If the PrepareAccess hook fails, use the DownloadFromScreen hook to retrieve
driver pixmap contents to a system RAM copy, perform software rendering on that
and copy the results back using the UploadToScreen hook. Use the classic
migration logic to minimize transfers (which as a bonus allows slightly
cleaning up some of the existing mixed pixmap code).
This enables things that weren't possible before with driver-allocated pixmap
storage: If some (or all) GPU pixmap storage can't be mapped directly by the
CPU, this can be handled between the PrepareAccess and
DownloadFrom/UploadToScreen hooks, e.g.:
* Radeon KMS on big endian machines can fail PrepareAccess if the pixmap
requires byte-swapping and swap bytes in DownloadFrom/UploadToScreen.
* Environments where GPU and CPU don't have a shared address space at all.
Here the driver PrepareAccess hook will always fail and leave all transfers
between GPU / CPU storage to the Download/From/UploadToScreen hooks.
Drivers which can handle all pixmaps in the PrepareAccess hook should notice
little if any difference.
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