On 15.09.2021 15:37, Roman Skakun wrote: >>> From: Roman Skakun <roman_skakun@xxxxxxxx> >>> >>> It is possible when default IO TLB size is not >>> enough to fit a long buffers as described here [1]. >>> >>> This patch makes a way to set this parameter >>> using cmdline instead of recompiling a kernel. >>> >>> [1] https://www.xilinx.com/support/answers/72694.html >> >> I'm not convinced the swiotlb use describe there falls under "intended >> use" - mapping a 1280x720 framebuffer in a single chunk? > > I had the same issue while mapping DMA chuck ~4MB for gem fb when > using xen vdispl. > I got the next log: > [ 142.030421] rcar-fcp fea2f000.fcp: swiotlb buffer is full (sz: > 3686400 bytes), total 32768 (slots), used 32 (slots) > > It happened when I tried to map bounce buffer, which has a large size. > The default size if 128(IO_TLB_SEGSIZE) * 2048(IO_TLB_SHIFT) = 262144 > bytes, but we requested 3686400 bytes. > When I change IO_TLB_SEGSIZE to 2048. (2048(IO_TLB_SEGSIZE) * > 2048(IO_TLB_SHIFT) = 4194304bytes). > It makes possible to retrieve a bounce buffer for requested size. > After changing this value, the problem is gone. But the question remains: Why does the framebuffer need to be mapped in a single giant chunk? >> In order to be sure to catch all uses like this one (including ones >> which make it upstream in parallel to yours), I think you will want >> to rename the original IO_TLB_SEGSIZE to e.g. IO_TLB_DEFAULT_SEGSIZE. > > I don't understand your point. Can you clarify this? There's a concrete present example: I have a patch pending adding another use of IO_TLB_SEGSIZE. This use would need to be replaced like you do here in several places. The need for the additional replacement would be quite obvious (from a build failure) if you renamed the manifest constant. Without renaming, it'll take someone running into an issue on a live system, which I consider far worse. This is because a simple re-basing of one of the patches on top of the other will not point out the need for the extra replacement, nor would a test build (with both patches in place). Jan
