linux-kernel - Re: [RFC] drm/fourcc: Add a modifier for contiguous memory

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Message-ID: <20221129124859.6aa3ff00@eldfell>
Date:   Tue, 29 Nov 2022 12:48:59 +0200
From:   Pekka Paalanen <ppaalanen@...il.com>
To:     Hsia-Jun Li <randy.li@...aptics.com>
Cc:     dri-devel@...ts.freedesktop.org, linux-media@...r.kernel.org,
        hverkuil@...all.nl, tfiga@...omium.org, nicolas@...fresne.ca,
        maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
        tzimmermann@...e.de, airlied@...il.com, daniel@...ll.ch,
        linux-kernel@...r.kernel.org, ayaka@...lik.info
Subject: Re: [RFC] drm/fourcc: Add a modifier for contiguous memory

On Tue, 29 Nov 2022 18:10:30 +0800
Hsia-Jun Li <randy.li@...aptics.com> wrote:

> From: "Hsia-Jun(Randy) Li" <randy.li@...aptics.com>
> 
> Hello All
> 
> Currently, we assume all the pixel formats are multiple planes,

Hi,

that's not true for any definition of "multiple planes" that I know of.

For example, DRM_FORMAT_XRGB8888 is a single-plane format by definition.

From below it sounds like you mean "physically non-contiguous". But no,
pixel formats make no such assumption at all. Contiguous or not is
independent of pixel formats.

> devices
> could support each component has its own memory plane.
> But that may not apply for any device in the world. We could have a
> device without IOMMU then this is not impossible.
> 
> Besides, when we export an handle through the PRIME, the upstream
> device(likes a capture card or camera) may not support non-contiguous
> memory. It would be better to allocate the handle in contiguous memory
> at the first time.
> 
> We may think the memory allocation is done in user space, we could do
> the trick there. But the dumb_create() sometimes is not the right API
> for that.
> 
> "Note that userspace is not allowed to use such objects for render
> acceleration - drivers must create their own private ioctls for such a
> use case."
> "Note that dumb objects may not be used for gpu acceleration, as has
> been attempted on some ARM embedded platforms. Such drivers really must
> have a hardware-specific ioctl to allocate suitable buffer objects."
> 
> We need to relay on those device custom APIs then. It would be helpful
> for their library to calculate the right size for contiguous memory. It
> would be useful for the driver supports rendering dumb buffer as well.
> 
> Signed-off-by: Hsia-Jun(Randy) Li <randy.li@...aptics.com>
> ---
>  include/uapi/drm/drm_fourcc.h | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
> index bc056f2d537d..ec039ced8257 100644
> --- a/include/uapi/drm/drm_fourcc.h
> +++ b/include/uapi/drm/drm_fourcc.h
> @@ -473,6 +473,11 @@ extern "C" {
>   */
>  #define DRM_FORMAT_MOD_LINEAR	fourcc_mod_code(NONE, 0)
>  
> +/*
> + * Contiguous memory
> + */
> +#define DRM_FORMAT_MOD_CONTIG_MEM	fourcc_mod_code(NONE, 1)

NAK. This is not what modifiers are for.

This also would not work in practise, because if this was a modifier,
you would not be able to use the actual modifiers.


Thanks,
pq

> +
>  /*
>   * Deprecated: use DRM_FORMAT_MOD_LINEAR instead
>   *


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