Re: [PATCH 2/6] drm/ttm: Store the bo_kmap_type in struct iosys_map

[Christian König <christian.koenig@xxxxxxx>]

Hi,

Am 17.06.24 um 14:32 schrieb Thomas Zimmermann:
> Hi
>
> Am 14.06.24 um 16:31 schrieb Christian König:
> > Am 14.06.24 um 15:21 schrieb Thomas Zimmermann:
> > > For each instances of struct iosys_map set up by ttm_bo_vmap(), store
> > > the type of allocation in the instance. Use this information to unmap
> > > the memory in ttm_bo_vunmap(). This change simplifies the unmap code
> > > and puts the complicated logic entirely into the map code.
> >
> > I'm not sure that's a good idea.
> >
> > The mapping information should already be available in the resource 
> > and storing it in the iosys_map structures duplicates that information.
> >
> > So we might run into the issue that the resource has changed and so 
> > we need a different approach now, but the iosys_map will say that we 
> > should unmap things for example.
>
> Patches 1 and 2 are only here to make patch 4 (add the kmap special 
> case) work. How can I distinguish between vmap'ed and kmap'ed memory? 
> It's not clear to me, whether there is a benefit from patch 4. The xe 
> driver makes it sound like that, but the rest of the kernel appears to 
> disagree.

Yeah, exactly that's the point.

The last time we talked about that we came to the conclusion that the 
kmap approach of mapping only a single page or range of pages isn't that 
useful in general.

The only use case where you actually need this is the 
ttm_bo_vm_access_kmap() helper and that is static and internal to TTM.

So what exactly is the use case xe tries to handle here?

Regards,
Christian.

> Best regards
> Thomas
>
> > Regards,
> > Christian.
> >
> > > Signed-off-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
> > > ---
> > >   drivers/gpu/drm/ttm/ttm_bo_util.c | 46 
> > > +++++++++++++++++++++----------
> > >   1 file changed, 31 insertions(+), 15 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/ttm/ttm_bo_util.c 
> > > b/drivers/gpu/drm/ttm/ttm_bo_util.c
> > > index 0b3f4267130c4..a9df0deff2deb 100644
> > > --- a/drivers/gpu/drm/ttm/ttm_bo_util.c
> > > +++ b/drivers/gpu/drm/ttm/ttm_bo_util.c
> > > @@ -36,6 +36,7 @@
> > >   #include <drm/ttm/ttm_tt.h>
> > >     #include <drm/drm_cache.h>
> > > +#include <drm/drm_device.h>
> > >     struct ttm_transfer_obj {
> > >       struct ttm_buffer_object base;
> > > @@ -479,24 +480,29 @@ int ttm_bo_vmap(struct ttm_buffer_object *bo, 
> > > struct iosys_map *map)
> > >         if (mem->bus.is_iomem) {
> > >           void __iomem *vaddr_iomem;
> > > +        u16 alloc_flags;
> > >   -        if (mem->bus.addr)
> > > +        if (mem->bus.addr) {
> > >               vaddr_iomem = (void __iomem *)mem->bus.addr;
> > > -        else if (mem->bus.caching == ttm_write_combined)
> > > -            vaddr_iomem = ioremap_wc(mem->bus.offset,
> > > -                         bo->base.size);
> > > +            alloc_flags = ttm_bo_map_premapped;
> > > +        } else if (mem->bus.caching == ttm_write_combined) {
> > > +            vaddr_iomem = ioremap_wc(mem->bus.offset, bo->base.size);
> > > +            alloc_flags = ttm_bo_map_iomap;
> > >   #ifdef CONFIG_X86
> > > -        else if (mem->bus.caching == ttm_cached)
> > > -            vaddr_iomem = ioremap_cache(mem->bus.offset,
> > > -                          bo->base.size);
> > > +        } else if (mem->bus.caching == ttm_cached) {
> > > +            vaddr_iomem = ioremap_cache(mem->bus.offset, 
> > > bo->base.size);
> > > +            alloc_flags = ttm_bo_map_iomap;
> > >   #endif
> > > -        else
> > > +        } else {
> > >               vaddr_iomem = ioremap(mem->bus.offset, bo->base.size);
> > > +            alloc_flags = ttm_bo_map_iomap;
> > > +        }
> > >             if (!vaddr_iomem)
> > >               return -ENOMEM;
> > >             iosys_map_set_vaddr_iomem(map, vaddr_iomem);
> > > +        map->alloc_flags = alloc_flags;
> > >         } else {
> > >           struct ttm_operation_ctx ctx = {
> > > @@ -506,6 +512,7 @@ int ttm_bo_vmap(struct ttm_buffer_object *bo, 
> > > struct iosys_map *map)
> > >           struct ttm_tt *ttm = bo->ttm;
> > >           pgprot_t prot;
> > >           void *vaddr;
> > > +        u16 alloc_flags;
> > >             ret = ttm_tt_populate(bo->bdev, ttm, &ctx);
> > >           if (ret)
> > > @@ -519,8 +526,10 @@ int ttm_bo_vmap(struct ttm_buffer_object *bo, 
> > > struct iosys_map *map)
> > >           vaddr = vmap(ttm->pages, ttm->num_pages, 0, prot);
> > >           if (!vaddr)
> > >               return -ENOMEM;
> > > +        alloc_flags = ttm_bo_map_vmap;
> > >             iosys_map_set_vaddr(map, vaddr);
> > > +        map->alloc_flags = alloc_flags;
> > >       }
> > >         return 0;
> > > @@ -537,20 +546,27 @@ EXPORT_SYMBOL(ttm_bo_vmap);
> > >    */
> > >   void ttm_bo_vunmap(struct ttm_buffer_object *bo, struct iosys_map 
> > > *map)
> > >   {
> > > -    struct ttm_resource *mem = bo->resource;
> > > -
> > >       dma_resv_assert_held(bo->base.resv);
> > >         if (iosys_map_is_null(map))
> > >           return;
> > >   -    if (!map->is_iomem)
> > > -        vunmap(map->vaddr);
> > > -    else if (!mem->bus.addr)
> > > +    switch (map->alloc_flags) {
> > > +    case ttm_bo_map_iomap:
> > >           iounmap(map->vaddr_iomem);
> > > -    iosys_map_clear(map);
> > > -
> > > +        break;
> > > +    case ttm_bo_map_vmap:
> > > +        vunmap(map->vaddr);
> > > +        break;
> > > +    case ttm_bo_map_premapped:
> > > +        break;
> > > +    default:
> > > +        drm_err(bo->base.dev, "Unsupported alloc_flags 0x%x\n", 
> > > map->alloc_flags);
> > > +        return;
> > > +    }
> > >       ttm_mem_io_free(bo->bdev, bo->resource);
> > > +
> > > +    iosys_map_clear(map);
> > >   }
> > >   EXPORT_SYMBOL(ttm_bo_vunmap);