linux-kernel - Re: [PATCH v2 10/11] drm/qxl: rework cursor plane

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Message-ID: <20210218115044.7tsi2szbdlw6lvdi@sirius.home.kraxel.org>
Date:   Thu, 18 Feb 2021 12:50:44 +0100
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>
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 v2 10/11] drm/qxl: rework cursor plane

  Hi,

> I'm still trying to wrap my head around the qxl cursor code.
> 
> Getting vmap out of the commit tail is good, but I feel like this isn't
> going in the right direction overall.
> 
> In ast, these helper functions were only good when converting the drvier to
> atomic modesetting. So I removed them in the latst patchset and did all the
> updates in the plane helpers directly.

I see the helper functions more as a way to get some structure into the
code flow.  The callbacks are easier to read if they just call helper
functions for stuff which needs more than a handful lines of code
(patch 9/11 exists for the same reason).

The helpers also make it easier move work from one callback to another,
but that is just a useful side-effect.

I had considered making that two separate patches, one factor out code
into functions and one moving the calls.  Turned out to not be that easy
though, because the old qxl_cursor_atomic_update() code was a rather
hairy mix of qxl_create_cursor() + qxl_primary_apply_cursor() +
qxl_primary_move_cursor().

> For cursor_bo itself, it seems to be transitional state that is only used
> during the plane update and crtc update . It should probably be stored in a
> plane-state structure.
> 
> Some of the primary plane's functions seem to deal with cursor handling.
> What's the role of the primary plane in cursor handling?

It's a quirk.  The qxl device will forget the cursor state on
qxl_io_create_primary(), so I have to remember the cursor state
and re-establish it by calling qxl_primary_apply_cursor() again.

So I'm not sure sticking this into plane state would work.  Because of
the quirk this is more than just a handover from prepare to commit.

> For now, I suggest to merge patch 1 to 8 and 11; and move the cursor patches
> into a new patchset.

I can merge 1-8, but 11 has to wait until the cursor is sorted.
There is a reason why 11 is last in the series ;)

> I'd like ot hear Daniel's opinion on this. Do you have
> further plans here?

Well.  I suspect I could easily spend a month cleaning up and party
redesign the qxl driver (specifically qxl_draw.c + qxl_image.c).

I'm not sure I'll find the time to actually do that anytime soon.
I have plenty of other stuff on my TODO list, and given that the
world is transitioning to virtio-gpu the priority for qxl isn't
that high.

So, no, I have no short-term plans for qxl beyond fixing pins +
reservations + lockdep.

take care,
  Gerd