Mitchell Horne a77e1f0f81 busdma: better handling of small segment bouncing ... Typically, when a DMA transaction requires bouncing, we will break up the request into segments that are, at maximum, page-sized. However, in the atypical case of a driver whose maximum segment size is smaller than PAGE_SIZE, we end up inefficiently assigning each segment its own bounce page. For example, the dwmmc driver has a maximum segment size of 2048 (PAGE_SIZE / 2); a 4-page transfer ends up requiring 8 bounce pages in the current scheme. We should attempt to batch segments into bounce pages more efficiently. This is achieved by pushing all considerations of the maximum segment size into the new _bus_dmamap_addsegs() function, which wraps _bus_dmamap_addseg(). Thus we allocate the minimal number of bounce pages required to complete the entire transfer, while still performing the transfer with smaller-sized transactions. For most drivers with a segment size >= PAGE_SIZE, this will have no impact. For drivers like dwmmc mentioned above, this improves the memory and performance efficiency when bouncing a large transfer. Co-authored-by: jhb Reviewed by: jhb MFC after: 1 month Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D45048		2024-05-07 13:02:57 -03:00
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