Hi Giuseppe, I initially believed there would be no distinction in performing R/W operations on DMA or SHM maps. However, in a previous email, David mentioned:
*> The relevant kwin expert, Xaver Hugl, stated in a chat:> "While the overhead on the compositor side would be lower, rendering> into a dmabuf with the CPU is pretty slow, especially on dedicated> GPUs and especially with QPainter."* Upon testing, I discovered that every R/W operation is not synchronized with the backing storage or importers of the buffer, which would potentially slow down QPainter operations. It appears there's a cache memory where written data is temporarily stored for quick reading, and the kernel subsystem schedules DMA transfers in a non-blocking manner. I believe Xaver Hugl was referring to the performance being lower in terms of transfers, as a fence mechanism is required to ensure the compositor finishes importing the buffer into the GPU. Best regards, Eduardo Hopperdietzel
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