Adding Jason to this discussion. Hi Bobby,
> Subject: [PATCH net-next v2 2/4] udmabuf: emit one sg entry per pinned > folio > > From: Bobby Eshleman <[email protected]> > > get_sg_table() emitted one PAGE_SIZE sg entry per page even when the > underlying folio was larger. > > Instead, walk folios[] and emit one sg entry per folio. When folios We have recently merged a patch (that will make it into 7.2) from Jason that replaced sg_set_folio() with sg_alloc_table_from_pages() in udmabuf driver: https://gitlab.freedesktop.org/drm/tip/-/commit/5bf888673e0dda5a53220fa0c4956271a46c353c Since you are relying on sg_set_folio(), the core argument against its usage in udmabuf is that it doesn't work well with offsets > PAGE_SIZE, resulting in a malformed scatterlist. Not sure if this can be fixed easily. > represent large pages (as is for MFD_HUGETLB), each sg entry is a large > page. Normal PAGE_SIZE sg tables are unchanged. > > This is helpful for importers like net/core/devmem that expect dmabuf sg IMO, udmabuf needs to detect whether importers can handle segments that are > PAGE_SIZE and set the entries appropriately. Please look into how the GPU drivers and other dmabuf exporters/importers handle this situation, so that we can adopt best practices to address this issue. Thanks, Vivek > entries to be size and length aligned. Prior to this patch udmabuf > handed over one PAGE_SIZE sg entry per page, so devmem only saw > PAGE_SIZE chunks regardless of the underlying folio size. > > dma_map_sgtable() does not always merge contiguous pages for us, so we > do this internally before exporting. > > Signed-off-by: Bobby Eshleman <[email protected]> > --- > drivers/dma-buf/udmabuf.c | 52 > ++++++++++++++++++++++++++++++++++++++++++----- > 1 file changed, 47 insertions(+), 5 deletions(-) > > diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c > index 94b8ecb892bb..9b751dd98b12 100644 > --- a/drivers/dma-buf/udmabuf.c > +++ b/drivers/dma-buf/udmabuf.c > @@ -141,26 +141,68 @@ static void vunmap_udmabuf(struct dma_buf > *buf, struct iosys_map *map) > vm_unmap_ram(map->vaddr, ubuf->pagecount); > } > > +/* Return the number of contiguous pages backed by the folio at @i. > + * A udmabuf may map only part of a folio, or reference the same folio > + * in multiple non-contiguous runs, so folio_nr_pages() can't be used. > + */ > +static pgoff_t udmabuf_folio_nr_pages(struct udmabuf *ubuf, pgoff_t i) > +{ > + struct folio *f = ubuf->folios[i]; > + pgoff_t j; > + > + for (j = 1; i + j < ubuf->pagecount; j++) { > + if (ubuf->folios[i + j] != f) > + break; > + /* Same folio, but not a sequential offset within it. */ > + if (ubuf->offsets[i + j] != ubuf->offsets[i] + j * PAGE_SIZE) > + break; > + } > + return j; > +} > + > +/* Count the contiguous folio runs in @ubuf, one sg entry per run. > + * > + * Coalescing folios into a single sg entry up front lets importers actually > + * see large chunks. We can't rely on dma_map_sgtable() to do this for us > as > + * the dma_map_direct() path preserves the input scatterlist lengths > verbatim. > + */ > +static unsigned int udmabuf_sg_nents(struct udmabuf *ubuf) > +{ > + unsigned int nents = 0; > + pgoff_t i; > + > + for (i = 0; i < ubuf->pagecount; i += udmabuf_folio_nr_pages(ubuf, > i)) > + nents++; > + return nents; > +} > + > static struct sg_table *get_sg_table(struct device *dev, struct dma_buf > *buf, > enum dma_data_direction direction) > { > struct udmabuf *ubuf = buf->priv; > - struct sg_table *sg; > struct scatterlist *sgl; > - unsigned int i = 0; > + struct sg_table *sg; > + pgoff_t i, run; > + unsigned int nents; > int ret; > > + nents = udmabuf_sg_nents(ubuf); > + > sg = kzalloc_obj(*sg); > if (!sg) > return ERR_PTR(-ENOMEM); > > - ret = sg_alloc_table(sg, ubuf->pagecount, GFP_KERNEL); > + ret = sg_alloc_table(sg, nents, GFP_KERNEL); > if (ret < 0) > goto err_alloc; > > - for_each_sg(sg->sgl, sgl, ubuf->pagecount, i) > - sg_set_folio(sgl, ubuf->folios[i], PAGE_SIZE, > + sgl = sg->sgl; > + for (i = 0; i < ubuf->pagecount; i += run) { > + run = udmabuf_folio_nr_pages(ubuf, i); > + sg_set_folio(sgl, ubuf->folios[i], run << PAGE_SHIFT, > ubuf->offsets[i]); > + sgl = sg_next(sgl); > + } > > ret = dma_map_sgtable(dev, sg, direction, 0); > if (ret < 0) > > -- > 2.53.0-Meta
