Hello, Laurent. There seems to have some advance in this area. Although there does not seem to be maintainer for mga in X.Org, I have seen similar reports with current kernels that claim to have overlay and DRI activated.
I do not have the hardware, so could you please test your problem in some G200 or G400 card with current xorg packages in unstable and send us to the bug address the result? Best regards, Ender. -- Interesante no, Carmiña. Estresante. -- Pazos (Airbag). -- Debian developer
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