linux-kernel - Re: [PATCH v3 10/23] drm/qxl: move qxl_primary_apply

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Date:   Mon, 28 Jan 2019 11:38:27 +0100
From:   Noralf Trønnes <noralf@...nnes.org>
To:     Gerd Hoffmann <kraxel@...hat.com>
Cc:     dri-devel@...ts.freedesktop.org, David Airlie <airlied@...ux.ie>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:DRM DRIVER FOR QXL VIRTUAL GPU" 
        <virtualization@...ts.linux-foundation.org>,
        "open list:DRM DRIVER FOR QXL VIRTUAL GPU" 
        <spice-devel@...ts.freedesktop.org>,
        Dave Airlie <airlied@...hat.com>
Subject: Re: [PATCH v3 10/23] drm/qxl: move qxl_primary_apply_cursor to
 correct place



Den 28.01.2019 09.10, skrev Gerd Hoffmann:
>>> The cursor must be set again after creating the primary surface.
>>> Also drop the error message.
> 
>>>  	if (!bo->is_primary) {
>>> -		if (!same_shadow)
>>> +		if (!same_shadow) {
>>>  			qxl_io_create_primary(qdev, 0, bo);
>>> +			qxl_primary_apply_cursor(plane);
>>> +		}
>>>  		bo->is_primary = true;
>>>  	}
>>>  
>>>
>>
>> I don't see how the commit message matches what you're doing. It gives
>> the impression that it must be applied under yet another condition, but
>> the condition for applying the cursor is changed from bo_old->is_primary
>> to !bo->is_primary.
> 
> The qxl device ties the cursor to the primary surface.  Therefore
> calling qxl_io_destroy_primary() and qxl_io_create_primary() to switch
> the framebuffer causes the cursor information being lost and the driver
> must re-apply it.
> 
> The correct call order to do that is qxl_io_destroy_primary() +
> qxl_io_create_primary() + qxl_primary_apply_cursor().
> 
> The old code did qxl_io_destroy_primary() + qxl_primary_apply_cursor() +
> qxl_io_create_primary().  Due to qxl_primary_apply_cursor request being
> queued in a ringbuffer and qxl_io_create_primary() trapping to the
> hypervisor instantly there is a high chance that qxl_io_create_primary()
> is processed first even with the wrong call order.  But it's racy and
> thus not reliable.
> 
>> It probably makes sense to someone that knows the driver.
> 
> If the above explains things better to you I should probably replace the
> commit message with that.
> 

This is actually my first review of a driver that I'm not familiar with.
I'm not quite sure how much in depth understanding that is required to
put my ack on it. Going further into the patchset I realised that
there's no way that I can verify the logic without being intimate with
the driver. So I have tried to verify things from a kms point of view.

I liked your expanded explanation better.

Noralf.

>> Acked-by: Noralf Trønnes <noralf@...nnes.org>
> 
> thanks,
>   Gerd
>