Hello, Thank you for the patches! On 2/27/24 13:14, Thomas Zimmermann wrote: > Dma-buf locking semantics require the caller of pin and unpin to hold > the buffer's reservation lock. Fix DRM to adhere to the specs. This > enables to fix the locking in DRM's console emulation. Similar changes > for vmap and mmap have been posted at [1][2] > > Most DRM drivers and memory managers acquire the buffer object's > reservation lock within their GEM pin and unpin callbacks. This > violates dma-buf locking semantics. We get away with it because PRIME > does not provide pin/unpin, but attach/detach, for which the locking > semantics is correct. > > Patches 1 to 8 rework DRM GEM code in various implementations to > acquire the reservation lock when entering the pin and unpin callbacks. > This prepares them for the next patch. Drivers that are not affected > by these patches either don't acquire the reservation lock (amdgpu) > or don't need preparation (loongson). > > Patch 9 moves reservation locking from the GEM pin/unpin callbacks > into drm_gem_pin() and drm_gem_unpin(). As PRIME uses these functions > internally it still gets the reservation lock. > > With the updated GEM callbacks, the rest of the patchset fixes the > fbdev emulation's buffer locking. Fbdev emulation needs to keep its > GEM buffer object inplace while updating its content. This required > a implicit pinning and apparently amdgpu didn't do this at all. > > Patch 10 introduces drm_client_buffer_vmap_local() and _vunmap_local(). > The former function map a GEM buffer into the kernel's address space > with regular vmap operations, but keeps holding the reservation lock. > The _vunmap_local() helper undoes the vmap and releases the lock. The > updated GEM callbacks make this possible. Between the two calls, the > fbdev emulation can update the buffer content without have the buffer > moved or evicted. Update fbdev-generic to use vmap_local helpers, > which fix amdgpu. The idea of adding a "local vmap" has previously been > attempted at [3] in a different form. > > Patch 11 adds implicit pinning to the DRM client's regular vmap > helper so that long-term vmap'ed buffers won't be evicted. This only > affects fbdev-dma, but GEM DMA helpers don't require pinning. So > there are no practical changes. > > Patches 12 and 13 remove implicit pinning from the vmap and vunmap > operations in gem-vram and qxl. These pin operations are not supposed > to be part of vmap code, but were required to keep the buffers in place > for fbdev emulation. With the conversion o ffbdev-generic to to > vmap_local helpers, that code can finally be removed. Isn't it a common behaviour for all DRM drivers to implicitly pin BO while it's vmapped? I was sure it should be common /o\ Why would you want to kmap BO that isn't pinned? Shouldn't TTM's vmap() be changed to do the pinning? I missed that TTM doesn't pin BO on vmap() and now surprised to see it. It should be a rather serious problem requiring backporting of the fixes, but I don't see the fixes tags on the patches (?) -- Best regards, Dmitry
