linux-kernel - Re: [PATCH drm-misc-next v7 4/7] drm/gpuvm: add an abstraction for a VM / BO combination

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Message-ID: <1043bb3c1156d08015db5478183888892dfeda88.camel@linux.intel.com>
Date:   Wed, 01 Nov 2023 10:41:48 +0100
From:   Thomas Hellström 
        <thomas.hellstrom@...ux.intel.com>
To:     Danilo Krummrich <dakr@...hat.com>, airlied@...il.com,
        daniel@...ll.ch, matthew.brost@...el.com, sarah.walker@...tec.com,
        donald.robson@...tec.com, boris.brezillon@...labora.com,
        christian.koenig@....com, faith@...strand.net
Cc:     dri-devel@...ts.freedesktop.org, nouveau@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH drm-misc-next v7 4/7] drm/gpuvm: add an abstraction for
 a VM / BO combination

Hi, Danilo,

On Tue, 2023-10-31 at 18:52 +0100, Danilo Krummrich wrote:
> On 10/31/23 17:45, Thomas Hellström wrote:
> > On Tue, 2023-10-31 at 17:39 +0100, Danilo Krummrich wrote:
> > > On 10/31/23 12:25, Thomas Hellström wrote:
> > > > On Mon, 2023-10-23 at 22:16 +0200, Danilo Krummrich wrote:
> > > > > Add an abstraction layer between the drm_gpuva mappings of a
> > > > > particular
> > > > > drm_gem_object and this GEM object itself. The abstraction
> > > > > represents
> > > > > a
> > > > > combination of a drm_gem_object and drm_gpuvm. The
> > > > > drm_gem_object
> > > > > holds
> > > > > a list of drm_gpuvm_bo structures (the structure representing
> > > > > this
> > > > > abstraction), while each drm_gpuvm_bo contains list of
> > > > > mappings
> > > > > of
> > > > > this
> > > > > GEM object.
> > > > > 
> > > > > This has multiple advantages:
> > > > > 
> > > > > 1) We can use the drm_gpuvm_bo structure to attach it to
> > > > > various
> > > > > lists
> > > > >      of the drm_gpuvm. This is useful for tracking external
> > > > > and
> > > > > evicted
> > > > >      objects per VM, which is introduced in subsequent
> > > > > patches.
> > > > > 
> > > > > 2) Finding mappings of a certain drm_gem_object mapped in a
> > > > > certain
> > > > >      drm_gpuvm becomes much cheaper.
> > > > > 
> > > > > 3) Drivers can derive and extend the structure to easily
> > > > > represent
> > > > >      driver specific states of a BO for a certain GPUVM.
> > > > > 
> > > > > The idea of this abstraction was taken from amdgpu, hence the
> > > > > credit
> > > > > for
> > > > > this idea goes to the developers of amdgpu.
> > > > > 
> > > > > Cc: Christian König <christian.koenig@....com>
> > > > > Signed-off-by: Danilo Krummrich <dakr@...hat.com>
> > > > > ---
> > > > >    drivers/gpu/drm/drm_gpuvm.c            | 335
> > > > > +++++++++++++++++++++--
> > > > > --
> > > > >    drivers/gpu/drm/nouveau/nouveau_uvmm.c |  64 +++--
> > > > >    include/drm/drm_gem.h                  |  32 +--
> > > > >    include/drm/drm_gpuvm.h                | 188
> > > > > +++++++++++++-
> > > > >    4 files changed, 533 insertions(+), 86 deletions(-)
> > > > 
> > > > That checkpatch.pl error still remains as well.
> > > 
> > > I guess you refer to:
> > > 
> > > ERROR: do not use assignment in if condition
> > > #633: FILE: drivers/gpu/drm/nouveau/nouveau_uvmm.c:1165:
> > > +                       if (!(op->gem.obj = obj))
> > > 
> > > This was an intentional decision, since in this specific case it
> > > seems to
> > > be more readable than the alternatives.
> > > 
> > > However, if we consider this to be a hard rule, which we never
> > > ever
> > > break,
> > > I'm fine changing it too.
> > 
> > With the errors, sooner or later they are going to start generate
> > patches to "fix" them. In this particular case also Xe CI is
> > complaining and abort building when I submit the Xe adaptation, so
> > it'd
> > be good to be checkpatch.pl conformant IMHO.
> 
> Ok, I will change this one.
> 
> However, in general my opinion on coding style is that we should
> preserve us
> the privilege to deviate from it when we agree it makes sense and
> improves
> the code quality.
> 
> Having a CI forcing people to *blindly* follow certain rules and even
> abort
> building isn't very beneficial in that respect.
> 
> Also, consider patches which partially change a line of code that
> already
> contains a coding style "issue" - the CI would also block you on that
> one I
> guess. Besides that it seems to block you on unrelated code, note
> that the
> assignment in question is from Nouveau and not from GPUVM.

Yes, I completely agree that having CI enforce error free coding style
checks is bad, and I'll see if I can get that changed on Xe CI. To my
Knowledge It hasn't always been like that.

But OTOH my take on this is that if there are coding style rules and
recommendations we should try to follow them unless there are *strong*
reasons not to. Sometimes that may result in code that may be a little
harder to read, but OTOH a reviewer won't have to read up on the
component's style flavor before reviewing and it will avoid future
style fix patches.

Thanks,
Thomas


> 
> - Danilo
> 
> > 
> > Thanks,
> > Thomas
> > 
> > 
> > 
> > 
> > > 
> > > > 
> > > > Thanks,
> > > > Thomas
> > > > 
> > > 
> > 
>