Hello,
I have a DRM driver which implements display protocol for Xen [1]
and this protocol has a dedicated XENDISPL_OP_PG_FLIP request which
tells the other end that some display buffer needs to be displayed,
e.g. it is issued by the frontend DRM driver to the corresponding
backend when the former wants to display a buffer.
Two notes:
1. Communication between two remote parties can obviously fail.
2. At the moment only primary plane is supported, so we can think of
the display buffer as of CRTC's primary fb.
There are two APIs available for user-space to initiate a page-flip
(please correct me if I am wrong here): legacy via DRM_IOCTL_MODE_PAGE_FLIP
and more recent via DRM_IOCTL_MODE_ATOMIC. My current implementation
(which is 4.9 based) uses drm_crtc_funcs.page_flip callback to send
XENDISPL_OP_PG_FLIP request to the backend, so if communication fails
I can return an error code, so the DRM core knows that page flip
cannot be done.
But now I am about to enable page flipping via DRM_IOCTL_MODE_ATOMIC for
which this happens without DRM_IOCTL_MODE_PAGE_FLIP, but via .atomic_check +
.atomic_flush callbacks.
The solution I have in mind is that I move the XENDISPL_OP_PG_FLIP request
to the .atomic_flush callback which seems to be the right place to serve
both DRM_IOCTL_MODE_PAGE_FLIP and DRM_IOCTL_MODE_ATOMIC (is this?).
The questions I have with this are:
1. What is the framebuffer I can send to the backend?
I assume I can use crtc->primary->fb from
drm_crtc_helper_funcs.atomic_flush,
is this assumption correct?
2. Is the check (either to send or not) for crtc->primary->fb != NULL
enough?
I assume there are other cases when .atomic_flush gets called,
so is there a way I can filter out unneeded cases? E.g. those which
are not for page flipping?
3. If communication with the backend fails there is no way for me
to tell the DRM core about this as .atomic_flush is called after
"the point of no return". Do you think I can remember the error
code and report it next time .atomic_check is called? So, other page
flips will not run on broken connection, for example.
4. As per my understanding I cannot put XENDISPL_OP_PG_FLIP request
into .atomic_check as the check doesn't guarantee that .atomic_flush
will follow. Is this correct or there is a more neat solution exists?
Thank you very much for you time,
Oleksandr Andrushchenko
[1]
http://elixir.free-electrons.com/linux/v4.15/source/include/xen/interface/io/displif.h
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