linux-kernel - Re: [PATCH drm-misc-next v4 6/8] drm/gpuvm: add drm_gpuvm_flags to drm

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Message-ID: <810dc476-8ead-19e6-23fc-0f9cf35ba2b2@redhat.com>
Date:   Wed, 27 Sep 2023 18:52:55 +0200
From:   Danilo Krummrich <dakr@...hat.com>
To:     Boris Brezillon <boris.brezillon@...labora.com>
Cc:     airlied@...il.com, daniel@...ll.ch, matthew.brost@...el.com,
        thomas.hellstrom@...ux.intel.com, sarah.walker@...tec.com,
        donald.robson@...tec.com, christian.koenig@....com,
        faith.ekstrand@...labora.com, dri-devel@...ts.freedesktop.org,
        nouveau@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH drm-misc-next v4 6/8] drm/gpuvm: add drm_gpuvm_flags to
 drm_gpuvm

On 9/22/23 13:58, Boris Brezillon wrote:
> On Wed, 20 Sep 2023 16:42:39 +0200
> Danilo Krummrich <dakr@...hat.com> wrote:
> 
>> +/**
>> + * enum drm_gpuvm_flags - flags for struct drm_gpuvm
>> + */
>> +enum drm_gpuvm_flags {
>> +	/**
>> +	 * @DRM_GPUVM_USERBITS: user defined bits
>> +	 */
>> +	DRM_GPUVM_USERBITS = (1 << 0),
> 
> Nit: I tried declaring driver-specific flags, and I find this
> counter-intuitive. You basically end up with something like:
> 
> enum my_gpuvm_flags {
> 	MY_FLAG_X = DRM_GPUVM_USERBITS,
> 	MY_FLAG_Y = DRM_GPUVM_USERBITS << 1,
> };
> 
> instead of the usual
> 
> enum my_gpuvm_flags {
> 	MY_FLAG_X = BIT(0),
> 	MY_FLAG_Y = BIT(1),
> };
> 
> pattern.

Right, same as with dma_fence flags.

> 
> Another issue I see coming is if we end up adding more core flags and
> drivers start falling short of bits for their own flags. This makes me
> wonder if we shouldn't kill this notion of USER flags and let drivers
> store their flags in some dedicated field, given they're likely to
> derive drm_gpuvm and drm_gpuva with their own object anyway.

The only reason I have this in the code is that Xe asked for this with
drm_gpuva_flags. Hence, for consistency reasons I added it for drm_gpuvm_flags
too.

Drivers can still have their own flag fields if needed, otherwise I guess it
doesn't really hurt to keep DRM_GPUVM_USERBITS in case someone wants to use it.

> 
>> +};
>> +
>