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Because pages within a single sg entry >>> are physically contiguous, we can populate the page array with a in an >>> inner loop using simple pointer math. This save a lot of time. >>> >>> The WARN_ON check is also pulled out of the loop to save branch >>> instructions. >>> >>> Performance results mapping a 2GB buffer on Radxa O6: >>> - Before: ~1440000 ns >>> - After: ~232000 ns >>> (~84% reduction in iteration time, or ~6.2x faster) >> >> Well real question is why do you care about the vmap performance? >> >> That should basically only be used for fbdev emulation (except for VMGFX) and we absolutely don't care about performance there. > > I agree that in mainline, dma_buf_vmap is not used very often. > Here’s what I was able to find: > > 1 1638 drivers/dma-buf/dma-buf.c <> > ret = dma_buf_vmap(dmabuf, map); > 2 376 drivers/gpu/drm/drm_gem_shmem_helper.c > <> > ret = dma_buf_vmap(obj->import_attach->dmabuf, map); > 3 85 drivers/gpu/drm/etnaviv/etnaviv_gem_prime.c > <> > ret = dma_buf_vmap(etnaviv_obj->base.import_attach->dmabuf, &map); > 4 433 drivers/gpu/drm/vmwgfx/vmwgfx_blit.c <> > ret = dma_buf_vmap(bo->tbo.base.dma_buf, map); > 5 88 drivers/gpu/drm/vmwgfx/vmwgfx_gem.c <> > ret = dma_buf_vmap(obj->import_attach->dmabuf, map); > > However, in the Android ecosystem, system_heap and similar heaps > are widely used across camera, NPU, and media drivers. Many of these > drivers are not in mainline but do use vmap() in real code paths. Well out of tree drivers are not a justification to make an upstream changes. Apart from a handful of workarounds which need to CPU access as fallback DMA-buf vmap is only used to provide fb dev emulation. The vmap interface has already given us quite a headache in the first place and there are a couple of unresolved problems regarding synchronization and coherency. When a driver would be pushed upstream which makes so frequent use of the dma_buf_vmap function that it matters for the performance I think there would be push back on that and the driver developer would require a very good explanation why that is necessary. So for now I have to reject that patch. Regards, Christian. > > As I can show you some of them from MTK platforms: > > 1: > [ 6.689849] system_heap_vmap+0x17c/0x254 [system_heap > 8d35d4ce35bb30d8a623f0b9863998a2528e4175] > [ 6.689859] dma_buf_vmap_unlocked+0xb8/0x130 > [ 6.689861] aov_core_init+0x310/0x718 [mtk_aov > 96e2e5e9457dcdacce3a7629b0600c5dbeca623b] > [ 6.689873] mtk_aov_probe+0x434/0x5b4 [mtk_aov > 96e2e5e9457dcdacce3a7629b0600c5dbeca623b] > > 2: > [ 116.181643] __vmap_pages_range_noflush+0x7c4/0x814 > [ 116.181645] vmap+0xb4/0x148 > [ 116.181647] system_heap_vmap+0x17c/0x254 [system_heap > 8d35d4ce35bb30d8a623f0b9863998a2528e4175] > [ 116.181651] dma_buf_vmap_unlocked+0xb8/0x130 > [ 116.181653] mtk_cam_vb2_vaddr+0xa0/0xfc [mtk_cam_isp8s > 0cf9be6c773a8f14aab9db9ebf53feacb499846a] > [ 116.181682] vb2_plane_vaddr+0x5c/0x78 > [ 116.181684] mtk_cam_job_fill_ipi_frame+0xa8c/0x128c [mtk_cam_isp8s > 0cf9be6c773a8f14aab9db9ebf53feacb499846a] > > 3: > [ 116.306178] __vmap_pages_range_noflush+0x7c4/0x814 > [ 116.306183] vmap+0xb4/0x148 > [ 116.306187] system_heap_vmap+0x17c/0x254 [system_heap > 8d35d4ce35bb30d8a623f0b9863998a2528e4175] > [ 116.306209] dma_buf_vmap_unlocked+0xb8/0x130 > [ 116.306212] apu_sysmem_alloc+0x168/0x360 [apusys > 8fb33cbce3b858d651b9da26fc370090a67cfb70] > [ 116.306468] mdw_mem_alloc+0xd8/0x314 [apusys > 8fb33cbce3b858d651b9da26fc370090a67cfb70] > [ 116.306591] mdw_mem_pool_chunk_add+0x11c/0x400 [apusys > 8fb33cbce3b858d651b9da26fc370090a67cfb70] > [ 116.306712] mdw_mem_pool_create+0x190/0x2c8 [apusys > 8fb33cbce3b858d651b9da26fc370090a67cfb70] > [ 116.306833] mdw_drv_open+0x21c/0x47c [apusys > 8fb33cbce3b858d651b9da26fc370090a67cfb70] > > While we may want to encourage more of these drivers to upstream, > some aspects are beyond our control (different SoC vendors), but we > can at least contribute upstream ourselves. > > Best Regards > Barry