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SubjectRe: [PATCH v2] drm: Bump encoder limit from 32 to 64
On Tue, Aug 20, 2019 at 08:16:55PM -0400, Lyude Paul wrote:
> Assuming that GPUs would never have even close to 32 separate video
> encoders is quite honestly a pretty reasonable assumption. Unfortunately
> we do not live in a reasonable world, as it looks like it is actually
> possible to find devices that will create more drm_encoder objects then
> this. Case in point: the ThinkPad P71's discrete GPU, which exposes 1
> eDP port and 5 DP ports. On the P71, nouveau attempts to create one
> encoder for the eDP port, and two encoders for each DP++/USB-C port
> along with 4 MST encoders for each DP port. This comes out to 35
> different encoders. Unfortunately, this can't really be optimized to
> make less encoders either.
>
> So, what if we bumped the limit to 64? Unfortunately this has one very
> awkward drawback: we already expose 32-bit bitmasks for encoders to
> userspace in drm_encoder->possible_clones. Yikes. While cloning is still
> (rarely) used in certain modern video hardware, it's mostly used in
> situations where memory bandwidth is so limited that it's not possible
> to scan out from 2 CRTCs at once.
>
> So, let's try to compromise here: allow encoders with indexes <32 to
> have non-zero values in drm_encoder->possible_clones, and don't allow
> encoders with higher indexes to set drm_encoder->possible_clones to a
> non-zero value. This allows us to avoid breaking UAPI and keep things
> working sanely for hardware which still uses cloning, while still being
> able to bump up the encoder limit.
>
> This also fixes driver probing for nouveau on the ThinkPad P71.
>
> Changes since v1:
> * Move index+possible_clones check out of drm_encoder_init() and into
> drm_encoder_register_all(), since encoder->possible_clones can get
> changed any time before registration - Daniel Vetter
> * Update the commit message a bit to accurately reflect modern day usage
> of hardware cloning, which as Daniel Stone pointed out is apparently a
> thing
>
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> Cc: nouveau@lists.freedesktop.org
> ---
> drivers/gpu/drm/drm_atomic.c | 2 +-
> drivers/gpu/drm/drm_encoder.c | 12 ++++++++++--
> include/drm/drm_crtc.h | 2 +-
> include/drm/drm_encoder.h | 20 +++++++++++++++-----
> 4 files changed, 27 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
> index 419381abbdd1..27ce988ef0cc 100644
> --- a/drivers/gpu/drm/drm_atomic.c
> +++ b/drivers/gpu/drm/drm_atomic.c
> @@ -392,7 +392,7 @@ static void drm_atomic_crtc_print_state(struct drm_printer *p,
> drm_printf(p, "\tcolor_mgmt_changed=%d\n", state->color_mgmt_changed);
> drm_printf(p, "\tplane_mask=%x\n", state->plane_mask);
> drm_printf(p, "\tconnector_mask=%x\n", state->connector_mask);
> - drm_printf(p, "\tencoder_mask=%x\n", state->encoder_mask);
> + drm_printf(p, "\tencoder_mask=%llx\n", state->encoder_mask);
> drm_printf(p, "\tmode: " DRM_MODE_FMT "\n", DRM_MODE_ARG(&state->mode));
>
> if (crtc->funcs->atomic_print_state)
> diff --git a/drivers/gpu/drm/drm_encoder.c b/drivers/gpu/drm/drm_encoder.c
> index 7fb47b7b8b44..9d443b45ebba 100644
> --- a/drivers/gpu/drm/drm_encoder.c
> +++ b/drivers/gpu/drm/drm_encoder.c
> @@ -71,6 +71,14 @@ int drm_encoder_register_all(struct drm_device *dev)
> int ret = 0;
>
> drm_for_each_encoder(encoder, dev) {
> + /*
> + * Since possible_clones has been exposed to userspace as a
> + * 32bit bitmask, we don't allow creating encoders with an
> + * index >=32 which are capable of cloning
> + */
> + if (WARN_ON(encoder->index >= 32 && encoder->possible_clones))
> + return -EINVAL;

I believe possible_clones was supposed to include the encoder itself. Not
really sure why though. I guess we've now decided that it's OK not to do
that?

git grep tells me drm_atomic_helper.c has some uses of drm_encoder_mask()
that need to be looked at.

> +
> if (encoder->funcs->late_register)
> ret = encoder->funcs->late_register(encoder);
> if (ret)
> @@ -112,8 +120,8 @@ int drm_encoder_init(struct drm_device *dev,
> {
> int ret;
>
> - /* encoder index is used with 32bit bitmasks */
> - if (WARN_ON(dev->mode_config.num_encoder >= 32))
> + /* encoder index is used with 64bit bitmasks */
> + if (WARN_ON(dev->mode_config.num_encoder >= 64))
> return -EINVAL;
>
> ret = drm_mode_object_add(dev, &encoder->base, DRM_MODE_OBJECT_ENCODER);
> diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
> index 7d14c11bdc0a..fd0b2438c3d5 100644
> --- a/include/drm/drm_crtc.h
> +++ b/include/drm/drm_crtc.h
> @@ -210,7 +210,7 @@ struct drm_crtc_state {
> * @encoder_mask: Bitmask of drm_encoder_mask(encoder) of encoders
> * attached to this CRTC.
> */
> - u32 encoder_mask;
> + u64 encoder_mask;
>
> /**
> * @adjusted_mode:
> diff --git a/include/drm/drm_encoder.h b/include/drm/drm_encoder.h
> index 70cfca03d812..3f9cb65694e1 100644
> --- a/include/drm/drm_encoder.h
> +++ b/include/drm/drm_encoder.h
> @@ -159,7 +159,15 @@ struct drm_encoder {
> * encoders can be used in a cloned configuration, they both should have
> * each another bits set.
> *
> - * In reality almost every driver gets this wrong.
> + * In reality almost every driver gets this wrong, and most modern
> + * display hardware does not have support for cloning. As well, while we
> + * expose this mask to userspace as 32bits long, we do sure purely to
> + * avoid breaking pre-existing UAPI since the limitation on the number
> + * of encoders has been increased from 32 bits to 64 bits. In order to
> + * maintain functionality for drivers which do actually support cloning,
> + * we only allow cloning with encoders that have an index <32. Encoders
> + * with indexes higher than 32 are not allowed to specify a non-zero
> + * value here.
> *
> * Note that since encoder objects can't be hotplugged the assigned indices
> * are stable and hence known before registering all objects.
> @@ -198,13 +206,15 @@ static inline unsigned int drm_encoder_index(const struct drm_encoder *encoder)
> }
>
> /**
> - * drm_encoder_mask - find the mask of a registered ENCODER
> + * drm_encoder_mask - find the mask of a registered encoder
> * @encoder: encoder to find mask for
> *
> - * Given a registered encoder, return the mask bit of that encoder for an
> - * encoder's possible_clones field.
> + * Returns:
> + * A bit mask with the nth bit set, where n is the index of the encoder. Take
> + * care when using this, as the DRM UAPI only allows for 32 bit encoder masks
> + * while internally encoder masks are 64 bits.
> */
> -static inline u32 drm_encoder_mask(const struct drm_encoder *encoder)
> +static inline u64 drm_encoder_mask(const struct drm_encoder *encoder)
> {
> return 1 << drm_encoder_index(encoder);
> }
> --
> 2.21.0
>
> _______________________________________________
> dri-devel mailing list
> dri-devel@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

--
Ville Syrjälä
Intel

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