在 2024-06-29星期六的 20:57 +0100，Jiaxun Yang写道：
>
>
> 在2024年6月29日六月 上午6:22，Icenowy Zheng写道：
> [...]
> > @@ -302,6 +302,10 @@ pgprot_t ttm_io_prot(struct ttm_buffer_object
> > *bo,
> > struct ttm_resource *res,
> > caching = res->bus.caching;
> > }
> >
> > + /* Downgrade cached mapping for non-snooping devices */
> > + if (!bo->bdev->dma_coherent && caching == ttm_cached)
> > + caching = ttm_write_combined;
> Hi Icenowy,
>
> Thanks for your patch! You saved many non-coh PCIe host
> implementations a day!.
>
> Unfortunately I don't think we can safely ttm_cached to
> ttm_write_comnined, we've
> had enough drama with write combine behaviour on all different
> platforms.
I think on these platforms, ttm_write_combined should be prevented to
be mapped to pgprot_writecombine instead, but downgrade ttm_cached to
ttm_write_combined (and then being treated same as ttm_uncached) is
acceptable.
>
> See drm_arch_can_wc_memory in drm_cache.h.
>
> Thanks
>
> > +
> > return ttm_prot_from_caching(caching, tmp);
> > }
> > EXPORT_SYMBOL(ttm_io_prot);
> > diff --git a/drivers/gpu/drm/ttm/ttm_tt.c
> > b/drivers/gpu/drm/ttm/ttm_tt.c
> > index 7b00ddf0ce49f..3335df45fba5e 100644
> > --- a/drivers/gpu/drm/ttm/ttm_tt.c
> > +++ b/drivers/gpu/drm/ttm/ttm_tt.c
> > @@ -152,6 +152,10 @@ static void ttm_tt_init_fields(struct ttm_tt
> > *ttm,
> > enum ttm_caching caching,
> > unsigned long extra_pages)
> > {
> > + /* Downgrade cached mapping for non-snooping devices */
> > + if (!bo->bdev->dma_coherent && caching == ttm_cached)
> > + caching = ttm_write_combined;
> > +
> > ttm->num_pages = (PAGE_ALIGN(bo->base.size) >> PAGE_SHIFT)
> > + extra_pages;
> > ttm->page_flags = page_flags;
> > ttm->dma_address = NULL;
> > diff --git a/include/drm/ttm/ttm_caching.h
> > b/include/drm/ttm/ttm_caching.h
> > index a18f43e93abab..f92d7911f50e4 100644
> > --- a/include/drm/ttm/ttm_caching.h
> > +++ b/include/drm/ttm/ttm_caching.h
> > @@ -47,7 +47,8 @@ enum ttm_caching {
> >
> > /**
> > * @ttm_cached: Fully cached like normal system memory,
> > requires that
> > - * devices snoop the CPU cache on accesses.
> > + * devices snoop the CPU cache on accesses. Downgraded to
> > + * ttm_write_combined when the snooping capaiblity is
> > missing.
> > */
> > ttm_cached
> > };
> > --
> > 2.45.2
>
