linux-kernel - Re: [PATCH v6 0/9] Fix cursor planes with virtualized drivers

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Message-ID: <219B7sJmmuzo8lj-2i5F6y0pc8XM03X6NdxHUq_R76N71AcTptEPcpjKLO9Rutriw88YtJDRNqibuR-YICIkhPnrBnQSM-Uu9YCc2uZoOiM=@emersion.fr>
Date:   Mon, 23 Oct 2023 08:19:05 +0000
From:   Simon Ser <contact@...rsion.fr>
To:     Albert Esteve <aesteve@...hat.com>
Cc:     qemu-devel@...gnu.org, zackr@...are.com, linux-doc@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        Maxime Ripard <mripard@...nel.org>, iforbes@...are.com,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Chia-I Wu <olvaffe@...il.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Hans de Goede <hdegoede@...hat.com>,
        Matt Roper <matthew.d.roper@...el.com>,
        David Airlie <airlied@...il.com>, banackm@...are.com,
        Rob Clark <robdclark@...il.com>, javierm@...hat.com,
        krastevm@...are.com, spice-devel@...ts.freedesktop.org,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Jonathan Corbet <corbet@....net>,
        David Airlie <airlied@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, mombasawalam@...are.com,
        Daniel Vetter <daniel@...ll.ch>, ppaalanen@...il.com,
        VMware Graphics Reviewers 
        <linux-graphics-maintainer@...are.com>,
        Gerd Hoffmann <kraxel@...hat.com>
Subject: Re: [PATCH v6 0/9] Fix cursor planes with virtualized drivers

On Monday, October 23rd, 2023 at 10:14, Albert Esteve <aesteve@...hat.com> wrote:

> On Mon, Oct 23, 2023 at 9:55 AM Simon Ser <contact@...rsion.fr> wrote:
> 
> > On Monday, October 23rd, 2023 at 09:46, Albert Esteve <aesteve@...hat.com> wrote:
> > 
> > > Link to the IGT test covering this patch (already merged):
> > > https://lists.freedesktop.org/archives/igt-dev/2023-July/058427.html
> > 
> > Hmm. IGT should not be merged before the kernel, because as long as the
> > kernel is not merged there might be some uAPI changes.
> 
> Right, but uAPI header was not updated on the IGT side. As per suggestion of the
> maintainers, I added a static variable that matches the definition on this patch:
> https://lists.freedesktop.org/archives/igt-dev/2023-August/058803.html
> 
> +/**
> + * Clients which do set cursor hotspot and treat the cursor plane
> + * like a mouse cursor should set this property.
> + */
> +#define LOCAL_DRM_CLIENT_CAP_CURSOR_PLANE_HOTSPOT	6
> 
> Once this patch gets upstreamed, the localized definition will be removed,
> replaced by the real one.

What if this patch gets delayed and another patch using the same number
is merged into the kernel first? What if someone finds a design flaw in
the uAPI and it needs to be completely changed? The IGT test would then
be completely broken.

As a rule of thumb: never merge user-space patches before kernel. As
soon as the kernel part is merged, it's fine to locally copy definitions
if desirable.

> > > Mutter patch:
> > > https://lists.freedesktop.org/archives/igt-dev/2023-July/058427.html
> > 
> > Seems like this link is same as IGT? Copy-pasta fail maybe?
> 
> Ah yes, my bad, this is the correct link:
> https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3337

Thanks!