: add optional invalidate_mappings callback
 v5

Am 18.04.19 um 10:08 schrieb Daniel Vetter:
> On Wed, Apr 17, 2019 at 09:13:22PM +0200, Christian König wrote:
>> Am 17.04.19 um 21:07 schrieb Daniel Vetter:
>>> On Tue, Apr 16, 2019 at 08:38:33PM +0200, Christian König wrote:
>>>> Each importer can now provide an invalidate_mappings callback.
>>>>
>>>> This allows the exporter to provide the mappings without the need to pin
>>>> the backing store.
>>>>
>>>> v2: don't try to invalidate mappings when the callback is NULL,
>>>>       lock the reservation obj while using the attachments,
>>>>       add helper to set the callback
>>>> v3: move flag for invalidation support into the DMA-buf,
>>>>       use new attach_info structure to set the callback
>>>> v4: use importer_priv field instead of mangling exporter priv.
>>>> v5: drop invalidation_supported flag
>>>>
>>>> Signed-off-by: Christian König <christian.koenig@....com>
>>>> ---
>>>>    drivers/dma-buf/dma-buf.c | 37 +++++++++++++++++++++++++++++++++++++
>>>>    include/linux/dma-buf.h   | 33 +++++++++++++++++++++++++++++++--
>>>>    2 files changed, 68 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
>>>> index 83c92bfd964c..a3738fab3927 100644
>>>> --- a/drivers/dma-buf/dma-buf.c
>>>> +++ b/drivers/dma-buf/dma-buf.c
>>>> @@ -563,6 +563,8 @@ struct dma_buf_attachment *dma_buf_attach(const struct dma_buf_attach_info *info
>>>>    	attach->dev = info->dev;
>>>>    	attach->dmabuf = dmabuf;
>>>> +	attach->importer_priv = info->importer_priv;
>>>> +	attach->invalidate = info->invalidate;
>>>>    	mutex_lock(&dmabuf->lock);
>>>> @@ -571,7 +573,9 @@ struct dma_buf_attachment *dma_buf_attach(const struct dma_buf_attach_info *info
>>>>    		if (ret)
>>>>    			goto err_attach;
>>>>    	}
>>>> +	reservation_object_lock(dmabuf->resv, NULL);
>>>>    	list_add(&attach->node, &dmabuf->attachments);
>>>> +	reservation_object_unlock(dmabuf->resv);
>>>>    	mutex_unlock(&dmabuf->lock);
>>>> @@ -615,7 +619,9 @@ void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *attach)
>>>>    					   DMA_BIDIRECTIONAL);
>>>>    	mutex_lock(&dmabuf->lock);
>>>> +	reservation_object_lock(dmabuf->resv, NULL);
>>>>    	list_del(&attach->node);
>>>> +	reservation_object_unlock(dmabuf->resv);
>>>>    	if (dmabuf->ops->detach)
>>>>    		dmabuf->ops->detach(dmabuf, attach);
>>>> @@ -653,7 +659,16 @@ dma_buf_map_attachment_locked(struct dma_buf_attachment *attach,
>>>>    	if (attach->sgt)
>>>>    		return attach->sgt;
>>>> +	/*
>>>> +	 * Mapping a DMA-buf can trigger its invalidation, prevent sending this
>>>> +	 * event to the caller by temporary removing this attachment from the
>>>> +	 * list.
>>>> +	 */
>>>> +	if (attach->invalidate)
>>>> +		list_del(&attach->node);
>>> Just noticed this: Why do we need this? invalidate needs the reservation
>>> lock, as does map_attachment. It should be impssoble to have someone else
>>> sneak in here.
>> I was having problems with self triggered invalidations.
>>
>> E.g. client A tries to map an attachment, that in turn causes the buffer to
>> move to a new place and client A is informed about that movement with an
>> invalidation.
> Uh, that sounds like a bug in ttm or somewhere else in the exporter. If
> you evict the bo that you're trying to map, that's bad.
>
> Or maybe it's a framework bug, and we need to track whether an attachment
> has a map or not. That would make more sense ...

Well neither, as far as I can see this is perfectly normal behavior.

We just don't want any invalidation send to a driver which is currently 
making a mapping.

If you want I can do this in the driver as well, but at least of hand it 
looks like a good idea to have that in common code.

Tracking the mappings could work as well, but the problem here is that I 
actually want the lifetime of old and new mappings to overlap for 
pipelining.

Regards,
Christian.

> -Daniel